We can shell out to the preprocessor

examples/nop-preprocessor.rs

@@ -3,8 +3,9 @@ extern crate mdbook;
 extern crate clap;
 
 use clap::{App, Arg, ArgMatches, SubCommand};
-use mdbook::preprocess::{Preprocessor, PreprocessorContext};
+use mdbook::preprocess::{Preprocessor, PreprocessorContext, CmdPreprocessor};
 use std::process;
+use std::io;
 
 fn main() {
     let matches = app().get_matches();
@@ -17,7 +18,18 @@ fn main() {
 }
 
 fn handle_preprocessing(args: &ArgMatches) {
-    unimplemented!()
+    let (ctx, book) = CmdPreprocessor::parse_input(io::stdin())
+        .expect("Couldn't parse the input");
+
+    // You can tell the preprocessor to blow up by setting a particular
+    // config value
+    if let Some(table) = ctx.config.get_preprocessor("nop-preprocessor") {
+        let should_blow_up = table.get("blow-up").is_some();
+
+        if should_blow_up {
+            panic!("Boom!!!1!");
+        }
+    }
 }
 
 fn handle_supports(sub_args: &ArgMatches) {

src/preprocess/cmd.rs

@@ -39,7 +39,7 @@ impl CmdPreprocessor {
         &self,
         child: &mut Child,
         book: Book,
-        ctx: PreprocessorContext,
+        ctx: &PreprocessorContext,
     ) {
         let mut stdin = child.stdin.take().expect("Child has stdin");
         let input = (ctx, book);
@@ -77,7 +77,25 @@ impl Preprocessor for CmdPreprocessor {
     }
 
     fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book> {
-        unimplemented!()
+        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(&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 {

tests/custom_preprocessors.rs

@@ -1,6 +1,8 @@
 extern crate mdbook;
 
 use mdbook::preprocess::{CmdPreprocessor, Preprocessor};
+use mdbook::MDBook;
+use std::path::Path;
 
 fn example() -> CmdPreprocessor {
     CmdPreprocessor::new("nop-preprocessor".to_string(), "cargo run --example nop-preprocessor --".to_string())
@@ -23,3 +25,25 @@ fn example_doesnt_support_not_supported() {
 
     assert_eq!(got, false);
 }
+
+#[test]
+fn ask_the_preprocessor_to_blow_up() {
+    let dummy_book = Path::new(env!("CARGO_MANIFEST_DIR")).join("tests").join("dummy_book");
+    let mut md = MDBook::load(&dummy_book).unwrap();
+    md.with_preprecessor(example());
+
+    md.config.set("preprocess.nop-preprocessor.blow-up", true).unwrap();
+
+    let got = md.build();
+
+    assert!(got.is_err());
+}
+
+#[test]
+fn process_the_dummy_book() {
+    let dummy_book = Path::new(env!("CARGO_MANIFEST_DIR")).join("tests").join("dummy_book");
+    let mut md = MDBook::load(&dummy_book).unwrap();
+    md.with_preprecessor(example());
+
+    md.build().unwrap();
+}