OPEN RECORD · 5 MEASURED CHECKS

Popen PIPE waiting vs bounded communicate and timeout cleanup

Waiting for a child while both PIPE streams remain unread conflicts with completion for this finite-output fixture. The same child completes after communicate drains its output.

When this evidence is relevant

Investigating subprocess PIPE, Popen wait, communicate timeout: compare the exact failure below with your public error, then check the measured scope.

Observed failure

finite_child_completes_during_selected_wait_strategy

Drain stdout and stderr with communicate. On TimeoutExpired, explicitly kill the direct child and communicate again to drain and reap. Use only bounded output because communicate buffers in memory.

Own synthetic Python child only, shell=False, no network, no descendants. 256 KiB per finite stream, a .25-second observed wait bound, and a separate partial-output sleeper with a 3-second self-exit bound. Same blocked child is drained to completion; timeout alone is not a general deadlock diagnosis. Direct-child cleanup only; no process-tree or unlimited-output claim. Every child has a six-second watchdog exit as an additional bound.

Measured 2026-09-12T13:41:04.198177+00:00 · Linux / x86_64. Historical package versions are not current production recommendations.

Primary references

The complete record and all reproduction files are open. No response is required.

Complete record

  1. finite_child_completes_during_selected_wait_strategy
  2. stdout_drained_exactly
  3. stderr_drained_exactly
  4. timeout_child_killed_and_reaped_before_return
  5. partial_stdout_preserved_after_timeout
Full execution record and environments
{
  "schema_version": 2,
  "test_id": "subprocess-pipe-drain-v1",
  "created_at": "2026-09-12T13:41:04.198177+00:00",
  "environment": {
    "libc": [
      "glibc",
      "2.39"
    ],
    "machine": "x86_64",
    "packages": {},
    "python_abi": "cpython-312-x86_64-linux-gnu",
    "python_implementation": "CPython",
    "python_version": "3.12.14",
    "system": "Linux"
  },
  "fixed_environment": {
    "libc": [
      "glibc",
      "2.39"
    ],
    "machine": "x86_64",
    "packages": {},
    "python_abi": "cpython-312-x86_64-linux-gnu",
    "python_implementation": "CPython",
    "python_version": "3.12.14",
    "system": "Linux"
  },
  "status": "passed",
  "failure": {
    "checks": [
      "stdout_drained_exactly",
      "stderr_drained_exactly",
      "timeout_child_killed_and_reaped_before_return",
      "partial_stdout_preserved_after_timeout"
    ],
    "error": "finite_child_completes_during_selected_wait_strategy",
    "failed_checks": [
      "finite_child_completes_during_selected_wait_strategy"
    ],
    "mode": "broken",
    "observations": {
      "child_self_exit_bound_seconds": 3,
      "finite_output": {
        "alive_at_timeout": false,
        "guard_child_reaped": true,
        "reaped_before_guard": true,
        "returncode": 0,
        "stderr_bytes": 262144,
        "stderr_sha256": "caf546901c2a1966d4530a5a6d0323691f474ee2d6b44286799c440458c7b77f",
        "stdout_bytes": 262144,
        "stdout_sha256": "4480ce8e19e155ff39079e3379eafd8f369c739e24931ddc684994c2cd12ca91",
        "timed_out": true
      },
      "output_limit_bytes": 524288,
      "timeout_child": {
        "alive_at_timeout": true,
        "guard_child_reaped": true,
        "reaped_before_guard": true,
        "returncode": -9,
        "stderr_bytes": 0,
        "stderr_sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
        "stdout_bytes": 11,
        "stdout_sha256": "3e5fda43a0585f02a96544ed0cbdf49513d15f25ed2f4370b9fb8f843ddd9317",
        "timed_out": true
      }
    },
    "status": "check_failed",
    "exit_code": 2
  },
  "success": {
    "checks": [
      "finite_child_completes_during_selected_wait_strategy",
      "stdout_drained_exactly",
      "stderr_drained_exactly",
      "timeout_child_killed_and_reaped_before_return",
      "partial_stdout_preserved_after_timeout"
    ],
    "error": "",
    "failed_checks": [],
    "mode": "fixed",
    "observations": {
      "child_self_exit_bound_seconds": 3,
      "finite_output": {
        "alive_at_timeout": false,
        "guard_child_reaped": true,
        "reaped_before_guard": true,
        "returncode": 0,
        "stderr_bytes": 262144,
        "stderr_sha256": "caf546901c2a1966d4530a5a6d0323691f474ee2d6b44286799c440458c7b77f",
