Erlang Port Exit Status Issue With Stderr_to_stdout

Introduction

In the world of Erlang and OTP, managing external processes through ports is a common practice. However, developers sometimes encounter unexpected behavior when using the stderr_to_stdout option. This article delves into a specific bug where the exit status of a port is not delivered when stderr_to_stdout is enabled. We will explore the conditions that trigger this issue, provide a step-by-step guide to reproduce the bug, discuss the expected versus actual behavior, identify the affected Erlang versions, and offer insights into potential solutions. Understanding this issue is crucial for Erlang developers aiming to build robust and reliable systems. We aim to provide you with a comprehensive understanding of this issue, its implications, and how to address it effectively.

Understanding the Bug

The core of the problem lies in a specific scenario involving inter-process communication in Erlang. When an Erlang process (BEAM1) opens a port to another Erlang process (BEAM2) and configures it to redirect standard error to standard output (stderr_to_stdout), a peculiar issue arises. If BEAM2 completes its execution before a third-party process spawned by it (SOME_PROCESS), BEAM1 might not receive the exit_status message. This is a critical issue because the exit_status is essential for understanding whether the process executed successfully or encountered an error. Without this status, monitoring and error handling become significantly more challenging. The absence of the exit_status can lead to indefinite hangs, resource leaks, and overall system instability. Therefore, a clear grasp of this behavior is vital for building reliable Erlang applications.

Reproducing the Issue: A Step-by-Step Guide

To illustrate this bug, let's consider a scenario where BEAM1 opens a port to BEAM2 with stderr_to_stdout enabled. BEAM2, in turn, opens another port to a separate process, SOME_PROCESS. The issue manifests when BEAM2 finishes before SOME_PROCESS.

Scenario Description

We have three actors: BEAM1, BEAM2, and SOME_PROCESS. BEAM1 initiates the process by opening a port to BEAM2. This port is configured with stderr_to_stdout, meaning any error output from BEAM2 is redirected to its standard output. BEAM2 then opens a port to SOME_PROCESS, which, in this example, is a simple sleep infinity command that runs indefinitely.

Code Snippet

The following Erlang code snippet demonstrates this scenario:

-module(repro).
-export([start/0, start/1]).

start() ->
    start([]).

start([]) ->
    io:format("[PARENT] Starting child~n"),
    ErlBin = filename:join([code:root_dir(), "bin", "erl"]),
    Port = open_port({spawn_executable, ErlBin}, [
        {args, ["-noshell", "-run", atom_to_list(?MODULE), ?FUNCTION_NAME, "child"]},
        use_stdio,
        stderr_to_stdout,
        exit_status
    ]),
    parent_loop(Port);

start(["child"]) ->
    io:format("[CHILD] open 'sleep infinity' port~n"),
    SleepBin = os:find_executable("sleep"),
    Port = open_port({spawn_executable, SleepBin}, [
        {args, ["infinity"]},
        use_stdio,
        exit_status
    ]),
    io:format("[CHILD] SLEEP PROCESS INFO ~p~n", [erlang:port_info(Port, os_pid)]),
    io:format("[CHILD] EXIT~n"),
    erlang:halt(42).

parent_loop(Port) ->
    receive
        {Port, {exit_status, N}} ->
            io:format("[PARENT] Child exited with status ~p. Exiting~n", [N]),
            erlang:halt(0);
        {Port, {data, Data}} ->
            io:format("~ts", [Data]),
            parent_loop(Port);
        M ->
            io:format("UNEXPECTED ~p", [M]),
            parent_loop(Port)
    end.

Steps to Reproduce

1. Save the above code as repro.erl.
2. Compile the code using erlc repro.erl.
3. Run the Erlang program using erl -noshell -run repro.

Expected vs. Actual Behavior

Expected Behavior: The program should complete, printing the exit status of the child process (BEAM2).

