Merge pull request #792 from rust-lang-nursery/custom-preprocessor

WIP: Custom Preprocessors

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
+
+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.
+
+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"]
+```
 
-A preprocessor is represented by the `Preprocessor` trait.
+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.
 
-```rust
-pub trait Preprocessor {
-    fn name(&self) -> &str;
-    fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book>;
-    fn supports_renderer(&self, _renderer: &str) -> bool {
-        true
-    }
-}
-```
+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.
 
-Where the `PreprocessorContext` is defined as
+<details>
+<summary>Example no-op preprocessor</summary>
 
 ```rust
-pub struct PreprocessorContext {
-    pub root: PathBuf,
-    pub config: Config,
-    /// The `Renderer` this preprocessor is being used with.
-    pub renderer: String,
-}
-```
-
-The `renderer` value allows you react accordingly, for example, PDF or HTML.
+// nop-preprocessors.rs
 
-## A complete Example
+{{#include ../../../examples/nop-preprocessor.rs}}
+```
+</details>
 
-The magic happens within the `run(...)` method of the
-[`Preprocessor`][preprocessor-docs] trait implementation.
+## Hints For Implementing A Preprocessor
 
-As direct access to the chapters is not possible, you will probably end up
-iterating them using `for_each_mut(...)`:
+By pulling in `mdbook` as a library, preprocessors can have access to the
+existing infrastructure for dealing with books.
 
-```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),
-          },
-      );
-  }
-});
-```
+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.
 
-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.
+Chapters can be accessed either directly (by recursively iterating over
+chapters) or via the `Book::for_each_mut()` convenience method.
 
-Finally you can use [`pulldown-cmark-to-cmark`][pctc] to transform these events
-back to a string.
+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, and do
-so safely.
+The following code block shows how to remove all emphasis from markdown,
+without accidentally breaking the document.
 
 ```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

@@ -1,36 +1,20 @@
-//! 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;
 
 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 {
-        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);
-    }
+    panic!("This example is intended to be part of a library");
 }
 
 struct Deemphasize;

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"
+        }
+    }
+}
+