fix: consume stderr on wait

src/child.rs

@@ -1,14 +1,12 @@
 //! Wrapper around `std::process::Child` containing a spawned FFmpeg command.
 
+use crate::iter::FfmpegIterator;
+use anyhow::Context;
 use std::{
-  io::{self, Write},
+  io::{self, copy, sink, Write},
   process::{Child, ChildStderr, ChildStdin, ChildStdout, ExitStatus},
 };
 
-use anyhow::Context;
-
-use crate::iter::FfmpegIterator;
-
 /// A wrapper around [`std::process::Child`] containing a spawned FFmpeg command.
 /// Provides interfaces for reading parsed metadata, progress updates, warnings and errors, and
 /// piped output frames if applicable.
@@ -100,6 +98,12 @@ impl FfmpegChild {
   ///
   /// Identical to `wait` in [`std::process::Child`].
   pub fn wait(&mut self) -> io::Result<ExitStatus> {
+    // If stderr hasn't already been consumed by a method like `iter()`,
+    // we need to run it to completion to avoid a deadlock.
+    if let Some(mut stderr) = self.take_stderr() {
+      copy(&mut stderr, &mut sink())?;
+    };
+
     self.inner.wait()
   }
 

src/test.rs

@@ -40,6 +40,35 @@ fn spawn_with_timeout(command: &mut FfmpegCommand, timeout: u64) -> anyhow::Resu
   }
 }
 
+/// Returns `Err` if the timeout thread finishes before the FFmpeg process
+/// Note: this variant leaves behind a hung FFmpeg child process + thread until
+/// the test suite exits.
+fn wait_with_timeout(command: &mut FfmpegCommand, timeout: u64) -> anyhow::Result<()> {
+  let (sender, receiver) = mpsc::channel();
+
+  // Thread 1: Waits for 1000ms and sends a message
+  let timeout_sender = sender.clone();
+  thread::spawn(move || {
+    thread::sleep(Duration::from_millis(timeout));
+    timeout_sender.send("timeout").ok();
+  });
+
+  // Thread 2: Wait for the child to exit in another thread
+  let mut ffmpeg_child = command.spawn()?;
+  thread::spawn(move || {
+    ffmpeg_child.wait().unwrap();
+    sender.send("ffmpeg").ok();
+  });
+
+  // Race the two threads
+  let finished_first = receiver.recv()?;
+  match finished_first {
+    "timeout" => anyhow::bail!("Timeout thread expired before FFmpeg"),
+    "ffmpeg" => Ok(()),
+    _ => anyhow::bail!("Unknown message received"),
+  }
+}
+
 #[test]
 fn test_installed() {
   assert!(ffmpeg_is_installed());
@@ -715,3 +744,30 @@ fn test_no_empty_events() -> anyhow::Result<()> {
 
   Ok(())
 }
+
+/// This command generates an warning on every frame, e.g.:
+///
+/// ```txt
+/// [Parsed_palettegen_4 @ 0x600001574bb0] [warning] The input frame is not in sRGB, colors may be off
+///```
+///
+/// When used in combination with `.wait()`, these error messages can completely
+/// fill the stderr buffer and cause a deadlock. The solution is to
+/// automatically drop the stderr channel when `.wait()` is called.
+///
+/// <https://github.com/nathanbabcock/ffmpeg-sidecar/issues/70>
+#[test]
+fn test_wait() -> anyhow::Result<()> {
+  let mut command = FfmpegCommand::new();
+  command
+    .args("-color_primaries 1".split(' '))
+    .args("-color_trc 1".split(' '))
+    .args("-colorspace 1".split(' '))
+    .format("lavfi")
+    .input("yuvtestsrc=size=64x64:rate=60:duration=60")
+    .args("-vf palettegen=max_colors=164".split(' '))
+    .codec_video("gif")
+    .format("null")
+    .output("-");
+  wait_with_timeout(&mut command, 5000)
+}