Updated the user guide's config section to mention specifying plugin commands

WIP: Custom Preprocessors

CONTRIBUTING.md

@@ -48,6 +48,57 @@ mdBook builds on stable Rust, if you want to build mdBook from source, here are
 
 The resulting binary can be found in `mdBook/target/debug/` under the name `mdBook` or `mdBook.exe`.
 
+### Code Quality
+
+We love code quality and Rust has some excellent tools to assist you with contributions.
+
+#### Formatting Code with rustfmt
+
+Before you make your Pull Request to the project, please run it through the `rustfmt` utility.
+This will ensure we have good quality source code that is better for us all to maintain.
+
+[rustfmt](https://github.com/rust-lang-nursery/rustfmt) has a lot more information on the project.
+The quick guide is
+
+1. Install it
+    ```
+    rustup component add rustfmt-preview
+    ```
+1. You can now run `rustfmt` on a single file simply by...
+    ```
+    rustfmt src/path/to/your/file.rs
+    ```
+   ... or you can format the entire project with
+   ```
+   cargo fmt
+   ```
+   When run through `cargo` it will format all bin and lib files in the current crate.
+
+For more information, such as running it from your favourite editor, please see the `rustfmt` project. [rustfmt](https://github.com/rust-lang-nursery/rustfmt)
+
+
+#### Finding Issues with Clippy
+
+Clippy is a code analyser/linter detecting mistakes, and therfore helps to improve your code.
+Like formatting your code with `rustfmt`, running clippy regularly and before your Pull Request will
+help us maintain awesome code.
+
+The best documentation can be found over at [rust-clippy](https://github.com/rust-lang-nursery/rust-clippy)
+
+1. To install
+    ```
+    rustup update
+    rustup install nightly
+    rustup component add clippy-preview --toolchain=nightly
+    ```
+2. Running clippy
+    As you may notice from the previous step, Clippy is on the nightly branch, so running it is like
+    ```
+    cargo +nightly clippy
+    ```
+
+Clippy has an ever growing list of checks, that are managed in [lint files](https://rust-lang-nursery.github.io/rust-clippy/master/index.html).
+
 ### Making a pull-request
 
 When you feel comfortable that your changes could be integrated into mdBook, you can create a pull-request on GitHub.

Cargo.lock

@@ -507,7 +507,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "mdbook"
-version = "0.2.1"
+version = "0.2.2-alpha.0"
 dependencies = [
  "ammonia 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "chrono 0.4.5 (registry+https://github.com/rust-lang/crates.io-index)",

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mdbook"
-version = "0.2.1"
+version = "0.2.2-alpha.0"
 authors = [
     "Mathieu David <mathieudavid@mathieudavid.org>", 
     "Michael-F-Bryan <michaelfbryan@gmail.com>",

README.md

@@ -145,6 +145,57 @@ explanation, check out the [User Guide].
 
     Delete directory in which generated book is located.
 
+### 3rd Party Plugins
+
+The way a book is loaded and rendered can be configured by the user via third
+party plugins. These plugins are just programs which will be invoked during the
+build process and are split into roughly two categories, *preprocessors* and
+*renderers*.
+
+Preprocessors are used to transform a book before it is sent to a renderer. 
+One example would be to replace all occurrences of 
+`{{#include some_file.ext}}` with the contents of that file. Some existing 
+preprocessors are:
+
+- `index` - a built-in preprocessor (enabled by default) which will transform
+  all `README.md` chapters to `index.md` so `foo/README.md` can be accessed via
+  the url `foo/` when published to a browser
+- `links` - a built-in preprocessor (enabled by default) for expanding the
+  `{{# playpen}}` and `{{# include}}` helpers in a chapter.
+
+Renderers are given the final book so they can do something with it. This is
+typically used for, as the name suggests, rendering the document in a particular
+format, however there's nothing stopping a renderer from doing static analysis
+of a book in order to validate links or run tests. Some existing renderers are:
+
+- `html` - the built-in renderer which will generate a HTML version of the book
+- [`linkcheck`] - a backend which will check that all links are valid
+- [`epub`] - an experimental EPUB generator
+
+> **Note for Developers:** Feel free to send us a PR if you've developed your
+> own plugin and want it mentioned here.
+
+A preprocessor or renderer is enabled by installing the appropriate program and
+then mentioning it in the book's `book.toml` file.
+
+```console
+$ cargo install mdbook-linkcheck
+$ edit book.toml && cat book.toml
+[book]
+title = "My Awesome Book"
+authors = ["Michael-F-Bryan"]
+
+[output.html]
+
+[output.linkcheck]  # enable the "mdbook-linkcheck" renderer
+
+$ mdbook build
+2018-10-20 13:57:51 [INFO] (mdbook::book): Book building has started
+2018-10-20 13:57:51 [INFO] (mdbook::book): Running the html backend
+2018-10-20 13:57:53 [INFO] (mdbook::book): Running the linkcheck backend
+```
+
+For more information on the plugin system, consult the [User Guide].
 
 ### As a library
 
@@ -189,3 +240,5 @@ All the code in this repository is released under the ***Mozilla Public License
 [Rust]: https://www.rust-lang.org/
 [CLI docs]: http://rust-lang-nursery.github.io/mdBook/cli/init.html
 [master-docs]: http://rust-lang-nursery.github.io/mdBook/mdbook/
+[`linkcheck`]: https://crates.io/crates/mdbook-linkcheck
+[`epub`]: https://crates.io/crates/mdbook-epub

book-example/src/continuous-integration.md

@@ -26,7 +26,7 @@ before_script:
   - cargo install-update -a
 
 script:
-  - cd path/to/mybook && mdbook build && mdbook test
+  - mdbook build path/to/mybook && mdbook test path/to/mybook
 ```
 
 ## Deploying Your Book to GitHub Pages

book-example/src/for_developers/README.md

@@ -42,5 +42,5 @@ explanation on the configuration system.
 
 
 [`MDBook`]: http://rust-lang-nursery.github.io/mdBook/mdbook/book/struct.MDBook.html
-[API Docs]: http://rust-lang-nursery.github.io/mdBook/mdbook/
+[API Docs]: https://docs.rs/mdbook/*/mdbook/
 [config]: file:///home/michael/Documents/forks/mdBook/target/doc/mdbook/config/index.html

book-example/src/for_developers/preprocessors.md

@@ -11,65 +11,79 @@ the book. Possible use cases are:
   mathjax equivalents
 
 
-## Implementing a Preprocessor
+## Hooking Into MDBook
 
-A preprocessor is represented by the `Preprocessor` trait.
+MDBook uses a fairly simple mechanism for discovering third party plugins.
+A new table is added to `book.toml` (e.g. `preprocessor.foo` for the `foo`
+preprocessor) and then `mdbook` will try to invoke the `mdbook-foo` program as
+part of the build process.
 
-```rust
-pub trait Preprocessor {
-    fn name(&self) -> &str;
-    fn run(&self, ctx: &PreprocessorContext, book: &mut Book) -> Result<()>;
-}
+While preprocessors can be hard-coded to specify which backend it should be run
+for (e.g. it doesn't make sense for MathJax to be used for non-HTML renderers)
+with the `preprocessor.foo.renderer` key.
+
+```toml
+[book]
+title = "My Book"
+authors = ["Michael-F-Bryan"]
+
+[preprocessor.foo]
+# The command can also be specified manually
+command = "python3 /path/to/foo.py"
+# Only run the `foo` preprocessor for the HTML and EPUB renderer
+renderer = ["html", "epub"]
 ```
 
-Where the `PreprocessorContext` is defined as
+In typical unix style, all inputs to the plugin will be written to `stdin` as
+JSON and `mdbook` will read from `stdout` if it is expecting output.
+
+The easiest way to get started is by creating your own implementation of the
+`Preprocessor` trait (e.g. in `lib.rs`) and then creating a shell binary which
+translates inputs to the correct `Preprocessor` method. For convenience, there
+is [an example no-op preprocessor] in the `examples/` directory which can easily
+be adapted for other preprocessors.
+
+<details>
+<summary>Example no-op preprocessor</summary>
 
 ```rust
-pub struct PreprocessorContext {
-    pub root: PathBuf,
-    pub config: Config,
-}
+// nop-preprocessors.rs
+
+{{#include ../../../examples/nop-preprocessor.rs}}
 ```
+</details>
 
-## A complete Example
+## Hints For Implementing A Preprocessor
 
-The magic happens within the `run(...)` method of the
-[`Preprocessor`][preprocessor-docs] trait implementation.
+By pulling in `mdbook` as a library, preprocessors can have access to the
+existing infrastructure for dealing with books.
 
