Rewrote a large proportion of the Preprocessor docs to be up-to-date

book-example/src/for_developers/preprocessors.md

@@ -11,68 +11,71 @@ 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
+While preprocessors can be hard-coded to specify which backend it should be run
-pub trait Preprocessor {
+for (e.g. it doesn't make sense for MathJax to be used for non-HTML renderers)
-    fn name(&self) -> &str;
+with the `preprocessor.foo.renderer` key.
-    fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book>;
+
-    fn supports_renderer(&self, _renderer: &str) -> bool {
+```toml
-        true
+[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 {
+// nop-preprocessors.rs
-    pub root: PathBuf,
+
-    pub config: Config,
+{{#include ../../../examples/nop-preprocessor.rs}}
-    /// The `Renderer` this preprocessor is being used with.
-    pub renderer: String,
-}
 ```
+</details>
 
-The `renderer` value allows you react accordingly, for example, PDF or HTML.
+## Hints For Implementing A Preprocessor
 
-## A complete Example
+By pulling in `mdbook` as a library, preprocessors can have access to the
+existing infrastructure for dealing with books.
 
-The magic happens within the `run(...)` method of the
+For example, a custom preprocessor could use the
-[`Preprocessor`][preprocessor-docs] trait implementation.
+[`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.
 
-As direct access to the chapters is not possible, you will probably end up
+Chapters can be accessed either directly (by recursively iterating over
-iterating them using `for_each_mut(...)`:
+chapters) or via the `Book::for_each_mut()` convenience method.
 
-```rust
+The `chapter.content` is just a string which happens to be markdown. While it's
-book.for_each_mut(|item: &mut BookItem| {
+entirely possible to use regular expressions or do a manual find & replace,
-    if let BookItem::Chapter(ref mut chapter) = *item {
+you'll probably want to process the input into something more computer-friendly.
-      eprintln!("{}: processing chapter '{}'", self.name(), chapter.name);
+The [`pulldown-cmark`][pc] crate implements a production-quality event-based
-      res = Some(
+Markdown parser, with the [`pulldown-cmark-to-cmark`][pctc] allowing you to
-          match Deemphasize::remove_emphasis(&mut num_removed_items, chapter) {
+translate events back into markdown text.
-              Ok(md) => {
-                  chapter.content = md;
-                  Ok(())
-              }
-              Err(err) => Err(err),
-          },
-      );
-  }
-});
-```
 
-The `chapter.content` is just a markdown formatted string, and you will have to
+The following code block shows how to remove all emphasis from markdown,
-process it in some way. Even though it's entirely possible to implement some
+without accidentally breaking the document.
-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(
@@ -107,3 +110,6 @@ For everything else, have a look [at the complete example][example].
 [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

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;
@@ -6,31 +8,13 @@ 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 do_it(book: OsString) -> Result<()> {
-    let mut book = MDBook::load(book)?;
-    book.with_preprecessor(Deemphasize);
-    book.build()
-}
-
 fn main() {
-    if args_os().count() != 2 {
+    panic!("This example is intended to be part of a library");
-        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);
-    }
 }
 
 struct Deemphasize;

examples/nop-preprocessor.rs

@@ -1,56 +1,64 @@
+extern crate clap;
 extern crate mdbook;
 extern crate serde_json;
-#[macro_use]
-extern crate clap;
 
 use clap::{App, Arg, ArgMatches, SubCommand};
-use mdbook::preprocess::CmdPreprocessor;
 use mdbook::book::Book;
-use std::process;
+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 = app().get_matches();
+    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(sub_args);
+        handle_supports(&preprocessor, sub_args);
     } else {
-        handle_preprocessing();
+        if let Err(e) = handle_preprocessing(&preprocessor) {
+            eprintln!("{}", e);
+            process::exit(1);
+        }
     }
 }
 
-fn handle_preprocessing() {
+fn handle_preprocessing(pre: &dyn Preprocessor) -> Result<(), Error> {
-    let (ctx, book) = CmdPreprocessor::parse_input(io::stdin())
+    let (ctx, book) = CmdPreprocessor::parse_input(io::stdin())?;
-        .expect("Couldn't parse the input");
 
-    // You can inspect the calling mdbook's version to check for compatibility
     if ctx.mdbook_version != mdbook::MDBOOK_VERSION {
-        panic!("The version check failed!");
+        // 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
+        );
     }
 
-    // In testing we want to tell the preprocessor to blow up by setting a
+    let processed_book = pre.run(&ctx, book)?;
-    // particular config value
+    serde_json::to_writer(io::stdout(), &processed_book)?;
-    if let Some(table) = ctx.config.get_preprocessor("nop-preprocessor") {
-        let should_blow_up = table.get("blow-up").is_some();
 
-        if should_blow_up {
+    Ok(())
-            panic!("Boom!!!1!");
-        }
 }
 
-    let processed_book = do_processing(book);
+fn handle_supports(pre: &dyn Preprocessor, sub_args: &ArgMatches) -> ! {
-
-    serde_json::to_writer(io::stdout(), &processed_book).unwrap();
-}
-
-fn do_processing(book: Book) -> Book {
-    // We *are* a nop preprocessor after all...
