Optimize tokio::child::unix::ChildImp::wait

Try avoiding the blocking thread if possible.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

src/tokio/child/unix.rs

@@ -1,6 +1,7 @@
 use std::{
 	convert::TryInto,
 	io::{Error, Result},
+	ops::ControlFlow,
 	os::unix::process::ExitStatusExt,
 	process::ExitStatus,
 };
@@ -69,7 +70,7 @@ impl ChildImp {
 		self.inner.id()
 	}
 
-	fn wait_imp(pgid: i32, flag: WaitPidFlag) -> Result<Option<ExitStatus>> {
+	fn wait_imp(pgid: i32, flag: WaitPidFlag) -> Result<ControlFlow<Option<ExitStatus>>> {
 		// Wait for processes in a loop until every process in this
 		// process group has exited (this ensures that we reap any
 		// zombies that may have been created if the parent exited after
@@ -85,14 +86,14 @@ impl ChildImp {
 				0 => {
 					// Zero should only happen if WNOHANG was passed in,
 					// and means that no processes have yet to exit.
-					return Ok(None);
+					return Ok(ControlFlow::Continue(()));
 				}
 				-1 => {
 					match Errno::last() {
 						Errno::ECHILD => {
 							// No more children to reap; this is a
 							// graceful exit.
-							return Ok(parent_exit_status);
+							return Ok(ControlFlow::Break(parent_exit_status));
 						}
 						errno => {
 							return Err(Error::from(errno));
@@ -115,24 +116,33 @@ impl ChildImp {
 	}
 
 	pub async fn wait(&mut self) -> Result<ExitStatus> {
-		if let Some(status) = self.try_wait()? {
-			return Ok(status);
-		}
+		const MAX_RETRY_ATTEMPT: usize = 10;
+
+		// Always wait for parent to exit first.
+		//
+		// It's likely that all its children has already exited and reaped by
+		// the time the parent exits.
+		let status = self.inner.wait().await?;
 
 		let pgid = self.pgid.as_raw();
-		match spawn_blocking(move || Self::wait_imp(pgid, WaitPidFlag::empty()))
-			.await?
-			.transpose()
-		{
-			None => self.inner.wait().await,
-			Some(status) => status,
+
+		// Try reaping all children, if there are some that are still alive after
+		// several attempts, then spawn a blocking task to reap them.
+		for retry_attempt in 1..=MAX_RETRY_ATTEMPT {
+			if Self::wait_imp(pgid, WaitPidFlag::WNOHANG)?.is_break() {
+				break;
+			} else if retry_attempt == MAX_RETRY_ATTEMPT {
+				spawn_blocking(move || Self::wait_imp(pgid, WaitPidFlag::empty())).await??;
+			}
 		}
+
+		Ok(status)
 	}
 
 	pub fn try_wait(&mut self) -> Result<Option<ExitStatus>> {
-		match Self::wait_imp(self.pgid.as_raw(), WaitPidFlag::WNOHANG) {
-			Ok(None) => self.inner.try_wait(),
-			otherwise => otherwise,
+		match Self::wait_imp(self.pgid.as_raw(), WaitPidFlag::WNOHANG)? {
+			ControlFlow::Break(res) => Ok(res),
+			ControlFlow::Continue(()) => self.inner.try_wait(),
 		}
 	}
 }