-As direct access to the chapters is not possible, you will probably end up
-iterating them using `for_each_mut(...)`:
+For example, a custom preprocessor could use the
+[`CmdPreprocessor::parse_input()`] function to deserialize the JSON written to
+`stdin`. Then each chapter of the `Book` can be mutated in-place via
+[`Book::for_each_mut()`], and then written to `stdout` with the `serde_json`
+crate.
+
+Chapters can be accessed either directly (by recursively iterating over
+chapters) or via the `Book::for_each_mut()` convenience method.
+
+The `chapter.content` is just a string which happens to be markdown. While it's
+entirely possible to use regular expressions or do a manual find & replace,
+you'll probably want to process the input into something more computer-friendly.
+The [`pulldown-cmark`][pc] crate implements a production-quality event-based
+Markdown parser, with the [`pulldown-cmark-to-cmark`][pctc] allowing you to
+translate events back into markdown text.
+
+The following code block shows how to remove all emphasis from markdown,
+without accidentally breaking the document.
 
 ```rust
-book.for_each_mut(|item: &mut BookItem| {
-    if let BookItem::Chapter(ref mut chapter) = *item {
-      eprintln!("{}: processing chapter '{}'", self.name(), chapter.name);
-      res = Some(
-          match Deemphasize::remove_emphasis(&mut num_removed_items, chapter) {
-              Ok(md) => {
-                  chapter.content = md;
-                  Ok(())
-              }
-              Err(err) => Err(err),
-          },
-      );
-  }
-});
-```
-
-The `chapter.content` is just a markdown formatted string, and you will have to
-process it in some way. Even though it's entirely possible to implement some
-sort of manual find & replace operation, if that feels too unsafe you can use
-[`pulldown-cmark`][pc] to parse the string into events and work on them instead.
-
-Finally you can use [`pulldown-cmark-to-cmark`][pctc] to transform these events
-back to a string.
-
-The following code block shows how to remove all emphasis from markdown, and do
-so safely.
-
-```rust
-fn remove_emphasis(num_removed_items: &mut i32, chapter: &mut Chapter) -> Result<String> {
+fn remove_emphasis(
+    num_removed_items: &mut usize,
+    chapter: &mut Chapter,
+) -> Result<String> {
     let mut buf = String::with_capacity(chapter.content.len());
+
     let events = Parser::new(&chapter.content).filter(|e| {
         let should_keep = match *e {
             Event::Start(Tag::Emphasis)
@@ -83,15 +97,19 @@ fn remove_emphasis(num_removed_items: &mut i32, chapter: &mut Chapter) -> Result
         }
         should_keep
     });
-    cmark(events, &mut buf, None)
-        .map(|_| buf)
-        .map_err(|err| Error::from(format!("Markdown serialization failed: {}", err)))
+
+    cmark(events, &mut buf, None).map(|_| buf).map_err(|err| {
+        Error::from(format!("Markdown serialization failed: {}", err))
+    })
 }
 ```
 
 For everything else, have a look [at the complete example][example].
 
-[preprocessor-docs]: https://docs.rs/mdbook/0.1.3/mdbook/preprocess/trait.Preprocessor.html
+[preprocessor-docs]: https://docs.rs/mdbook/latest/mdbook/preprocess/trait.Preprocessor.html
 [pc]: https://crates.io/crates/pulldown-cmark
 [pctc]: https://crates.io/crates/pulldown-cmark-to-cmark
 [example]: https://github.com/rust-lang-nursery/mdBook/blob/master/examples/de-emphasize.rs
+[an example no-op preprocessor]: https://github.com/rust-lang-nursery/mdBook/blob/master/examples/nop-preprocessor.rs
+[`CmdPreprocessor::parse_input()`]: https://docs.rs/mdbook/latest/mdbook/preprocess/trait.Preprocessor.html#method.parse_input
+[`Book::for_each_mut()`]: https://docs.rs/mdbook/latest/mdbook/book/struct.Book.html#method.for_each_mut

book-example/src/format/config.md

@@ -14,6 +14,10 @@ description = "The example book covers examples."
 build-dir = "my-example-book"
 create-missing = false
 
+[preprocess.index]
+
+[preprocess.links]
+
 [output.html]
 additional-css = ["custom.css"]
 
@@ -27,7 +31,6 @@ It is important to note that **any** relative path specified in the in the
 configuration will always be taken relative from the root of the book where the
 configuration file is located.
 
-
 ### General metadata
 
 This is general information about your book.
@@ -59,15 +62,25 @@ This controls the build process of your book.
   will be created when the book is built (i.e. `create-missing = true`). If this
   is `false` then the build process will instead exit with an error if any files
   do not exist.
-- **preprocess:** Specify which preprocessors to be applied. Default is
-  `["links", "index"]`. To disable default preprocessors, pass an empty array
-  `[]` in.
+- **use-default-preprocessors:** Disable the default preprocessors of (`links` &
+  `index`) by setting this option to `false`.
 
+  If you have the same, and/or other preprocessors declared via their table
+  of configuration, they will run instead.
+
+  - For clarity, with no preprocessor configuration, the default `links` and 
+    `index` will run.
+  - Setting `use-default-preprocessors = false` will disable these
+    default preprocessors from running.
+  - Adding `[preprocessor.links]`, for example, will ensure, regardless of 
+    `use-default-preprocessors` that `links` it will run.
+
+## Configuring Preprocessors
 
 The following preprocessors are available and included by default:
 
