Add optional field for preprocessors

This adds the `optional` field to the preprocessor configuration to
mirror the same option for the `output` table. Missing preprocessors are
now an error unless the `optional` field is set. This should help with
inadvertently building a book when a missing preprocessor that you
expect to be installed.

crates/mdbook-driver/src/builtin_preprocessors/cmd.rs

@@ -2,7 +2,7 @@ use anyhow::{Context, Result, ensure};
 use log::{debug, trace, warn};
 use mdbook_core::book::Book;
 use mdbook_preprocessor::{Preprocessor, PreprocessorContext};
-use std::io::{self, Write};
+use std::io::Write;
 use std::path::PathBuf;
 use std::process::{Child, Stdio};
 
@@ -34,12 +34,18 @@ pub struct CmdPreprocessor {
     name: String,
     cmd: String,
     root: PathBuf,
+    optional: bool,
 }
 
 impl CmdPreprocessor {
     /// Create a new `CmdPreprocessor`.
-    pub fn new(name: String, cmd: String, root: PathBuf) -> CmdPreprocessor {
-        CmdPreprocessor { name, cmd, root }
+    pub fn new(name: String, cmd: String, root: PathBuf, optional: bool) -> CmdPreprocessor {
+        CmdPreprocessor {
+            name,
+            cmd,
+            root,
+            optional,
+        }
     }
 
     fn write_input_to_child(&self, child: &mut Child, book: &Book, ctx: &PreprocessorContext) {
@@ -75,18 +81,29 @@ impl Preprocessor for CmdPreprocessor {
     fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book> {
         let mut cmd = crate::compose_command(&self.cmd, &ctx.root)?;
 
-        let mut child = cmd
+        let mut child = match cmd
             .stdin(Stdio::piped())
             .stdout(Stdio::piped())
             .stderr(Stdio::inherit())
             .current_dir(&self.root)
             .spawn()
-            .with_context(|| {
-                format!(
-                    "Unable to start the \"{}\" preprocessor. Is it installed?",
-                    self.name()
-                )
-            })?;
+        {
+            Ok(c) => c,
+            Err(e) => {
+                crate::handle_command_error(
+                    e,
+                    self.optional,
+                    "preprocessor",
+                    "preprocessor",
+                    &self.name,
+                    &self.cmd,
+                )?;
+                // This should normally not be reached, since the validation
+                // for NotFound should have already happened when running the
+                // "supports" command.
+                return Ok(book);
+            }
+        };
 
         self.write_input_to_child(&mut child, &book, ctx);
 
@@ -123,7 +140,7 @@ impl Preprocessor for CmdPreprocessor {
 
         let mut cmd = crate::compose_command(&self.cmd, &self.root)?;
 
-        let outcome = cmd
+        match cmd
             .arg("supports")
             .arg(renderer)
             .stdin(Stdio::null())
@@ -131,19 +148,20 @@ impl Preprocessor for CmdPreprocessor {
             .stderr(Stdio::inherit())
             .current_dir(&self.root)
             .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);
+        {
+            Ok(status) => Ok(status.code() == Some(0)),
+            Err(e) => {
+                crate::handle_command_error(
+                    e,
+                    self.optional,
+                    "preprocessor",
+                    "preprocessor",
+                    &self.name,
+                    &self.cmd,
+                )?;
+                Ok(false)
             }
         }
-
-        Ok(outcome.unwrap_or(false))
     }
 }
 
@@ -161,7 +179,12 @@ mod tests {
     #[test]
     fn round_trip_write_and_parse_input() {
         let md = guide();
-        let cmd = CmdPreprocessor::new("test".to_string(), "test".to_string(), md.root.clone());
+        let cmd = CmdPreprocessor::new(
+            "test".to_string(),
+            "test".to_string(),
+            md.root.clone(),
+            false,
+        );
         let ctx = PreprocessorContext::new(
             md.root.clone(),
             md.config.clone(),

crates/mdbook-driver/src/mdbook.rs

@@ -437,6 +437,8 @@ struct PreprocessorConfig {
     before: Vec<String>,
     #[serde(default)]
     after: Vec<String>,
+    #[serde(default)]
+    optional: bool,
 }
 
