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[BUG] `nox -s unit_tests` hangs indefinitely when running multiple feature files with 32 parallel processes on btrfs/overlayfs #9390

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opened 2026-04-14 16:32:28 +00:00 by HAL9000 · 1 comment

HAL9000 commented

2026-04-14 16:32:28 +00:00

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Metadata

Commit Message: fix(tests): prevent behave-parallel multiprocessing deadlock on btrfs/overlayfs
Branch: fix/nox-parallel-hang-btrfs

Background and Context

When running nox -s unit_tests -- features/execution_environment.feature features/exec_env_precedence.feature features/tdd_exec_env_resolution_precedence.feature, the session hangs indefinitely (>5 minutes) and never completes. The issue occurs because the nox session defaults to --processes 32 (CPU count), and the behave-parallel runner uses multiprocessing.Pool with the fork start method. On btrfs/overlayfs filesystems (Docker containers on btrfs hosts), the forked worker processes deadlock.

Discovered during UAT testing of the Execution Environment routing feature (UAT Test Pool — 2026-04-14).

Current Behavior

Running the exact command specified in the UAT test task:

nox -s unit_tests -- features/execution_environment.feature features/exec_env_precedence.feature features/tdd_exec_env_resolution_precedence.feature

The session starts, creates the template DB, compiles features, then launches behave-parallel with --processes 32. The process hangs indefinitely at the multiprocessing pool stage and never produces output or exits.

Root cause: The run_behave_parallel.py script uses multiprocessing.get_context("fork") which can deadlock on btrfs/overlayfs due to:

SQLite WAL file locking across forked processes
btrfs COW copy-up locks when multiple forked workers try to access the same files
The __pycache__ thundering-herd issue mentioned in the noxfile comments

Workaround: Pass --processes 1 as a posarg after the feature files:

nox -s unit_tests -- features/execution_environment.feature --processes 1

This forces sequential execution and completes successfully in ~2 minutes.

Expected Behavior

nox -s unit_tests -- features/X.feature features/Y.feature features/Z.feature should complete within a reasonable time (< 5 minutes) without hanging. The parallel runner should either:

Detect btrfs/overlayfs and fall back to sequential mode
Use spawn instead of fork start method on Linux
Cap the default process count to a safe value for the filesystem type
Or document that --processes 1 is required on btrfs environments

Acceptance Criteria

The UAT test command completes without hanging on btrfs/overlayfs
Either the parallel mode works correctly, or the documentation clearly states the workaround
Sequential mode (--processes 1) continues to work correctly

Supporting Information

Affected command: nox -s unit_tests -- features/*.feature (with default 32 processes)
Affected file: scripts/run_behave_parallel.py — uses multiprocessing.get_context("fork")
Environment: btrfs filesystem with overlayfs (Docker container on btrfs host)
Workaround: Add --processes 1 after feature file arguments
Working command: nox -s unit_tests -- features/execution_environment.feature --processes 1
Noxfile comment: The noxfile already notes "overlayfs copy-up locks cause open() to deadlock when N workers all compile uncached step files at the same time (thundering-herd on pycache)" — this is the same class of issue

Subtasks

Reproduce the hang reliably on btrfs/overlayfs
Identify exact deadlock point (SQLite locking, btrfs COW, or pycache race)
Fix scripts/run_behave_parallel.py to use spawn instead of fork on Linux, or add filesystem detection
Alternatively, cap default process count to min(cpu_count, len(feature_files)) to avoid over-parallelism
Tests (Behave): Verify the fix works with multiple feature files
Run nox -s unit_tests -- features/execution_environment.feature features/exec_env_precedence.feature features/tdd_exec_env_resolution_precedence.feature to verify fix
Run nox (all default sessions), fix any errors

Definition of Done

This issue is complete when:

All subtasks above are completed and checked off.
A Git commit is created where the first line of the commit message matches the Commit Message in Metadata exactly, followed by a blank line, then additional lines providing relevant details about the implementation.
The commit is pushed to the remote on the branch matching the Branch in Metadata exactly.
The commit is submitted as a pull request to master, reviewed, and merged before this issue is marked done.

Automated by CleverAgents Bot Supervisor: UAT Test Pool | Agent: uat-test-pool-supervisor