-- `links`: Expand the `{{# playpen}}` and `{{# include}}` handlebars helpers in
-  a chapter.
+- `links`: Expand the `{{ #playpen }}` and `{{ #include }}` handlebars
+  helpers in a chapter to include the contents of a file.
 - `index`: Convert all chapter files named `README.md` into `index.md`. That is
   to say, all `README.md` would be rendered to an index file `index.html` in the
   rendered book.
@@ -78,10 +91,53 @@ The following preprocessors are available and included by default:
 [build]
 build-dir = "build"
 create-missing = false
-preprocess = ["links", "index"]
+
+[preprocess.links]
+
+[preprocess.index]
 ```
 
+### Custom Preprocessor Configuration
+
+Like renderers, preprocessor will need to be given its own table (e.g.
+`[preprocessor.mathjax]`). In the section, you may then pass extra
+configuration to the preprocessor by adding key-value pairs to the table.
+
+For example
+
+```
+[preprocess.links]
+# set the renderers this preprocessor will run for
+renderers = ["html"]
+some_extra_feature = true
+```
+
+#### Locking a Preprocessor dependency to a renderer
+
+You can explicitly specify that a preprocessor should run for a renderer by
+binding the two together.
+
+```
+[preprocessor.mathjax]
+renderers = ["html"]  # mathjax only makes sense with the HTML renderer
+```
+
+### Provide Your Own Command
+
+By default when you add a `[preprocessor.foo]` table to your `book.toml` file,
+`mdbook` will try to invoke the `mdbook-foo` executa`ble. If you want to use a
+different program name or pass in command-line arguments, this behaviour can
+be overridden by adding a `command` field.
+
+```toml
+[preprocessor.random]
+command = "python random.py"
+```
+
+## Configuring Renderers
+
 ### HTML renderer options
+
 The HTML renderer has a couple of options as well. All the options for the
 renderer need to be specified under the TOML table `[output.html]`.
 
@@ -139,7 +195,8 @@ Available configuration options for the `[output.html.search]` table:
 - **copy-js:** Copy JavaScript files for the search implementation to the output
   directory. Defaults to `true`.
 
-This shows all available options in the **book.toml**:
+This shows all available HTML output options in the **book.toml**:
+
 ```toml
 [book]
 title = "Example book"
@@ -176,6 +233,16 @@ heading-split-level = 3
 copy-js = true
 ```
 
+### Custom Renderers
+
+A custom renderer can be enabled by adding a `[output.foo]` table to your 
+`book.toml`. Similar to [preprocessors](#configuring-preprocessors) this will 
+instruct `mdbook` to pass a representation of the book to `mdbook-foo` for 
+rendering.
+
+Custom renderers will have access to all configuration within their table
+(i.e. anything under `[output.foo]`), and the command to be invoked can be 
+manually specified with the `command` field.
 
 ## Environment Variables
 
@@ -214,4 +281,4 @@ book's title without needing to touch your `book.toml`.
 
 The latter case may be useful in situations where `mdbook` is invoked from a
 script or CI, where it sometimes isn't possible to update the `book.toml` before
-building.
+building.

examples/de-emphasize.rs

@@ -1,4 +1,6 @@
-//! This program removes all forms of emphasis from the markdown of the book.
+//! An example preprocessor for removing all forms of emphasis from a markdown
+//! book.
+
 extern crate mdbook;
 extern crate pulldown_cmark;
 extern crate pulldown_cmark_to_cmark;
@@ -6,89 +8,75 @@ extern crate pulldown_cmark_to_cmark;
 use mdbook::book::{Book, BookItem, Chapter};
 use mdbook::errors::{Error, Result};
 use mdbook::preprocess::{Preprocessor, PreprocessorContext};
-use mdbook::MDBook;
 use pulldown_cmark::{Event, Parser, Tag};
 use pulldown_cmark_to_cmark::fmt::cmark;
 
-use std::env::{args, args_os};
-use std::ffi::OsString;
-use std::process;
+const NAME: &str = "md-links-to-html-links";
+
+fn main() {
+    panic!("This example is intended to be part of a library");
+}
 
 struct Deemphasize;
 
 impl Preprocessor for Deemphasize {
     fn name(&self) -> &str {
-        "md-links-to-html-links"
+        NAME
     }
 
-    fn run(&self, _ctx: &PreprocessorContext, book: &mut Book) -> Result<()> {
+    fn run(&self, _ctx: &PreprocessorContext, mut book: Book) -> Result<Book> {
         eprintln!("Running '{}' preprocessor", self.name());
-        let mut res: Option<_> = None;
         let mut num_removed_items = 0;
-        book.for_each_mut(|item: &mut BookItem| {
-            if let Some(Err(_)) = res {
-                return;
-            }
-            if let BookItem::Chapter(ref mut chapter) = *item {
-                eprintln!("{}: processing chapter '{}'", self.name(), chapter.name);
-                res = Some(
-                    match Deemphasize::remove_emphasis(&mut num_removed_items, chapter) {
-                        Ok(md) => {
-                            chapter.content = md;
-                            Ok(())
-                        }
-                        Err(err) => Err(err),
-                    },
-                );
-            }
-        });
+
+        process(&mut book.sections, &mut num_removed_items)?;
+
         eprintln!(
             "{}: removed {} events from markdown stream.",
             self.name(),
             num_removed_items
         );
-        match res {
-            Some(res) => res,
-            None => Ok(()),
+
+        Ok(book)
+    }
+}
+
+fn process<'a, I>(items: I, num_removed_items: &mut usize) -> Result<()>
+where
+    I: IntoIterator<Item = &'a mut BookItem> + 'a,
+{
+    for item in items {
+        if let BookItem::Chapter(ref mut chapter) = *item {
+            eprintln!("{}: processing chapter '{}'", NAME, chapter.name);
+
+            let md = remove_emphasis(num_removed_items, chapter)?;
+            chapter.content = md;
         }
     }
+
+    Ok(())
 }
 
-fn do_it(book: OsString) -> Result<()> {
-    let mut book = MDBook::load(book)?;
-    book.with_preprecessor(Deemphasize);
-    book.build()
-}
+fn remove_emphasis(
+    num_removed_items: &mut usize,
+    chapter: &mut Chapter,
+) -> Result<String> {
+    let mut buf = String::with_capacity(chapter.content.len());
 
-fn main() {
-    if args_os().count() != 2 {
-        eprintln!("USAGE: {} <book>", args().next().expect("executable"));
-        return;
-    }
-    if let Err(e) = do_it(args_os().skip(1).next().expect("one argument")) {
-        eprintln!("{}", e);
-        process::exit(1);
-    }
-}
+    let events = Parser::new(&chapter.content).filter(|e| {
+        let should_keep = match *e {
+            Event::Start(Tag::Emphasis)
+            | Event::Start(Tag::Strong)
+            | Event::End(Tag::Emphasis)
+            | Event::End(Tag::Strong) => false,
+            _ => true,
+        };
+        if !should_keep {
+            *num_removed_items += 1;
+        }
+        should_keep
+    });
 
-impl Deemphasize {
-    fn remove_emphasis(num_removed_items: &mut i32, chapter: &mut Chapter) -> Result<String> {
-        let mut buf = String::with_capacity(chapter.content.len());
-        let events = Parser::new(&chapter.content).filter(|e| {
-            let should_keep = match *e {
-                Event::Start(Tag::Emphasis)
-                | Event::Start(Tag::Strong)
-                | Event::End(Tag::Emphasis)
-                | Event::End(Tag::Strong) => false,
-                _ => true,
-            };
-            if !should_keep {
-                *num_removed_items += 1;
-            }
-            should_keep
-        });
-        cmark(events, &mut buf, None)
-            .map(|_| buf)
-            .map_err(|err| Error::from(format!("Markdown serialization failed: {}", err)))
-    }
+    cmark(events, &mut buf, None).map(|_| buf).map_err(|err| {
+        Error::from(format!("Markdown serialization failed: {}", err))
+    })
 }

examples/nop-preprocessor.rs

@@ -0,0 +1,112 @@
+extern crate clap;
+extern crate mdbook;
+extern crate serde_json;
+
+use clap::{App, Arg, ArgMatches, SubCommand};
+use mdbook::book::Book;
+use mdbook::errors::Error;
+use mdbook::preprocess::{CmdPreprocessor, Preprocessor, PreprocessorContext};
+use std::io;
+use std::process;
+use nop_lib::Nop;
+
+pub fn make_app() -> App<'static, 'static> {
+    App::new("nop-preprocessor")
+        .about("A mdbook preprocessor which does precisely nothing")
+        .subcommand(
+            SubCommand::with_name("supports")
+                .arg(Arg::with_name("renderer").required(true))
+                .about("Check whether a renderer is supported by this preprocessor"))
+}
+
