🐛 fix: prefork children exit immediately in Docker containers by meruiden · Pull Request #4133 · gofiber/fiber

meruiden · 2026-03-07T20:47:17Z

Summary

watchMaster() kills child processes when os.Getppid() == 1, assuming the parent died and the child was reparented to init. In Docker containers, the master process commonly is PID 1, so children immediately see Getppid() == 1 and call os.Exit(1) after the first 500ms tick.

Fix

Store the original parent PID at startup and detect parent death by checking if the parent PID changed, rather than comparing against a hardcoded value of 1.

// Before
for range time.NewTicker(watchInterval).C {
    if os.Getppid() == 1 {
        os.Exit(1)
    }
}

// After
masterPID := os.Getppid()
for range time.NewTicker(watchInterval).C {
    if os.Getppid() != masterPID {
        os.Exit(1)
    }
}

Testing

All existing prefork tests pass
Verified prefork works in Docker containers without needing an external init process (tini)

watchMaster() assumed PID 1 always means the parent died and the child was reparented to init. In Docker containers, the master process itself commonly runs as PID 1, causing all children to exit after the first 500ms health check. Fix by storing the original parent PID at startup and detecting parent death by checking if the parent PID changed, rather than comparing against a hardcoded value of 1. Fixes gofiber#4132 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gemini-code-assist · 2026-03-07T20:47:30Z

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical bug affecting prefork child processes when running in Docker containers. Previously, child processes would prematurely exit due to an incorrect assumption about the master process's PID. The fix introduces a more robust mechanism for detecting master process termination, ensuring stable operation within containerized environments.

Highlights

Docker Compatibility Fix: Addressed an issue where prefork child processes in Docker containers would immediately exit. This occurred because the master process, often PID 1 in Docker, caused os.Getppid() == 1 to incorrectly trigger an exit condition for children.
Improved Parent Process Tracking: Modified the watchMaster() function to store the initial parent PID at startup. It now detects master process death by checking if the parent PID changes from this original value, providing a more robust mechanism than hardcoding a check for PID 1.

Changelog

prefork.go
- Updated the watchMaster function to correctly identify when the master process has exited, especially in containerized environments where the master might be PID 1.
- Changed the logic from checking os.Getppid() == 1 to comparing os.Getppid() against the initial masterPID.

Activity

No human activity has been recorded on this pull request yet.

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coderabbitai · 2026-03-07T20:47:35Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Review profile: CHILL

Plan: Pro

Run ID: 43101173-b561-461a-8852-87a13aede197

📥 Commits

Reviewing files that changed from the base of the PR and between a2e1f82 and f40dfd0.

📒 Files selected for processing (1)

prefork.go

Walkthrough

Replaces the hardcoded PID-1 check with dynamic parent tracking: the child captures the master's PID at startup and uses it in watchMaster(masterPID int) to detect parent death by comparing subsequent Getppid() values against the stored master PID (Windows code path adjusted to use provided PID).

Changes

Cohort / File(s)	Summary
Prefork parent tracking `prefork.go`	Reworked master-death detection: child captures master's PID and calls `watchMaster(masterPID int)`; `watchMaster` now compares `os.Getppid()` against the stored `masterPID` (Windows path updated to use `masterPID`). Removed hardcoded `ppid == 1` check.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 I stored the parent's number tight,
No longer fooled by PID-one's might,
Children watch the PID they knew,
And only exit when it’s true.
A tiny hop, a safer night.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the bug fix (prefork children exiting immediately) and specifies the context (Docker containers), accurately summarizing the main change.
Description check	✅ Passed	The PR description provides a clear problem statement, specific before/after code comparison, and testing verification. However, it does not follow the repository's required template structure with checklist items.
Linked Issues check	✅ Passed	The code changes implement the exact fix suggested in issue `#4132`: capturing the original parent PID at startup and detecting parent death by PID change rather than comparing against hardcoded 1.
Out of Scope Changes check	✅ Passed	All changes in prefork.go are directly related to the parent PID detection mechanism described in issue `#4132`, with no extraneous modifications.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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gemini-code-assist

Code Review

This pull request fixes a bug where child processes in a prefork setup would exit immediately when running inside a Docker container. The fix correctly changes the logic to check if the parent PID has changed, instead of checking if it is 1. While the core logic of the fix is correct for its intended purpose, I've identified a race condition where the child process may fail to detect the master's death if the master exits at a specific moment during child process initialization. I've left a comment with a suggestion to make the implementation more robust by eliminating this race condition.