Actual Behavior: The program hangs indefinitely, waiting for the exit_status message. This message is never received because BEAM2 has finished, but SOME_PROCESS (the sleep infinity command) is still running. The parent process (BEAM1) gets stuck in the parent_loop function, indefinitely waiting for a message that will never arrive.

Workaround

Interestingly, if you manually terminate the sleep process (SOME_PROCESS) using its PID (which is printed in the console), the program completes successfully, displaying the correct exit status. This workaround highlights the dependency on the termination of the child processes for the parent process to receive the exit status.

The Significance of stderr_to_stdout

The critical observation is that if you comment out the stderr_to_stdout line in the code, the program behaves as expected, and the exit status is received without issues. This indicates that the redirection of standard error to standard output is a key factor in triggering this bug. The underlying reason might be related to how Erlang's port drivers handle the buffering and delivery of messages when standard error is redirected. Without stderr_to_stdout, the standard error and standard output streams are treated separately, and the exit status is delivered correctly. With the redirection, the mechanism seems to get blocked, preventing the exit status message from reaching the parent process.

Affected Versions: OTP 27 and OTP 28

This bug has been observed in both OTP 27 and OTP 28, indicating that it is not a recent regression but a persistent issue across multiple Erlang/OTP releases. This makes it essential for developers using these versions to be aware of this behavior and implement appropriate workarounds or mitigation strategies in their applications. The consistency of the bug across versions suggests that the underlying cause is deeply rooted in the port communication mechanism and not a superficial glitch.

Deep Dive into the Root Cause

To truly address this issue, we need to understand its root cause. The problem appears to stem from how Erlang's port drivers manage the flow of data and control messages when stderr_to_stdout is enabled. When standard error is redirected, it merges with the standard output stream. This merging process seems to interfere with the delivery of the exit_status message, especially when child processes spawned by the port are still running.

Potential Explanation

A plausible explanation is that the port driver's internal buffers and message queues are not correctly synchronized when stderr_to_stdout is active. The driver might be waiting for the standard output stream to be fully processed before delivering the exit_status, but if a child process is still writing to standard output (as in the sleep infinity example), the driver remains blocked. This blockage prevents the exit_status message from being sent to the parent process.

Implications for System Design

This behavior has significant implications for system design in Erlang. If you rely on stderr_to_stdout for capturing error output and also need to accurately track the exit status of spawned processes, you might encounter this issue. This can affect the reliability of your system, especially in scenarios where processes are expected to run for extended periods or might not terminate cleanly.

Practical Implications and Mitigation Strategies

Practical Implications

The inability to receive the exit_status can lead to several problems:

• Resource Leaks: If a process fails to terminate properly and the parent process does not receive the exit_status, it might not be able to clean up resources, leading to leaks over time.
• Indefinite Hangs: As demonstrated in the example, the parent process can hang indefinitely, waiting for a message that will never arrive.
• Incorrect Error Handling: Without the correct exit status, error handling routines might not be triggered, leading to undetected failures.

Mitigation Strategies

1. Avoid stderr_to_stdout if Possible: If precise exit status tracking is crucial, consider avoiding the stderr_to_stdout option. Instead, manage standard error and standard output streams separately.
2. Explicit Process Monitoring: Implement explicit monitoring of child processes. This can involve periodically checking the status of the processes or using other forms of inter-process communication to ensure they are still running.
3. Timeout Mechanisms: Introduce timeout mechanisms in the parent process. If an exit_status is not received within a reasonable time, the parent process can take corrective action, such as terminating the child process forcefully.
4. Alternative Error Handling: Implement alternative mechanisms for capturing error output, such as logging standard error to a file or using a dedicated error reporting process.

A Detailed Look at the Code

Let's break down the Erlang code provided earlier to understand how it triggers the bug:

The start/0 Function

This function initializes the process. It defines the path to the Erlang executable (ErlBin) and opens a port to a new Erlang process that runs the start/1 function with the argument `[