+fn main() {
+    let matches = make_app().get_matches();
+
+    // Users will want to construct their own preprocessor here
+    let preprocessor = Nop::new();
+
+    if let Some(sub_args) = matches.subcommand_matches("supports") {
+        handle_supports(&preprocessor, sub_args);
+    } else {
+        if let Err(e) = handle_preprocessing(&preprocessor) {
+            eprintln!("{}", e);
+            process::exit(1);
+        }
+    }
+}
+
+fn handle_preprocessing(pre: &dyn Preprocessor) -> Result<(), Error> {
+    let (ctx, book) = CmdPreprocessor::parse_input(io::stdin())?;
+
+    if ctx.mdbook_version != mdbook::MDBOOK_VERSION {
+        // We should probably use the `semver` crate to check compatibility
+        // here...
+        eprintln!(
+            "Warning: The {} plugin was built against version {} of mdbook, \
+             but we're being called from version {}",
+            pre.name(),
+            mdbook::MDBOOK_VERSION,
+            ctx.mdbook_version
+        );
+    }
+
+    let processed_book = pre.run(&ctx, book)?;
+    serde_json::to_writer(io::stdout(), &processed_book)?;
+
+    Ok(())
+}
+
+fn handle_supports(pre: &dyn Preprocessor, sub_args: &ArgMatches) -> ! {
+    let renderer = sub_args.value_of("renderer").expect("Required argument");
+    let supported = pre.supports_renderer(&renderer);
+
+    // Signal whether the renderer is supported by exiting with 1 or 0.
+    if supported {
+        process::exit(0);
+    } else {
+        process::exit(1);
+    }
+}
+
+/// The actual implementation of the `Nop` preprocessor. This would usually go
+/// in your main `lib.rs` file.
+mod nop_lib {
+    use super::*;
+
+    /// A no-op preprocessor.
+    pub struct Nop;
+
+    impl Nop {
+        pub fn new() -> Nop {
+            Nop
+        }
+    }
+
+    impl Preprocessor for Nop {
+        fn name(&self) -> &str {
+            "nop-preprocessor"
+        }
+
+        fn run(
+            &self,
+            ctx: &PreprocessorContext,
+            book: Book,
+        ) -> Result<Book, Error> {
+            // In testing we want to tell the preprocessor to blow up by setting a
+            // particular config value
+            if let Some(nop_cfg) = ctx.config.get_preprocessor(self.name()) {
+                if nop_cfg.contains_key("blow-up") {
+                    return Err("Boom!!1!".into());
+                }
+            }
+
+            // we *are* a no-op preprocessor after all
+            Ok(book)
+        }
+
+        fn supports_renderer(&self, renderer: &str) -> bool {
+            renderer != "not-supported"
+        }
+    }
+}
+

src/book/mod.rs

@@ -20,7 +20,8 @@ use tempfile::Builder as TempFileBuilder;
 use toml::Value;
 
 use errors::*;
-use preprocess::{IndexPreprocessor, LinkPreprocessor, Preprocessor, PreprocessorContext};
+use preprocess::{IndexPreprocessor, LinkPreprocessor, Preprocessor,
+    PreprocessorContext, CmdPreprocessor};
 use renderer::{CmdRenderer, HtmlHandlebars, RenderContext, Renderer};
 use utils;
 
@@ -149,23 +150,39 @@ impl MDBook {
     pub fn build(&self) -> Result<()> {
         info!("Book building has started");
 
-        let mut preprocessed_book = self.book.clone();
-        let preprocess_ctx = PreprocessorContext::new(self.root.clone(), self.config.clone());
-
-        for preprocessor in &self.preprocessors {
-            debug!("Running the {} preprocessor.", preprocessor.name());
-            preprocessor.run(&preprocess_ctx, &mut preprocessed_book)?;
-        }
-
         for renderer in &self.renderers {
-            info!("Running the {} backend", renderer.name());
-            self.run_renderer(&preprocessed_book, renderer.as_ref())?;
+            self.execute_build_process(&**renderer)?;
         }
 
         Ok(())
     }
 
-    fn run_renderer(&self, preprocessed_book: &Book, renderer: &Renderer) -> Result<()> {
+    /// Run the entire build process for a particular `Renderer`.
+    fn execute_build_process(&self, renderer: &Renderer) -> Result<()> {
+        let mut preprocessed_book = self.book.clone();
+        let preprocess_ctx = PreprocessorContext::new(self.root.clone(),
+                                     self.config.clone(),
+                                     renderer.name().to_string());
+
+        for preprocessor in &self.preprocessors {
+            if preprocessor_should_run(&**preprocessor, renderer, &self.config) {
+                debug!("Running the {} preprocessor.", preprocessor.name());
+                preprocessed_book =
+                    preprocessor.run(&preprocess_ctx, preprocessed_book)?;
+            }
+        }
+
+        info!("Running the {} backend", renderer.name());
+        self.render(&preprocessed_book, renderer)?;
+
+        Ok(())
+    }
+
+    fn render(
+        &self,
+        preprocessed_book: &Book,
+        renderer: &Renderer,
+    ) -> Result<()> {
         let name = renderer.name();
         let build_dir = self.build_dir_for(name);
         if build_dir.exists() {
@@ -215,13 +232,16 @@ impl MDBook {
 
         let temp_dir = TempFileBuilder::new().prefix("mdbook-").tempdir()?;
 
-        let preprocess_context = PreprocessorContext::new(self.root.clone(), self.config.clone());
+        // FIXME: Is "test" the proper renderer name to use here?
+        let preprocess_context = PreprocessorContext::new(self.root.clone(),
+                                                          self.config.clone(),
+                                                          "test".to_string());
 
-        LinkPreprocessor::new().run(&preprocess_context, &mut self.book)?;
+        let book = LinkPreprocessor::new().run(&preprocess_context, self.book.clone())?;
         // Index Preprocessor is disabled so that chapter paths continue to point to the
         // actual markdown files.
 
-        for item in self.iter() {
+        for item in book.iter() {
             if let BookItem::Chapter(ref ch) = *item {
                 if !ch.path.as_os_str().is_empty() {
                     let path = self.source_dir().join(&ch.path);
@@ -330,30 +350,55 @@ fn default_preprocessors() -> Vec<Box<Preprocessor>> {
     ]
 }
 
+fn is_default_preprocessor(pre: &Preprocessor) -> bool {
+    let name = pre.name();
+    name == LinkPreprocessor::NAME || name == IndexPreprocessor::NAME
+}
+
 /// Look at the `MDBook` and try to figure out what preprocessors to run.
 fn determine_preprocessors(config: &Config) -> Result<Vec<Box<Preprocessor>>> {
-    let preprocess_list = match config.build.preprocess {
-        Some(ref p) => p,
-        // If no preprocessor field is set, default to the LinkPreprocessor and
-        // IndexPreprocessor. This allows you to disable default preprocessors
-        // by setting "preprocess" to an empty list.
-        None => return Ok(default_preprocessors()),
-    };
+    let mut preprocessors = Vec::new();
 
-    let mut preprocessors: Vec<Box<Preprocessor>> = Vec::new();
+    if config.build.use_default_preprocessors {
+        preprocessors.extend(default_preprocessors());
+    }
 
-    for key in preprocess_list {
-        match key.as_ref() {
-            "links" => preprocessors.push(Box::new(LinkPreprocessor::new())),
-            "index" => preprocessors.push(Box::new(IndexPreprocessor::new())),
-            _ => bail!("{:?} is not a recognised preprocessor", key),
+    if let Some(preprocessor_table) =
+        config.get("preprocessor").and_then(|v| v.as_table())
+    {
+        for key in preprocessor_table.keys() {
+            match key.as_ref() {
+                "links" => {
+                    preprocessors.push(Box::new(LinkPreprocessor::new()))
+                }
+                "index" => {
+                    preprocessors.push(Box::new(IndexPreprocessor::new()))
+                }
+                name => preprocessors.push(interpret_custom_preprocessor(
+                    name,
+                    &preprocessor_table[name],
+                )),
+            }
         }
     }
 
     Ok(preprocessors)
 }
 
-fn interpret_custom_renderer(key: &str, table: &Value) -> Box<Renderer> {
+fn interpret_custom_preprocessor(
+    key: &str,
+    table: &Value,
+) -> Box<CmdPreprocessor> {
+    let command = table
+        .get("command")
+        .and_then(|c| c.as_str())
+        .map(|s| s.to_string())
+        .unwrap_or_else(|| format!("mdbook-{}", key));
+
+    Box::new(CmdPreprocessor::new(key.to_string(), command.to_string()))
+}
+
+fn interpret_custom_renderer(key: &str, table: &Value) -> Box<CmdRenderer> {
