fix: drain stdout before resolving when a child holds the pipe past exit (#5753)

packages/coding-agent/src/utils/child-process.ts

@@ -38,10 +38,13 @@ export function spawnProcessSync(
 /**
  * Wait for a child process to terminate without hanging on inherited stdio handles.
  *
- * On Windows, daemonized descendants can inherit the child's stdout/stderr pipe
- * handles. In that case the child emits `exit`, but `close` can hang forever even
- * though the original process is already gone. We wait briefly for stdio to end,
- * then forcibly stop tracking the inherited handles.
+ * A short-lived child can `exit` while a detached descendant keeps its stdout/stderr
+ * pipe open. We must not resolve and destroy the streams on a fixed deadline measured
+ * from `exit`, or output still being written past that deadline is silently lost
+ * (earendil-works/pi#5303). Instead, after `exit` we wait for the pipes to fall idle:
+ * the grace timer is re-armed on every chunk, so an actively writing descendant keeps
+ * us reading, while a quiet inherited handle (e.g. a Windows daemonized descendant
+ * that never lets `close` fire) still releases us after the grace elapses.
  */
 export function waitForChildProcess(child: ChildProcess): Promise<number | null> {
 	return new Promise((resolve, reject) => {
@@ -62,6 +65,8 @@ export function waitForChildProcess(child: ChildProcess): Promise<number | null>
 			child.removeListener("close", onClose);
 			child.stdout?.removeListener("end", onStdoutEnd);
 			child.stderr?.removeListener("end", onStderrEnd);
+			child.stdout?.removeListener("data", onData);
+			child.stderr?.removeListener("data", onData);
 		};
 
 		const finalize = (code: number | null) => {
@@ -80,6 +85,17 @@ export function waitForChildProcess(child: ChildProcess): Promise<number | null>
 			}
 		};
 
+		const armIdleTimer = () => {
+			if (postExitTimer) clearTimeout(postExitTimer);
+			postExitTimer = setTimeout(() => finalize(exitCode), EXIT_STDIO_GRACE_MS);
+		};
+
+		const onData = () => {
+			// Output is still arriving after exit; defer finalizing so we don't
+			// destroy the stream mid-write and truncate the tail.
+			if (exited && !settled) armIdleTimer();
+		};
+
 		const onStdoutEnd = () => {
 			stdoutEnded = true;
 			maybeFinalizeAfterExit();
@@ -102,7 +118,7 @@ export function waitForChildProcess(child: ChildProcess): Promise<number | null>
 			exitCode = code;
 			maybeFinalizeAfterExit();
 			if (!settled) {
-				postExitTimer = setTimeout(() => finalize(code), EXIT_STDIO_GRACE_MS);
+				armIdleTimer();
 			}
 		};
 
@@ -112,6 +128,8 @@ export function waitForChildProcess(child: ChildProcess): Promise<number | null>
 
 		child.stdout?.once("end", onStdoutEnd);
 		child.stderr?.once("end", onStderrEnd);
+		child.stdout?.on("data", onData);
+		child.stderr?.on("data", onData);
 		child.once("error", onError);
 		child.once("exit", onExit);
 		child.once("close", onClose);