Exit if a preprocessor command is not found

examples/nop-preprocessor.rs

@@ -57,7 +57,7 @@ fn handle_supports(pre: &dyn Preprocessor, sub_args: &ArgMatches) -> ! {
     let renderer = sub_args
         .get_one::<String>("renderer")
         .expect("Required argument");
-    let supported = pre.supports_renderer(renderer);
+    let supported = pre.supports_renderer(renderer, false);
 
     // Signal whether the renderer is supported by exiting with 1 or 0.
     if supported {
@@ -99,7 +99,7 @@ mod nop_lib {
             Ok(book)
         }
 
-        fn supports_renderer(&self, renderer: &str) -> bool {
+        fn supports_renderer(&self, renderer: &str, _error_on_missing_preprocessor: bool) -> bool {
             renderer != "not-supported"
         }
     }

src/book/mod.rs

@@ -606,7 +606,8 @@ fn preprocessor_should_run(
 ) -> 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());
+        return preprocessor
+            .supports_renderer(renderer.name(), cfg.build.error_on_missing_preprocessor);
     }
 
     let key = format!("preprocessor.{}.renderers", preprocessor.name());
@@ -619,7 +620,7 @@ fn preprocessor_should_run(
             .any(|name| name == renderer_name);
     }
 
-    preprocessor.supports_renderer(renderer_name)
+    preprocessor.supports_renderer(renderer_name, cfg.build.error_on_missing_preprocessor)
 }
 
 #[cfg(test)]
@@ -877,7 +878,7 @@ mod tests {
             unimplemented!()
         }
 
-        fn supports_renderer(&self, _renderer: &str) -> bool {
+        fn supports_renderer(&self, _renderer: &str, _error_on_missing_preprocessor: bool) -> bool {
             self.0
         }
     }

src/cmd/init.rs

@@ -1,8 +1,7 @@
 use crate::get_book_dir;
 use clap::{arg, ArgMatches, Command as ClapCommand};
-use mdbook::config;
 use mdbook::errors::Result;
-use mdbook::MDBook;
+use mdbook::{Config, MDBook};
 use std::io;
 use std::io::Write;
 use std::process::Command;
@@ -31,7 +30,12 @@ pub fn make_subcommand() -> ClapCommand {
 pub fn execute(args: &ArgMatches) -> Result<()> {
     let book_dir = get_book_dir(args);
     let mut builder = MDBook::init(&book_dir);
-    let mut config = config::Config::default();
+
+    let mut config = Config::default();
+    // We want new books to raise an error if a preprocessor is missing, but we want old books to simply emit a warning.
+    // This is why BuildConfig::default() sets it to false and why we force it to true here
+    config.build.error_on_missing_preprocessor = true;
+
     // If flag `--theme` is present, copy theme to src
     if args.get_flag("theme") {
         let theme_dir = book_dir.join("theme");

src/config.rs

@@ -480,6 +480,12 @@ pub struct BuildConfig {
     pub use_default_preprocessors: bool,
     /// Extra directories to trigger rebuild when watching/serving
     pub extra_watch_dirs: Vec<PathBuf>,
+    /// Should missing a preprocessor be considered an error?
+    /// By default, the application raises a warning instead and continue generation,
+    /// even if the book may be generated incorrectly.
+    /// Set this flag to  ̀false` to consider this an error, and exits the application
+    /// if a preprocessor is missing.
+    pub error_on_missing_preprocessor: bool,
 }
 
 impl Default for BuildConfig {
@@ -489,6 +495,7 @@ impl Default for BuildConfig {
             create_missing: true,
             use_default_preprocessors: true,
             extra_watch_dirs: Vec::new(),
+            error_on_missing_preprocessor: false,
         }
     }
 }
@@ -815,6 +822,7 @@ mod tests {
         build-dir = "outputs"
         create-missing = false
         use-default-preprocessors = true
+        error-on-missing-preprocessor = false
 