     // look for the `command` field, falling back to using the key
     // prepended by "mdbook-"
     let table_dot_command = table
@@ -361,11 +406,37 @@ fn interpret_custom_renderer(key: &str, table: &Value) -> Box<Renderer> {
         .and_then(|c| c.as_str())
         .map(|s| s.to_string());
 
-    let command = table_dot_command.unwrap_or_else(|| format!("mdbook-{}", key));
+    let command =
+        table_dot_command.unwrap_or_else(|| format!("mdbook-{}", key));
 
     Box::new(CmdRenderer::new(key.to_string(), command.to_string()))
 }
 
+/// Check whether we should run a particular `Preprocessor` in combination
+/// with the renderer, falling back to `Preprocessor::supports_renderer()`
+/// method if the user doesn't say anything.
+///
+/// The `build.use-default-preprocessors` config option can be used to ensure
+/// default preprocessors always run if they support the renderer.
+fn preprocessor_should_run(preprocessor: &Preprocessor, renderer: &Renderer, cfg: &Config) -> bool {
+    // default preprocessors should be run by default (if supported)
+    if cfg.build.use_default_preprocessors && is_default_preprocessor(preprocessor) {
+        return preprocessor.supports_renderer(renderer.name());
+    }
+
+    let key = format!("preprocessor.{}.renderers", preprocessor.name());
+    let renderer_name = renderer.name();
+
+    if let Some(Value::Array(ref explicit_renderers)) = cfg.get(&key) {
+        return explicit_renderers.into_iter()
+            .filter_map(|val| val.as_str())
+            .any(|name| name == renderer_name);
+    }
+
+    preprocessor.supports_renderer(renderer_name)
+}
+
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -413,8 +484,8 @@ mod tests {
     fn config_defaults_to_link_and_index_preprocessor_if_not_set() {
         let cfg = Config::default();
 
-        // make sure we haven't got anything in the `output` table
-        assert!(cfg.build.preprocess.is_none());
+        // make sure we haven't got anything in the `preprocessor` table
+        assert!(cfg.get("preprocessor").is_none());
 
         let got = determine_preprocessors(&cfg);
 
@@ -425,47 +496,107 @@ mod tests {
     }
 
     #[test]
-    fn config_doesnt_default_if_empty() {
-        let cfg_str: &'static str = r#"
-        [book]
-        title = "Some Book"
+    fn use_default_preprocessors_works() {
+        let mut cfg = Config::default();
+        cfg.build.use_default_preprocessors = false;
 
-        [build]
-        build-dir = "outputs"
-        create-missing = false
-        preprocess = []
-        "#;
+        let got = determine_preprocessors(&cfg).unwrap();
 
-        let cfg = Config::from_str(cfg_str).unwrap();
-
-        // make sure we have something in the `output` table
-        assert!(cfg.build.preprocess.is_some());
-
-        let got = determine_preprocessors(&cfg);
-
-        assert!(got.is_ok());
-        assert!(got.unwrap().is_empty());
+        assert_eq!(got.len(), 0);
     }
 
     #[test]
-    fn config_complains_if_unimplemented_preprocessor() {
+    fn can_determine_third_party_preprocessors() {
         let cfg_str: &'static str = r#"
         [book]
         title = "Some Book"
 
+        [preprocessor.random]
+
         [build]
         build-dir = "outputs"
         create-missing = false
-        preprocess = ["random"]
         "#;
 
         let cfg = Config::from_str(cfg_str).unwrap();
 
-        // make sure we have something in the `output` table
-        assert!(cfg.build.preprocess.is_some());
+        // make sure the `preprocessor.random` table exists
+        assert!(cfg.get_preprocessor("random").is_some());
 
-        let got = determine_preprocessors(&cfg);
+        let got = determine_preprocessors(&cfg).unwrap();
 
-        assert!(got.is_err());
+        assert!(got.into_iter().any(|p| p.name() == "random"));
+    }
+
+    #[test]
+    fn preprocessors_can_provide_their_own_commands() {
+        let cfg_str = r#"
+        [preprocessor.random]
+        command = "python random.py"
+        "#;
+
+        let cfg = Config::from_str(cfg_str).unwrap();
+
+        // make sure the `preprocessor.random` table exists
+        let random = cfg.get_preprocessor("random").unwrap();
+        let random = interpret_custom_preprocessor(
+            "random",
+            &Value::Table(random.clone()),
+        );
+
+        assert_eq!(random.cmd(), "python random.py");
+    }
+
+    #[test]
+    fn config_respects_preprocessor_selection() {
+        let cfg_str: &'static str = r#"
+        [preprocessor.links]
+        renderers = ["html"]
+        "#;
+
+        let cfg = Config::from_str(cfg_str).unwrap();
+
+        // double-check that we can access preprocessor.links.renderers[0]
+        let html = cfg.get_preprocessor("links")
+            .and_then(|links| links.get("renderers"))
+            .and_then(|renderers| renderers.as_array())
+            .and_then(|renderers| renderers.get(0))
+            .and_then(|renderer| renderer.as_str())
+            .unwrap();
+        assert_eq!(html, "html");
+        let html_renderer = HtmlHandlebars::default();
+        let pre = LinkPreprocessor::new();
+
+        let should_run = preprocessor_should_run(&pre, &html_renderer, &cfg);
+        assert!(should_run);
+    }
+
+    struct BoolPreprocessor(bool);
+    impl Preprocessor for BoolPreprocessor {
+        fn name(&self) -> &str {
+            "bool-preprocessor"
+        }
+
+        fn run(&self, _ctx: &PreprocessorContext, _book: Book) -> Result<Book> {
+            unimplemented!()
+        }
+
+        fn supports_renderer(&self, _renderer: &str) -> bool {
+            self.0
+        }
+    }
+
+    #[test]
+    fn preprocessor_should_run_falls_back_to_supports_renderer_method() {
+        let cfg = Config::default();
+        let html = HtmlHandlebars::new();
+
+        let should_be = true;
+        let got = preprocessor_should_run(&BoolPreprocessor(should_be), &html, &cfg);
+        assert_eq!(got, should_be);
+
+        let should_be = false;
+        let got = preprocessor_should_run(&BoolPreprocessor(should_be), &html, &cfg);
+        assert_eq!(got, should_be);
     }
 }

src/book/summary.rs

@@ -292,6 +292,7 @@ impl<'a> SummaryParser<'a> {
     /// already been consumed by a previous parser.
     fn parse_numbered(&mut self) -> Result<Vec<SummaryItem>> {
         let mut items = Vec::new();
+        let mut root_items = 0;
         let root_number = SectionNumber::default();
 
         // we need to do this funny loop-match-if-let dance because a rule will
@@ -308,9 +309,11 @@ impl<'a> SummaryParser<'a> {
             // if we've resumed after something like a rule the root sections
             // will be numbered from 1. We need to manually go back and update
             // them
-            update_section_numbers(&mut bunch_of_items, 0, items.len() as u32);
+            update_section_numbers(&mut bunch_of_items, 0, root_items);
+            root_items += bunch_of_items.len() as u32;
             items.extend(bunch_of_items);
 
+
             match self.next_event() {
                 Some(Event::Start(Tag::Paragraph)) => {
                     // we're starting the suffix chapters
@@ -724,4 +727,40 @@ mod tests {
         let got = parser.parse_numbered();
         assert!(got.is_err());
     }
+
+    /// Regression test for https://github.com/rust-lang-nursery/mdBook/issues/779
+    /// Ensure section numbers are correctly incremented after a horizontal separator.
+    #[test]
+    fn keep_numbering_after_separator() {
+        let src = "- [First](./first.md)\n---\n- [Second](./second.md)\n---\n- [Third](./third.md)\n";
+        let should_be = vec![
+            SummaryItem::Link(Link {
+                name: String::from("First"),
+                location: PathBuf::from("./first.md"),
+                number: Some(SectionNumber(vec![1])),
+                nested_items: Vec::new(),
+            }),
+            SummaryItem::Separator,
+            SummaryItem::Link(Link {
+                name: String::from("Second"),
+                location: PathBuf::from("./second.md"),
+                number: Some(SectionNumber(vec![2])),
+                nested_items: Vec::new(),
+            }),
+            SummaryItem::Separator,
+            SummaryItem::Link(Link {
+                name: String::from("Third"),
+                location: PathBuf::from("./third.md"),
+                number: Some(SectionNumber(vec![3])),
+                nested_items: Vec::new(),
+            }),
+        ];
+
