fix(langchain): resolve race condition in ShellSession.execute() (#34535)

Addresses a flaky test

When executing `exit 1` as a startup command, the shell process
terminates immediately. The code then tries to write a marker command
(`printf '...'`) to stdin, but the pipe is already broken because the
shell has exited, causing `BrokenPipeError`.

libs/langchain_v1/langchain/agents/middleware/shell_tool.py

@@ -211,9 +211,14 @@ class ShellSession:
         with self._lock:
             self._drain_queue()
             payload = command if command.endswith("\n") else f"{command}\n"
-            self._stdin.write(payload)
-            self._stdin.write(f"printf '{marker} %s\\n' $?\n")
-            self._stdin.flush()
+            try:
+                self._stdin.write(payload)
+                self._stdin.write(f"printf '{marker} %s\\n' $?\n")
+                self._stdin.flush()
+            except (BrokenPipeError, OSError):
+                # The shell exited before we could write the marker command.
+                # This happens when commands like 'exit 1' terminate the shell.
+                return self._collect_output_after_exit(deadline)
 
             return self._collect_output(marker, deadline, timeout)
 
@@ -304,6 +309,80 @@ class ShellSession:
             total_bytes=total_bytes,
         )
 
+    def _collect_output_after_exit(self, deadline: float) -> CommandExecutionResult:
+        """Collect output after the shell exited unexpectedly.
+
+        Called when a `BrokenPipeError` occurs while writing to stdin, indicating the
+        shell process terminated (e.g., due to an 'exit' command).
+
+        Args:
+            deadline: Absolute time by which collection must complete.
+
+        Returns:
+            `CommandExecutionResult` with collected output and the process exit code.
+        """
+        collected: list[str] = []
+        total_lines = 0
+        total_bytes = 0
+        truncated_by_lines = False
+        truncated_by_bytes = False
+
+        # Give reader threads a brief moment to enqueue any remaining output.
+        drain_timeout = 0.1
+        drain_deadline = min(time.monotonic() + drain_timeout, deadline)
+
+        while True:
+            remaining = drain_deadline - time.monotonic()
+            if remaining <= 0:
+                break
+            try:
+                source, data = self._queue.get(timeout=remaining)
+            except queue.Empty:
+                break
+
+            if data is None:
+                # EOF marker from a reader thread; continue draining.
+                continue
+
+            total_lines += 1
+            encoded = data.encode("utf-8", "replace")
+            total_bytes += len(encoded)
+
+            if total_lines > self._policy.max_output_lines:
+                truncated_by_lines = True
+                continue
+
+            if (
+                self._policy.max_output_bytes is not None
+                and total_bytes > self._policy.max_output_bytes
+            ):
+                truncated_by_bytes = True
+                continue
+
+            if source == "stderr":
+                stripped = data.rstrip("\n")
+                collected.append(f"[stderr] {stripped}")
+                if data.endswith("\n"):
+                    collected.append("\n")
+            else:
+                collected.append(data)
+
+        # Get exit code from the terminated process.
+        exit_code: int | None = None
+        if self._process:
+            exit_code = self._process.poll()
+
+        output = "".join(collected)
+        return CommandExecutionResult(
+            output=output,
+            exit_code=exit_code,
+            timed_out=False,
+            truncated_by_lines=truncated_by_lines,
+            truncated_by_bytes=truncated_by_bytes,
+            total_lines=total_lines,
+            total_bytes=total_bytes,
+        )
+
     def _kill_process(self) -> None:
         if not self._process:
             return