         [output.html]
         theme = "./themedir"
@@ -856,6 +864,7 @@ mod tests {
             create_missing: false,
             use_default_preprocessors: true,
             extra_watch_dirs: Vec::new(),
+            error_on_missing_preprocessor: false,
         };
         let rust_should_be = RustConfig { edition: None };
         let playground_should_be = Playground {
@@ -1067,6 +1076,8 @@ mod tests {
             create_missing: true,
             use_default_preprocessors: true,
             extra_watch_dirs: Vec::new(),
+            error_on_missing_preprocessor: false, // This flag is missing from "src" string,
+                                                  // so it should be false to ensure backward compatibility
         };
 
         let html_should_be = HtmlConfig {

src/preprocess/cmd.rs

@@ -1,7 +1,7 @@
 use super::{Preprocessor, PreprocessorContext};
 use crate::book::Book;
 use crate::errors::*;
-use log::{debug, trace, warn};
+use log::{debug, error, trace, warn};
 use shlex::Shlex;
 use std::io::{self, Read, Write};
 use std::process::{Child, Command, Stdio};
@@ -134,7 +134,7 @@ impl Preprocessor for CmdPreprocessor {
         })
     }
 
-    fn supports_renderer(&self, renderer: &str) -> bool {
+    fn supports_renderer(&self, renderer: &str, error_on_missing_preprocessor: bool) -> bool {
         debug!(
             "Checking if the \"{}\" preprocessor supports \"{}\"",
             self.name(),
@@ -164,11 +164,17 @@ impl Preprocessor for CmdPreprocessor {
 
         if let Err(ref e) = outcome {
             if e.kind() == io::ErrorKind::NotFound {
-                warn!(
-                    "The command wasn't found, is the \"{}\" preprocessor installed?",
-                    self.name
+                let message = format!(
+                    "The command \"{}\" wasn't found, is the \"{}\" preprocessor installed?",
+                    self.cmd, self.name
                 );
-                warn!("\tCommand: {}", self.cmd);
+
+                if error_on_missing_preprocessor {
+                    error!("{message}");
+                    std::process::exit(1);
+                } else {
+                    warn!("{message}");
+                }
             }
         }
 

src/preprocess/mod.rs

@@ -63,7 +63,9 @@ pub trait Preprocessor {
     /// particular renderer.
     ///
     /// By default, always returns `true`.
-    fn supports_renderer(&self, _renderer: &str) -> bool {
+    /// Set `error_on_missing_preprocessor` to `true` to exit the application is a preprocessor is missing,
+    /// or to `false` to simply raise a warning and continue generation.
+    fn supports_renderer(&self, _renderer: &str, _error_on_missing_preprocessor: bool) -> bool {
         true
     }
 }

tests/custom_preprocessors.rs

@@ -15,7 +15,7 @@ fn example() -> CmdPreprocessor {
 fn example_supports_whatever() {
     let cmd = example();
 
-    let got = cmd.supports_renderer("whatever");
+    let got = cmd.supports_renderer("whatever", false);
 
     assert_eq!(got, true);
 }
@@ -24,7 +24,7 @@ fn example_supports_whatever() {
 fn example_doesnt_support_not_supported() {
     let cmd = example();
 
-    let got = cmd.supports_renderer("not-supported");
+    let got = cmd.supports_renderer("not-supported", false);
 
     assert_eq!(got, false);
 }

tests/init.rs

@@ -94,7 +94,7 @@ fn run_mdbook_init_with_custom_book_and_src_locations() {
     let contents = fs::read_to_string(temp.path().join("book.toml")).unwrap();
     assert_eq!(
         contents,
-        "[book]\nauthors = []\nlanguage = \"en\"\nmultilingual = false\nsrc = \"in\"\n\n[build]\nbuild-dir = \"out\"\ncreate-missing = true\nextra-watch-dirs = []\nuse-default-preprocessors = true\n"
+        "[book]\nauthors = []\nlanguage = \"en\"\nmultilingual = false\nsrc = \"in\"\n\n[build]\nbuild-dir = \"out\"\ncreate-missing = true\nerror-on-missing-preprocessor = false\nextra-watch-dirs = []\nuse-default-preprocessors = true\n"
     );
 }