 /// Look at the `MDBook` and try to figure out what preprocessors to run.
@@ -513,7 +515,12 @@ fn determine_preprocessors(config: &Config, root: &Path) -> Result<Vec<Box<dyn P
                         .command
                         .to_owned()
                         .unwrap_or_else(|| format!("mdbook-{name}"));
-                    Box::new(CmdPreprocessor::new(name, command, root.to_owned()))
+                    Box::new(CmdPreprocessor::new(
+                        name,
+                        command,
+                        root.to_owned(),
+                        table.optional,
+                    ))
                 }
             };
             preprocessors.push(preprocessor);

guide/src/format/configuration/preprocessors.md

@@ -64,6 +64,18 @@ be overridden by adding a `command` field.
 command = "python random.py"
 ```
 
+### Optional preprocessors
+
+If you enable a preprocessor that isn't installed, the default behavior is to throw an error.
+This behavior can be changed by marking the preprocessor as optional:
+
+```toml
+[preprocessor.example]
+optional = true
+```
+
+This demotes the error to a warning.
+
 ## Require A Certain Order
 
 The order in which preprocessors are run can be controlled with the `before` and `after` fields.

tests/testsuite/preprocessor.rs

@@ -76,6 +76,7 @@ fn example() -> CmdPreprocessor {
         "nop-preprocessor".to_string(),
         "cargo run --quiet --example nop-preprocessor --".to_string(),
         std::env::current_dir().unwrap(),
+        false,
     )
 }
 
@@ -154,11 +155,25 @@ fn relative_command_path() {
 #[test]
 fn missing_preprocessor() {
     BookTest::from_dir("preprocessor/missing_preprocessor").run("build", |cmd| {
-        cmd.expect_stdout(str![[""]])
+        cmd.expect_failure()
+            .expect_stdout(str![[""]])
             .expect_stderr(str![[r#"
 [TIMESTAMP] [INFO] (mdbook_driver::mdbook): Book building has started
-[TIMESTAMP] [WARN] (mdbook_driver::builtin_preprocessors::cmd): The command wasn't found, is the "missing" preprocessor installed?
-[TIMESTAMP] [WARN] (mdbook_driver::builtin_preprocessors::cmd): [TAB]Command: trduyvbhijnorgevfuhn
+[TIMESTAMP] [ERROR] (mdbook_driver): The command `trduyvbhijnorgevfuhn` wasn't found, is the `missing` preprocessor installed? If you want to ignore this error when the `missing` preprocessor is not installed, set `optional = true` in the `[preprocessor.missing]` section of the book.toml configuration file.
+[TIMESTAMP] [ERROR] (mdbook_core::utils): Error: Unable to run the preprocessor `missing`
+[TIMESTAMP] [ERROR] (mdbook_core::utils): [TAB]Caused By: [NOT_FOUND]
+
+"#]]);
+    });
+}
+
+// Optional missing is not an error.
+#[test]
+fn missing_optional_not_fatal() {
+    BookTest::from_dir("preprocessor/missing_optional_not_fatal").run("build", |cmd| {
+        cmd.expect_stdout(str![[""]]).expect_stderr(str![[r#"
+[TIMESTAMP] [INFO] (mdbook_driver::mdbook): Book building has started
+[TIMESTAMP] [WARN] (mdbook_driver): The command `trduyvbhijnorgevfuhn` for preprocessor `missing` was not found, but is marked as optional.
 [TIMESTAMP] [INFO] (mdbook_driver::mdbook): Running the html backend
 [TIMESTAMP] [INFO] (mdbook_html::html_handlebars::hbs_renderer): HTML book written to `[ROOT]/book`
 

tests/testsuite/preprocessor/missing_optional_not_fatal/book.toml

@@ -0,0 +1,3 @@
+[preprocessor.missing]
+command = "trduyvbhijnorgevfuhn"
+optional = true