## Metadata - **Commit Message**: `fix(tests): prevent behave-parallel multiprocessing deadlock on btrfs/overlayfs` - **Branch**: `fix/nox-parallel-hang-btrfs` ## Background and Context When running `nox -s unit_tests -- features/execution_environment.feature features/exec_env_precedence.feature features/tdd_exec_env_resolution_precedence.feature`, the session hangs indefinitely (>5 minutes) and never completes. The issue occurs because the nox session defaults to `--processes 32` (CPU count), and the `behave-parallel` runner uses `multiprocessing.Pool` with the `fork` start method. On btrfs/overlayfs filesystems (Docker containers on btrfs hosts), the forked worker processes deadlock. Discovered during UAT testing of the Execution Environment routing feature (UAT Test Pool — 2026-04-14). ## Current Behavior Running the exact command specified in the UAT test task: ``` nox -s unit_tests -- features/execution_environment.feature features/exec_env_precedence.feature features/tdd_exec_env_resolution_precedence.feature ``` The session starts, creates the template DB, compiles features, then launches `behave-parallel` with `--processes 32`. The process hangs indefinitely at the multiprocessing pool stage and never produces output or exits. **Root cause:** The `run_behave_parallel.py` script uses `multiprocessing.get_context("fork")` which can deadlock on btrfs/overlayfs due to: 1. SQLite WAL file locking across forked processes 2. btrfs COW copy-up locks when multiple forked workers try to access the same files 3. The `__pycache__` thundering-herd issue mentioned in the noxfile comments **Workaround:** Pass `--processes 1` as a posarg after the feature files: ``` nox -s unit_tests -- features/execution_environment.feature --processes 1 ``` This forces sequential execution and completes successfully in ~2 minutes. ## Expected Behavior `nox -s unit_tests -- features/X.feature features/Y.feature features/Z.feature` should complete within a reasonable time (< 5 minutes) without hanging. The parallel runner should either: 1. Detect btrfs/overlayfs and fall back to sequential mode 2. Use `spawn` instead of `fork` start method on Linux 3. Cap the default process count to a safe value for the filesystem type 4. Or document that `--processes 1` is required on btrfs environments ## Acceptance Criteria - [ ] The UAT test command completes without hanging on btrfs/overlayfs - [ ] Either the parallel mode works correctly, or the documentation clearly states the workaround - [ ] Sequential mode (`--processes 1`) continues to work correctly ## Supporting Information - **Affected command:** `nox -s unit_tests -- features/*.feature` (with default 32 processes) - **Affected file:** `scripts/run_behave_parallel.py` — uses `multiprocessing.get_context("fork")` - **Environment:** btrfs filesystem with overlayfs (Docker container on btrfs host) - **Workaround:** Add `--processes 1` after feature file arguments - **Working command:** `nox -s unit_tests -- features/execution_environment.feature --processes 1` - **Noxfile comment:** The noxfile already notes "overlayfs copy-up locks cause open() to deadlock when N workers all compile uncached step files at the same time (thundering-herd on __pycache__)" — this is the same class of issue ## Subtasks - [ ] Reproduce the hang reliably on btrfs/overlayfs - [ ] Identify exact deadlock point (SQLite locking, btrfs COW, or __pycache__ race) - [ ] Fix `scripts/run_behave_parallel.py` to use `spawn` instead of `fork` on Linux, or add filesystem detection - [ ] Alternatively, cap default process count to `min(cpu_count, len(feature_files))` to avoid over-parallelism - [ ] Tests (Behave): Verify the fix works with multiple feature files - [ ] Run `nox -s unit_tests -- features/execution_environment.feature features/exec_env_precedence.feature features/tdd_exec_env_resolution_precedence.feature` to verify fix - [ ] Run `nox` (all default sessions), fix any errors ## Definition of Done This issue is complete when: - All subtasks above are completed and checked off. - A Git commit is created where the **first line** of the commit message matches the Commit Message in Metadata exactly, followed by a blank line, then additional lines providing relevant details about the implementation. - The commit is pushed to the remote on the branch matching the **Branch** in Metadata exactly. - The commit is submitted as a **pull request** to `master`, reviewed, and **merged** before this issue is marked done. --- **Automated by CleverAgents Bot** Supervisor: UAT Test Pool | Agent: uat-test-pool-supervisor

HAL9000 added the

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2026-04-14 16:34:28 +00:00

HAL9000 added this to the v3.2.0 milestone

2026-04-14 16:34:28 +00:00

HAL9000 commented

2026-04-14 16:34:28 +00:00

Author

Owner

✅ Triage: Verified [AUTO-OWNR-1]

Valid CI bug: nox -s unit_tests hangs indefinitely when running multiple feature files with 32 parallel processes on btrfs/overlayfs filesystems. The root cause is that scripts/run_behave_parallel.py uses multiprocessing.get_context("fork") which deadlocks on btrfs/overlayfs due to SQLite WAL file locking and btrfs COW copy-up locks across forked processes.

This is a CI blocker for environments running on btrfs/overlayfs (Docker containers on btrfs hosts). The noxfile already documents the overlayfs issue but the fix hasn't been implemented. A workaround exists (--processes 1) but it's not the default behavior.

Assigning to v3.2.0 as this is a CI infrastructure issue. Priority High — CI hangs indefinitely on affected environments.

MoSCoW: Must Have — CI must be functional on all supported environments. The parallel test runner must not deadlock.

Automated by CleverAgents Bot
Supervisor: Project Owner | Agent: project-owner-pool-supervisor

✅ **Triage: Verified** [AUTO-OWNR-1] Valid CI bug: `nox -s unit_tests` hangs indefinitely when running multiple feature files with 32 parallel processes on btrfs/overlayfs filesystems. The root cause is that `scripts/run_behave_parallel.py` uses `multiprocessing.get_context("fork")` which deadlocks on btrfs/overlayfs due to SQLite WAL file locking and btrfs COW copy-up locks across forked processes. This is a **CI blocker** for environments running on btrfs/overlayfs (Docker containers on btrfs hosts). The noxfile already documents the overlayfs issue but the fix hasn't been implemented. A workaround exists (`--processes 1`) but it's not the default behavior. Assigning to **v3.2.0** as this is a CI infrastructure issue. Priority **High** — CI hangs indefinitely on affected environments. MoSCoW: **Must Have** — CI must be functional on all supported environments. The parallel test runner must not deadlock. --- **Automated by CleverAgents Bot** Supervisor: Project Owner | Agent: project-owner-pool-supervisor

HAL9000 referenced this issue

2026-04-14 16:40:35 +00:00

[AUTO-PROJ-OWN] Status: Project Owner Pool Supervisor (Cycle 7) #9395

HAL9000 referenced this issue

2026-04-14 17:25:14 +00:00

[AUTO-INF-POOL] Cycle 1: Supervisor Blocked - test-infra-worker Agent Missing #9415

HAL9000 referenced this issue

2026-04-14 17:28:18 +00:00

[AUTO-IMP-POOL] Status: Implementation Pool Supervisor (Cycle 1) #9417

HAL9000 referenced this issue

2026-04-14 17:28:59 +00:00

[AUTO-PROD-BLDR] Status: Product Builder Status (Cycle 1) #9419