+        let mut parser = SummaryParser::new(src);
+        let _ = parser.stream.next();
+
+        let got = parser.parse_numbered().unwrap();
+
+        assert_eq!(got, should_be);
+    }
 }

src/config.rs

@@ -209,6 +209,18 @@ impl Config {
         Ok(())
     }
 
+    /// Get the table associated with a particular renderer.
+    pub fn get_renderer<I: AsRef<str>>(&self, index: I) -> Option<&Table> {
+        let key = format!("output.{}", index.as_ref());
+        self.get(&key).and_then(|v| v.as_table())
+    }
+
+    /// Get the table associated with a particular preprocessor.
+    pub fn get_preprocessor<I: AsRef<str>>(&self, index: I) -> Option<&Table> {
+        let key = format!("preprocessor.{}", index.as_ref());
+        self.get(&key).and_then(|v| v.as_table())
+    }
+
     fn from_legacy(mut table: Value) -> Config {
         let mut cfg = Config::default();
 
@@ -382,8 +394,9 @@ pub struct BuildConfig {
     /// Should non-existent markdown files specified in `SETTINGS.md` be created
     /// if they don't exist?
     pub create_missing: bool,
-    /// Which preprocessors should be applied
-    pub preprocess: Option<Vec<String>>,
+    /// Should the default preprocessors always be used when they are
+    /// compatible with the renderer?
+    pub use_default_preprocessors: bool,
 }
 
 impl Default for BuildConfig {
@@ -391,7 +404,7 @@ impl Default for BuildConfig {
         BuildConfig {
             build_dir: PathBuf::from("book"),
             create_missing: true,
-            preprocess: None,
+            use_default_preprocessors: true,
         }
     }
 }
@@ -551,7 +564,7 @@ mod tests {
         [build]
         build-dir = "outputs"
         create-missing = false
-        preprocess = ["first_preprocessor", "second_preprocessor"]
+        use-default-preprocessors = true
 
         [output.html]
         theme = "./themedir"
@@ -562,6 +575,10 @@ mod tests {
         [output.html.playpen]
         editable = true
         editor = "ace"
+
+        [preprocess.first]
+
+        [preprocess.second]
         "#;
 
     #[test]
@@ -579,10 +596,7 @@ mod tests {
         let build_should_be = BuildConfig {
             build_dir: PathBuf::from("outputs"),
             create_missing: false,
-            preprocess: Some(vec![
-                "first_preprocessor".to_string(),
-                "second_preprocessor".to_string(),
-            ]),
+            use_default_preprocessors: true,
         };
         let playpen_should_be = Playpen {
             editable: true,
@@ -684,7 +698,7 @@ mod tests {
         let build_should_be = BuildConfig {
             build_dir: PathBuf::from("my-book"),
             create_missing: true,
-            preprocess: None,
+            use_default_preprocessors: true,
         };
 
         let html_should_be = HtmlConfig {

src/lib.rs

@@ -114,6 +114,12 @@ pub mod renderer;
 pub mod theme;
 pub mod utils;
 
+/// The current version of `mdbook`.
+///
+/// This is provided as a way for custom preprocessors and renderers to do
+/// compatibility checks.
+pub const MDBOOK_VERSION: &str = env!("CARGO_PKG_VERSION");
+
 pub use book::BookItem;
 pub use book::MDBook;
 pub use config::Config;

src/preprocess/cmd.rs

@@ -0,0 +1,154 @@
+use super::{Preprocessor, PreprocessorContext};
+use book::Book;
+use errors::*;
+use serde_json;
+use shlex::Shlex;
+use std::io::{self, Read, Write};
+use std::process::{Child, Command, Stdio};
+
+/// A custom preprocessor which will shell out to a 3rd-party program.
+///
+/// # Preprocessing Protocol
+///
+/// When the `supports_renderer()` method is executed, `CmdPreprocessor` will
+/// execute the shell command `$cmd supports $renderer`. If the renderer is
+/// supported, custom preprocessors should exit with a exit code of `0`,
+/// any other exit code be considered as unsupported.
+///
+/// The `run()` method is implemented by passing a `(PreprocessorContext, Book)`
+/// tuple to the spawned command (`$cmd`) as JSON via `stdin`. Preprocessors
+/// should then "return" a processed book by printing it to `stdout` as JSON.
+/// For convenience, the `CmdPreprocessor::parse_input()` function can be used
+/// to parse the input provided by `mdbook`.
+///
+/// Exiting with a non-zero exit code while preprocessing is considered an
+/// error. `stderr` is passed directly through to the user, so it can be used
+/// for logging or emitting warnings if desired.
+///
+/// # Examples
+///
+/// An example preprocessor is available in this project's `examples/`
+/// directory.
+#[derive(Debug, Clone, PartialEq)]
+pub struct CmdPreprocessor {
+    name: String,
+    cmd: String,
+}
+
+impl CmdPreprocessor {
+    /// Create a new `CmdPreprocessor`.
+    pub fn new(name: String, cmd: String) -> CmdPreprocessor {
+        CmdPreprocessor { name, cmd }
+    }
+
+    /// A convenience function custom preprocessors can use to parse the input
+    /// written to `stdin` by a `CmdRenderer`.
+    pub fn parse_input<R: Read>(
+        reader: R,
+    ) -> Result<(PreprocessorContext, Book)> {
+        serde_json::from_reader(reader)
+            .chain_err(|| "Unable to parse the input")
+    }
+
+    fn write_input_to_child(
+        &self,
+        child: &mut Child,
+        book: &Book,
+        ctx: &PreprocessorContext,
+    ) {
+        let stdin = child.stdin.take().expect("Child has stdin");
+
+        if let Err(e) = self.write_input(stdin, &book, &ctx) {
+            // Looks like the backend hung up before we could finish
+            // sending it the render context. Log the error and keep going
+            warn!("Error writing the RenderContext to the backend, {}", e);
+        }
+    }
+
+    fn write_input<W: Write>(&self, writer: W, book: &Book, ctx: &PreprocessorContext) -> Result<()> {
+        serde_json::to_writer(writer, &(ctx, book))
+            .map_err(Into::into)
+    }
+
+    /// The command this `Preprocessor` will invoke.
+    pub fn cmd(&self) -> &str {
+        &self.cmd
+    }
+
+    fn command(&self) -> Result<Command> {
+        let mut words = Shlex::new(&self.cmd);
+        let executable = match words.next() {
+            Some(e) => e,
+            None => bail!("Command string was empty"),
+        };
+
+        let mut cmd = Command::new(executable);
+
+        for arg in words {
+            cmd.arg(arg);
+        }
+
+        Ok(cmd)
+    }
+}
+
+impl Preprocessor for CmdPreprocessor {
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book> {
+        let mut cmd = self.command()?;
+
+        let mut child = cmd
+            .stdin(Stdio::piped())
+            .stdout(Stdio::piped())
+            .stderr(Stdio::inherit())
+            .spawn()
+            .chain_err(|| format!("Unable to start the \"{}\" preprocessor. Is it installed?", self.name()))?;
+
+        self.write_input_to_child(&mut child, &book, ctx);
+
+        let output = child
+            .wait_with_output()
+            .chain_err(|| "Error waiting for the preprocessor to complete")?;
+
+        trace!("{} exited with output: {:?}", self.cmd, output);
+        ensure!(output.status.success(), "The preprocessor exited unsuccessfully");
+
+        serde_json::from_slice(&output.stdout).chain_err(|| "Unable to parse the preprocessed book")
+    }
+
+    fn supports_renderer(&self, renderer: &str) -> bool {
+        debug!("Checking if the \"{}\" preprocessor supports \"{}\"", self.name(), renderer);
+
+        let mut cmd = match self.command() {
+            Ok(c) => c,
+            Err(e) => {
+                warn!("Unable to create the command for the \"{}\" preprocessor, {}", self.name(), e);
+                return false;
+            }
+        };
+
+        let outcome = cmd
+            .arg("supports")
+            .arg(renderer)
+            .stdin(Stdio::null())
+            .stdout(Stdio::inherit())
+            .stderr(Stdio::inherit())
+            .status()
+            .map(|status| status.code() == Some(0));
+
+        if let Err(ref e) = outcome {
+            if e.kind() == io::ErrorKind::NotFound {
+                warn!(
+                    "The command wasn't found, is the \"{}\" preprocessor installed?",
+                    self.name
+                );
+                warn!("\tCommand: {}", self.cmd);
+            }
+        }
+
+        outcome.unwrap_or(false)
+    }
+}

src/preprocess/index.rs

@@ -11,6 +11,8 @@ use book::{Book, BookItem};
 pub struct IndexPreprocessor;
 
 impl IndexPreprocessor {
+    pub(crate) const NAME: &'static str = "index";
+