-    book
-}
-
-fn handle_supports(sub_args: &ArgMatches) {
     let renderer = sub_args.value_of("renderer").expect("Required argument");
-    let supported = renderer_is_supported(&renderer);
+    let supported = pre.supports_renderer(&renderer);
 
     // Signal whether the renderer is supported by exiting with 1 or 0.
     if supported {
@@ -60,17 +68,45 @@ fn handle_supports(sub_args: &ArgMatches) {
     }
 }
 
-fn renderer_is_supported(renderer: &str) -> bool {
+/// The actual implementation of the `Nop` preprocessor. This would usually go
-    // We support everything except the `not-supported` renderer
+/// in your main `lib.rs` file.
-    renderer != "not-supported"
+mod nop_lib {
+    use super::*;
+
+    /// A no-op preprocessor.
+    pub struct Nop;
+
+    impl Nop {
+        pub fn new() -> Nop {
+            Nop
+        }
     }
 
-fn app() -> App<'static, 'static> {
+    impl Preprocessor for Nop {
-    app_from_crate!().subcommand(
+        fn name(&self) -> &str {
-        SubCommand::with_name("supports")
+            "nop-preprocessor"
-            .arg(Arg::with_name("renderer").required(true))
-            .about(
-                "Check whether a renderer is supported by this 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/preprocess/cmd.rs

@@ -3,7 +3,7 @@ use book::Book;
 use errors::*;
 use serde_json;
 use shlex::Shlex;
-use std::io::{self, Read};
+use std::io::{self, Read, Write};
 use std::process::{Child, Command, Stdio};
 
 /// A custom preprocessor which will shell out to a 3rd-party program.
@@ -50,23 +50,24 @@ impl CmdPreprocessor {
             .chain_err(|| "Unable to parse the input")
     }
 
-    fn write_input(
+    fn write_input_to_child(
         &self,
         child: &mut Child,
-        book: Book,
+        book: &Book,
         ctx: &PreprocessorContext,
     ) {
-        let mut stdin = child.stdin.take().expect("Child has stdin");
+        let stdin = child.stdin.take().expect("Child has stdin");
-        let input = (ctx, book);
 
-        if let Err(e) = serde_json::to_writer(&mut stdin, &input) {
+        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);
         }
+    }
 
-        // explicitly close the `stdin` file handle
+    fn write_input<W: Write>(&self, writer: W, book: &Book, ctx: &PreprocessorContext) -> Result<()> {
-        drop(stdin);
+        serde_json::to_writer(writer, &(ctx, book))
+            .map_err(Into::into)
     }
 
     /// The command this `Preprocessor` will invoke.
@@ -106,7 +107,7 @@ impl Preprocessor for CmdPreprocessor {
             .spawn()
             .chain_err(|| format!("Unable to start the \"{}\" preprocessor. Is it installed?", self.name()))?;
 
-        self.write_input(&mut child, book, ctx);
+        self.write_input_to_child(&mut child, &book, ctx);
 
         let output = child
             .wait_with_output()

src/preprocess/links.rs

@@ -69,7 +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);
                     }
@@ -83,6 +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;
@@ -109,10 +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::IncludeRange(p, _) => {
-            LinkType::IncludeRangeFrom(p, _) => Some(return_relative_path(base, &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::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)),
         }
     }
@@ -182,11 +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,
@@ -207,20 +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))
+            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)),
+                    .chain_err(|| {
-            LinkType::IncludeRangeFrom(ref pat, ref range) => file_to_string(base.join(pat))
+                        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)),
+                    .chain_err(|| {
-            LinkType::IncludeRangeTo(ref pat, ref range) => file_to_string(base.join(pat))
+                        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)),
+                    .chain_err(|| {
-            LinkType::IncludeRangeFull(ref pat, _) => file_to_string(base.join(pat))
+                        format!(
-                .chain_err(|| format!("Could not read file for link {}", self.link_text)),
+                            "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))
+                let target = base.join(pat);
-                    .chain_err(|| format!("Could not read file for link {}", self.link_text))?;
+
+                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",
@@ -465,7 +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 {
@@ -475,7 +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}}",
                 },
             ]
         );