Found a race condition when backends exit really quickly

Author: Michael-F-Bryan

Cargo.toml

@@ -32,6 +32,7 @@ open = "1.1"
 regex = "0.2.1"
 tempdir = "0.3.4"
 itertools = "0.7.4"
+tempfile = "2.2.0"
 
 # Watch feature
 notify = { version = "4.0", optional = true }

src/lib.rs

@@ -110,6 +110,7 @@ extern crate serde_derive;
 #[macro_use]
 extern crate serde_json;
 extern crate tempdir;
+extern crate tempfile;
 extern crate toml;
 
 #[cfg(test)]

src/renderer/mod.rs

@@ -17,8 +17,9 @@ mod html_handlebars;
 
 use std::io::Read;
 use std::path::PathBuf;
-use std::process::{Command, Stdio};
+use std::process::Command;
 use serde_json;
+use tempfile;
 
 use errors::*;
 use config::Config;
@@ -135,17 +136,20 @@ impl Renderer for CmdRenderer {
     fn render(&self, ctx: &RenderContext) -> Result<()> {
         info!("Invoking the \"{}\" renderer", self.cmd);
 
-        let mut child = Command::new(&self.cmd)
-            .stdin(Stdio::piped())
-            .spawn()
+        // We need to write the RenderContext to a temporary file here instead
+        // of passing it in via a pipe. This prevents a race condition where
+        // some quickly executing command (e.g. `/bin/true`) may exit before we
+        // finish writing the render context (closing the stdin pipe and
+        // throwing a write error).
+        let mut temp = tempfile::tempfile().chain_err(|| "Unable to create a temporary file")?;
+        serde_json::to_writer(&mut temp, &ctx)
+            .chain_err(|| "Unable to serialize the RenderContext")?;
+
+        let status = Command::new(&self.cmd)
+            .stdin(temp)
+            .status()
             .chain_err(|| "Unable to start the renderer")?;
 
-        serde_json::to_writer(
-            child.stdin.as_mut().expect("stdin is always attached"),
-            &ctx,
-        ).chain_err(|| "Error occurred while sending the render context to the renderer")?;
-
-        let status = child.wait()?;
         trace!("{} exited with output: {:?}", self.cmd, status);
 
         if !status.success() {

tests/alternate_backends.rs

@@ -0,0 +1,45 @@
+//! Integration tests to make sure alternate backends work.
+
+extern crate mdbook;
+extern crate tempdir;
+
+use tempdir::TempDir;
+use mdbook::config::Config;
+use mdbook::MDBook;
+
+#[test]
+fn passing_alternate_backend() {
+    let (md, _temp) = dummy_book_with_backend("passing", "true");
+
+    md.build().unwrap();
+}
+
+#[test]
+fn failing_alternate_backend() {
+    let (md, _temp) = dummy_book_with_backend("failing", "false");
+
+    md.build().unwrap_err();
+}
+
+#[test]
+fn alternate_backend_with_arguments() {
+    let (md, _temp) = dummy_book_with_backend("arguments", "echo Hello World!");
+
+    md.build().unwrap();
+}
+
+fn dummy_book_with_backend(name: &str, command: &str) -> (MDBook, TempDir) {
+    let temp = TempDir::new("mdbook").unwrap();
+
+    let mut config = Config::default();
+    config
+        .set(format!("output.{}.command", name), command)
+        .unwrap();
+
+    let md = MDBook::init(temp.path())
+        .with_config(config)
+        .build()
+        .unwrap();
+
+    (md, temp)
+}