     /// Create a new `IndexPreprocessor`.
     pub fn new() -> Self {
         IndexPreprocessor
@@ -19,10 +21,10 @@ impl IndexPreprocessor {
 
 impl Preprocessor for IndexPreprocessor {
     fn name(&self) -> &str {
-        "index"
+        Self::NAME
     }
 
-    fn run(&self, ctx: &PreprocessorContext, book: &mut Book) -> Result<()> {
+    fn run(&self, ctx: &PreprocessorContext, mut book: Book) -> Result<Book> {
         let source_dir = ctx.root.join(&ctx.config.book.src);
         book.for_each_mut(|section: &mut BookItem| {
             if let BookItem::Chapter(ref mut ch) = *section {
@@ -37,7 +39,7 @@ impl Preprocessor for IndexPreprocessor {
             }
         });
 
-        Ok(())
+        Ok(book)
     }
 }
 

src/preprocess/links.rs

@@ -16,6 +16,8 @@ const MAX_LINK_NESTED_DEPTH: usize = 10;
 pub struct LinkPreprocessor;
 
 impl LinkPreprocessor {
+    pub(crate) const NAME: &'static str = "links";
+
     /// Create a new `LinkPreprocessor`.
     pub fn new() -> Self {
         LinkPreprocessor
@@ -24,10 +26,10 @@ impl LinkPreprocessor {
 
 impl Preprocessor for LinkPreprocessor {
     fn name(&self) -> &str {
-        "links"
+        Self::NAME
     }
 
-    fn run(&self, ctx: &PreprocessorContext, book: &mut Book) -> Result<()> {
+    fn run(&self, ctx: &PreprocessorContext, mut book: Book) -> Result<Book> {
         let src_dir = ctx.root.join(&ctx.config.book.src);
 
         book.for_each_mut(|section: &mut BookItem| {
@@ -43,7 +45,7 @@ impl Preprocessor for LinkPreprocessor {
             }
         });
 
-        Ok(())
+        Ok(book)
     }
 }
 
@@ -67,7 +69,12 @@ where
             Ok(new_content) => {
                 if depth < MAX_LINK_NESTED_DEPTH {
                     if let Some(rel_path) = playpen.link.relative_path(path) {
-                        replaced.push_str(&replace_all(&new_content, rel_path, source, depth + 1));
+                        replaced.push_str(&replace_all(
+                            &new_content,
+                            rel_path,
+                            source,
+                            depth + 1,
+                        ));
                     } else {
                         replaced.push_str(&new_content);
                     }
@@ -81,6 +88,10 @@ where
             }
             Err(e) => {
                 error!("Error updating \"{}\", {}", playpen.link_text, e);
+                for cause in e.iter().skip(1) {
+                    warn!("Caused By: {}", cause);
+                }
+
                 // This should make sure we include the raw `{{# ... }}` snippet
                 // in the page content if there are any errors.
                 previous_end_index = playpen.start_index;
@@ -107,10 +118,18 @@ impl<'a> LinkType<'a> {
         let base = base.as_ref();
         match self {
             LinkType::Escaped => None,
-            LinkType::IncludeRange(p, _) => Some(return_relative_path(base, &p)),
-            LinkType::IncludeRangeFrom(p, _) => Some(return_relative_path(base, &p)),
-            LinkType::IncludeRangeTo(p, _) => Some(return_relative_path(base, &p)),
-            LinkType::IncludeRangeFull(p, _) => Some(return_relative_path(base, &p)),
+            LinkType::IncludeRange(p, _) => {
+                Some(return_relative_path(base, &p))
+            }
+            LinkType::IncludeRangeFrom(p, _) => {
+                Some(return_relative_path(base, &p))
+            }
+            LinkType::IncludeRangeTo(p, _) => {
+                Some(return_relative_path(base, &p))
+            }
+            LinkType::IncludeRangeFull(p, _) => {
+                Some(return_relative_path(base, &p))
+            }
             LinkType::Playpen(p, _) => Some(return_relative_path(base, &p)),
         }
     }
@@ -180,11 +199,15 @@ impl<'a> Link<'a> {
 
                 match (typ.as_str(), file_arg) {
                     ("include", Some(pth)) => Some(parse_include_path(pth)),
-                    ("playpen", Some(pth)) => Some(LinkType::Playpen(pth.into(), props)),
+                    ("playpen", Some(pth)) => {
+                        Some(LinkType::Playpen(pth.into(), props))
+                    }
                     _ => None,
                 }
             }
-            (Some(mat), None, None) if mat.as_str().starts_with(ESCAPE_CHAR) => {
+            (Some(mat), None, None)
+                if mat.as_str().starts_with(ESCAPE_CHAR) =>
+            {
                 Some(LinkType::Escaped)
             }
             _ => None,
@@ -205,20 +228,65 @@ impl<'a> Link<'a> {
         match self.link {
             // omit the escape char
             LinkType::Escaped => Ok((&self.link_text[1..]).to_owned()),
-            LinkType::IncludeRange(ref pat, ref range) => file_to_string(base.join(pat))
-                .map(|s| take_lines(&s, range.clone()))
-                .chain_err(|| format!("Could not read file for link {}", self.link_text)),
-            LinkType::IncludeRangeFrom(ref pat, ref range) => file_to_string(base.join(pat))
-                .map(|s| take_lines(&s, range.clone()))
-                .chain_err(|| format!("Could not read file for link {}", self.link_text)),
-            LinkType::IncludeRangeTo(ref pat, ref range) => file_to_string(base.join(pat))
-                .map(|s| take_lines(&s, range.clone()))
-                .chain_err(|| format!("Could not read file for link {}", self.link_text)),
-            LinkType::IncludeRangeFull(ref pat, _) => file_to_string(base.join(pat))
-                .chain_err(|| format!("Could not read file for link {}", self.link_text)),
+            LinkType::IncludeRange(ref pat, ref range) => {
+                let target = base.join(pat);
+
+                file_to_string(&target)
+                    .map(|s| take_lines(&s, range.clone()))
+                    .chain_err(|| {
+                        format!(
+                            "Could not read file for link {} ({})",
+                            self.link_text,
+                            target.display(),
+                        )
+                    })
+            }
+            LinkType::IncludeRangeFrom(ref pat, ref range) => {
+                let target = base.join(pat);
+
+                file_to_string(&target)
+                    .map(|s| take_lines(&s, range.clone()))
+                    .chain_err(|| {
+                        format!(
+                            "Could not read file for link {} ({})",
+                            self.link_text,
+                            target.display(),
+                        )
+                    })
+            }
+            LinkType::IncludeRangeTo(ref pat, ref range) => {
+                let target = base.join(pat);
+
+                file_to_string(&target)
+                    .map(|s| take_lines(&s, range.clone()))
+                    .chain_err(|| {
+                        format!(
+                            "Could not read file for link {} ({})",
+                            self.link_text,
+                            target.display(),
+                        )
+                    })
+            }
+            LinkType::IncludeRangeFull(ref pat, _) => {
+                let target = base.join(pat);
+
+                file_to_string(&target).chain_err(|| {
+                    format!("Could not read file for link {} ({})",
+                            self.link_text,
+                            target.display())
+                })
+            }
             LinkType::Playpen(ref pat, ref attrs) => {
-                let contents = file_to_string(base.join(pat))
-                    .chain_err(|| format!("Could not read file for link {}", self.link_text))?;
+                let target = base.join(pat);
+
+                let contents =
+                file_to_string(&target).chain_err(|| {
+                        format!(
+                            "Could not read file for link {} ({})",
+                            self.link_text,
+                            target.display()
+                        )
+                    })?;
                 let ftype = if !attrs.is_empty() { "rust," } else { "rust" };
                 Ok(format!(
                     "```{}{}\n{}\n```\n",
@@ -463,7 +531,10 @@ mod tests {
                 Link {
                     start_index: 38,
                     end_index: 68,
-                    link: LinkType::Playpen(PathBuf::from("file.rs"), vec!["editable"]),
+                    link: LinkType::Playpen(
+                        PathBuf::from("file.rs"),