Copilot

Pull request overview

Fixes prefork child processes exiting immediately when running inside Docker containers where the master process is commonly PID 1, by changing the master-liveness check in watchMaster() to detect PPID changes instead of comparing against a hardcoded PID value.

Changes:

Update watchMaster() to store the initial parent PID and exit only when the PPID changes.
Expand inline documentation to explain the reparenting-based detection approach and the Docker PID 1 scenario.

- Capture masterPID in caller before spawning goroutine to eliminate race condition window between goroutine scheduling and Getppid() call - Use masterPID for Windows FindProcess path too, for consistency - Update comment to mention subreapers, not just init/PID 1 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

codecov · 2026-03-07T21:05:24Z

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.05%. Comparing base (c493e59) to head (f40dfd0).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
prefork.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4133      +/-   ##
==========================================
- Coverage   91.10%   91.05%   -0.06%     
==========================================
  Files         119      119              
  Lines       11385    11386       +1     
==========================================
- Hits        10372    10367       -5     
- Misses        642      647       +5     
- Partials      371      372       +1

Flag	Coverage Δ
unittests	`91.05% <60.00%> (-0.06%)`	⬇️

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Copilot

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

welcome · 2026-03-15T14:07:56Z

Congrats on merging your first pull request! 🎉 We here at Fiber are proud of you! If you need help or want to chat with us, join us on Discord https://gofiber.io/discord

ReneWerner87 · 2026-03-15T14:08:46Z

@meruiden How does the fasthttp prefork work? Is there the same issue there? We’re planning to switch directly to this version soon.

meruiden · 2026-03-15T14:33:05Z

@ReneWerner87 I checked fasthttp's prefork — it doesn't have the same issue, but it has the inverse problem.

fasthttp's prefork.go has no watchMaster() at all. Children never poll os.Getppid(), so they won't falsely exit in Docker (the bug this PR fixes). However, that means if the master process dies unexpectedly, children become orphans — they get reparented to PID 1 and keep running silently with no supervision.

I've opened a PR to fix it: valyala/fasthttp#2158

It uses the same pattern as this PR — store the initial PPID at startup and exit when it changes.

meruiden requested a review from a team as a code owner March 7, 2026 20:47

meruiden requested review from ReneWerner87, Copilot, efectn, gaby and sixcolors March 7, 2026 20:47

ReneWerner87 added v3 ☢️ Bug labels Mar 7, 2026

ReneWerner87 added this to v3 Mar 7, 2026

ReneWerner87 added this to the v3 milestone Mar 7, 2026

Copilot started reviewing on behalf of meruiden March 7, 2026 20:47 View session

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gaby requested a review from Copilot March 11, 2026 00:58

Copilot started reviewing on behalf of gaby March 11, 2026 00:59 View session

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gaby approved these changes Mar 11, 2026

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ReneWerner87 approved these changes Mar 15, 2026

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ReneWerner87 merged commit e6353ef into gofiber:main Mar 15, 2026
23 of 24 checks passed

github-project-automation Bot moved this to Done in v3 Mar 15, 2026

meruiden mentioned this pull request Mar 15, 2026

fix: detect master process death in prefork children valyala/fasthttp#2158

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ReneWerner87 removed this from the v3 milestone Apr 25, 2026

ReneWerner87 added this to the v3.2.0 milestone Apr 25, 2026

coderabbitai Bot mentioned this pull request May 8, 2026

🔥 feat(prefork): delegate implementation to fasthttp/prefork #4210

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🐛 fix: prefork children exit immediately in Docker containers#4133

🐛 fix: prefork children exit immediately in Docker containers#4133
ReneWerner87 merged 2 commits into
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meruiden:fix/prefork-pid1-detection

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