+                        vec!["editable"]
+                    ),
                     link_text: "{{#playpen file.rs editable }}",
                 },
                 Link {
@@ -473,7 +544,8 @@ mod tests {
                         PathBuf::from("my.rs"),
                         vec!["editable", "no_run", "should_panic"],
                     ),
-                    link_text: "{{#playpen my.rs editable no_run should_panic}}",
+                    link_text:
+                        "{{#playpen my.rs editable no_run should_panic}}",
                 },
             ]
         );

src/preprocess/mod.rs

@@ -2,9 +2,11 @@
 
 pub use self::index::IndexPreprocessor;
 pub use self::links::LinkPreprocessor;
+pub use self::cmd::CmdPreprocessor;
 
 mod index;
 mod links;
+mod cmd;
 
 use book::Book;
 use config::Config;
@@ -14,17 +16,29 @@ use std::path::PathBuf;
 
 /// Extra information for a `Preprocessor` to give them more context when
 /// processing a book.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct PreprocessorContext {
     /// The location of the book directory on disk.
     pub root: PathBuf,
     /// The book configuration (`book.toml`).
     pub config: Config,
+    /// The `Renderer` this preprocessor is being used with.
+    pub renderer: String,
+    /// The calling `mdbook` version.
+    pub mdbook_version: String,
+    __non_exhaustive: (),
 }
 
 impl PreprocessorContext {
     /// Create a new `PreprocessorContext`.
-    pub(crate) fn new(root: PathBuf, config: Config) -> Self {
-        PreprocessorContext { root, config }
+    pub(crate) fn new(root: PathBuf, config: Config, renderer: String) -> Self {
+        PreprocessorContext {
+            root,
+            config,
+            renderer,
+            mdbook_version: ::MDBOOK_VERSION.to_string(),
+            __non_exhaustive: (),
+        }
     }
 }
 
@@ -36,5 +50,13 @@ pub trait Preprocessor {
 
     /// Run this `Preprocessor`, allowing it to update the book before it is
     /// given to a renderer.
-    fn run(&self, ctx: &PreprocessorContext, book: &mut Book) -> Result<()>;
+    fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book>;
+
+    /// A hint to `MDBook` whether this preprocessor is compatible with a
+    /// particular renderer.
+    ///
+    /// By default, always returns `true`.
+    fn supports_renderer(&self, _renderer: &str) -> bool {
+        true
+    }
 }

src/renderer/mod.rs

@@ -26,8 +26,6 @@ use book::Book;
 use config::Config;
 use errors::*;
 
-const MDBOOK_VERSION: &str = env!("CARGO_PKG_VERSION");
-
 /// An arbitrary `mdbook` backend.
 ///
 /// Although it's quite possible for you to import `mdbook` as a library and
@@ -66,6 +64,7 @@ pub struct RenderContext {
     /// renderers to cache intermediate results, this directory is not
     /// guaranteed to be empty or even exist.
     pub destination: PathBuf,
+    __non_exhaustive: (),
 }
 
 impl RenderContext {
@@ -78,9 +77,10 @@ impl RenderContext {
         RenderContext {
             book: book,
             config: config,
-            version: MDBOOK_VERSION.to_string(),
+            version: ::MDBOOK_VERSION.to_string(),
             root: root.into(),
             destination: destination.into(),
+            __non_exhaustive: (),
         }
     }
 

src/theme/index.hbs

@@ -27,7 +27,7 @@
 
         <!-- Custom theme stylesheets -->
         {{#each additional_css}}
-        <link rel="stylesheet" href="{{ this }}">
+        <link rel="stylesheet" href="{{ ../path_to_root }}{{ this }}">
         {{/each}}
 
         {{#if mathjax_support}}
@@ -235,7 +235,7 @@
 
         <!-- Custom JS scripts -->
         {{#each additional_js}}
-        <script type="text/javascript" src="{{ path_to_root }}{{this}}"></script>
+        <script type="text/javascript" src="{{ ../path_to_root }}{{this}}"></script>
         {{/each}}
 
         {{#if is_print}}

tests/build_process.rs

@@ -0,0 +1,80 @@
+extern crate mdbook;
+
+mod dummy_book;
+
+use dummy_book::DummyBook;
+use mdbook::book::Book;
+use mdbook::config::Config;
+use mdbook::errors::*;
+use mdbook::preprocess::{Preprocessor, PreprocessorContext};
+use mdbook::renderer::{RenderContext, Renderer};
+use mdbook::MDBook;
+use std::sync::{Arc, Mutex};
+
+struct Spy(Arc<Mutex<Inner>>);
+
+#[derive(Debug, Default)]
+struct Inner {
+    run_count: usize,
+    rendered_with: Vec<String>,
+}
+
+impl Preprocessor for Spy {
+    fn name(&self) -> &str {
+        "dummy"
+    }
+
+    fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book> {
+        let mut inner = self.0.lock().unwrap();
+        inner.run_count += 1;
+        inner.rendered_with.push(ctx.renderer.clone());
+        Ok(book)
+    }
+}
+
+impl Renderer for Spy {
+    fn name(&self) -> &str {
+        "dummy"
+    }
+
+    fn render(&self, _ctx: &RenderContext) -> Result<()> {
+        let mut inner = self.0.lock().unwrap();
+        inner.run_count += 1;
+        Ok(())
+    }
+}
+
+#[test]
+fn mdbook_runs_preprocessors() {
+    let spy: Arc<Mutex<Inner>> = Default::default();
+
+    let temp = DummyBook::new().build().unwrap();
+    let cfg = Config::default();
+
+    let mut book = MDBook::load_with_config(temp.path(), cfg).unwrap();
+    book.with_preprecessor(Spy(Arc::clone(&spy)));
+    book.build().unwrap();
+
+    let inner = spy.lock().unwrap();
+    assert_eq!(inner.run_count, 1);
+    assert_eq!(inner.rendered_with.len(), 1);
+    assert_eq!(
+        "html", inner.rendered_with[0],
+        "We should have been run with the default HTML renderer"
+    );
+}
+
+#[test]
+fn mdbook_runs_renderers() {
+    let spy: Arc<Mutex<Inner>> = Default::default();
+
+    let temp = DummyBook::new().build().unwrap();
+    let cfg = Config::default();
+
+    let mut book = MDBook::load_with_config(temp.path(), cfg).unwrap();
+    book.with_renderer(Spy(Arc::clone(&spy)));
+    book.build().unwrap();
+
+    let inner = spy.lock().unwrap();
+    assert_eq!(inner.run_count, 1);
+}

tests/custom_preprocessors.rs

@@ -0,0 +1,53 @@
+extern crate mdbook;
+
+mod dummy_book;
+
+use dummy_book::DummyBook;
+use mdbook::preprocess::{CmdPreprocessor, Preprocessor};
+use mdbook::MDBook;
+
+fn example() -> CmdPreprocessor {
+    CmdPreprocessor::new("nop-preprocessor".to_string(), "cargo run --example nop-preprocessor --".to_string())
+}
+
+#[test]
+fn example_supports_whatever() {
+    let cmd = example();
+
+    let got = cmd.supports_renderer("whatever");
+
+    assert_eq!(got, true);
+}
+
+#[test]
+fn example_doesnt_support_not_supported() {
+    let cmd = example();
+
+    let got = cmd.supports_renderer("not-supported");
+
+    assert_eq!(got, false);
+}
+
+#[test]
+fn ask_the_preprocessor_to_blow_up() {
+    let dummy_book = DummyBook::new();
+    let temp = dummy_book.build().unwrap();
+    let mut md = MDBook::load(temp.path()).unwrap();
+    md.with_preprecessor(example());
+
+    md.config.set("preprocessor.nop-preprocessor.blow-up", true).unwrap();
+
+    let got = md.build();
+
+    assert!(got.is_err());
+}
+
+#[test]
+fn process_the_dummy_book() {
+    let dummy_book = DummyBook::new();
+    let temp = dummy_book.build().unwrap();
+    let mut md = MDBook::load(temp.path()).unwrap();
+    md.with_preprecessor(example());
+
+    md.build().unwrap();
+}

tests/testing.rs

@@ -4,14 +4,8 @@ mod dummy_book;
 
 use dummy_book::DummyBook;
 
-use mdbook::book::Book;
-use mdbook::config::Config;
-use mdbook::errors::*;
-use mdbook::preprocess::{Preprocessor, PreprocessorContext};
 use mdbook::MDBook;
 
-use std::sync::{Arc, Mutex};
-
 #[test]
 fn mdbook_can_correctly_test_a_passing_book() {
     let temp = DummyBook::new().with_passing_test(true).build().unwrap();
@@ -27,30 +21,3 @@ fn mdbook_detects_book_with_failing_tests() {
 
     assert!(md.test(vec![]).is_err());
 }
-
-#[test]
-fn mdbook_runs_preprocessors() {
-    let has_run: Arc<Mutex<bool>> = Arc::new(Mutex::new(false));
-
-    struct DummyPreprocessor(Arc<Mutex<bool>>);
-
-    impl Preprocessor for DummyPreprocessor {
-        fn name(&self) -> &str {
-            "dummy"
-        }
-
-        fn run(&self, _ctx: &PreprocessorContext, _book: &mut Book) -> Result<()> {
-            *self.0.lock().unwrap() = true;
-            Ok(())
-        }
-    }
-
-    let temp = DummyBook::new().build().unwrap();
-    let cfg = Config::default();
-
-    let mut book = MDBook::load_with_config(temp.path(), cfg).unwrap();
-    book.with_preprecessor(DummyPreprocessor(Arc::clone(&has_run)));
-    book.build().unwrap();
-
-    assert!(*has_run.lock().unwrap())
-}