        "stdout_bytes": 262144,
        "stdout_sha256": "4480ce8e19e155ff39079e3379eafd8f369c739e24931ddc684994c2cd12ca91",
        "timed_out": false
      },
      "output_limit_bytes": 524288,
      "timeout_child": {
        "alive_at_timeout": true,
        "guard_child_reaped": true,
        "reaped_before_guard": true,
        "returncode": -9,
        "stderr_bytes": 0,
        "stderr_sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
        "stdout_bytes": 11,
        "stdout_sha256": "3e5fda43a0585f02a96544ed0cbdf49513d15f25ed2f4370b9fb8f843ddd9317",
        "timed_out": true
      }
    },
    "status": "passed",
    "exit_code": 0
  },
  "checks": [
    "finite_child_completes_during_selected_wait_strategy",
    "stdout_drained_exactly",
    "stderr_drained_exactly",
    "timeout_child_killed_and_reaped_before_return",
    "partial_stdout_preserved_after_timeout"
  ],
  "negative_controls": {
    "timeout-only": {
      "checks": [
        "finite_child_completes_during_selected_wait_strategy",
        "stdout_drained_exactly",
        "stderr_drained_exactly"
      ],
      "error": "timeout_child_killed_and_reaped_before_return, partial_stdout_preserved_after_timeout",
      "failed_checks": [
        "timeout_child_killed_and_reaped_before_return",
        "partial_stdout_preserved_after_timeout"
      ],
      "mode": "timeout-only",
      "observations": {
        "child_self_exit_bound_seconds": 3,
        "finite_output": {
          "alive_at_timeout": false,
          "guard_child_reaped": true,
          "reaped_before_guard": true,
          "returncode": 0,
          "stderr_bytes": 262144,
          "stderr_sha256": "caf546901c2a1966d4530a5a6d0323691f474ee2d6b44286799c440458c7b77f",
          "stdout_bytes": 262144,
          "stdout_sha256": "4480ce8e19e155ff39079e3379eafd8f369c739e24931ddc684994c2cd12ca91",
          "timed_out": false
        },
        "output_limit_bytes": 524288,
        "timeout_child": {
          "alive_at_timeout": true,
          "guard_child_reaped": true,
          "reaped_before_guard": false,
          "returncode": null,
          "stderr_bytes": 0,
          "stderr_sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
          "stdout_bytes": 0,
          "stdout_sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
          "timed_out": true
        }
      },
      "status": "check_failed",
      "exit_code": 2
    }
  },
  "test_sha256": "ad0459606e42f61402a605a2412728fbd282775aee43c1666b7b0a692b20aef1",
  "fixture_sha256": "89cde701fe6a7e9d5392fc84cb56b969c60c52b4a0dc63401368a6b14e740655",
  "requirements_fixed": "# No pip dependencies. CPython 3.11+; measured version in record.json.\n",
  "scope": "Own synthetic Python child only, shell=False, no network, no descendants. 256 KiB per finite stream, a .25-second observed wait bound, and a separate partial-output sleeper with a 3-second self-exit bound. Same blocked child is drained to completion; timeout alone is not a general deadlock diagnosis. Direct-child cleanup only; no process-tree or unlimited-output claim. Every child has a six-second watchdog exit as an additional bound.",
  "sources": [
    {
      "title": "Python 3.12 official behavior reference",
      "url": "https://docs.python.org/3.12/library/subprocess.html"
    }
  ],
  "artifact_sha256": {
    "README.md": "5991288edbe50c9b27c1b3a0ebe1a04740be913d041eb30c0f2b5f7cdde00e7d",
    "child.py": "04f9de8bc287a99d1ecf0af3126f6bfc85f1dfb0377ff92b537ca23a47043597",
    "fixture.json": "89cde701fe6a7e9d5392fc84cb56b969c60c52b4a0dc63401368a6b14e740655",
    "pipeline.py": "ad0459606e42f61402a605a2412728fbd282775aee43c1666b7b0a692b20aef1",
    "probe.py": "688abd4f4f6ad33173f2c839ca331ff621bb30a0067f74dee62e76c48ce83698",
    "requirements-fixed.txt": "c4f1f77998ac5b6df8b96effb120595ce438ddd2b1e25e18943c2e30e2e3c20b",
    "runs/broken.stderr.log": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
    "runs/broken.stdout.log": "e8580f959409112928caa44eac49c8ce986b290914334712d5dbe8706a400d80",
    "runs/fixed.stderr.log": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
    "runs/fixed.stdout.log": "d05577ca8aa4f94269302a057a539e0f7f79a6dc38f37e61bbcc50319470caa4",
    "runs/probe.stderr.log": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
    "runs/probe.stdout.log": "c8557c489211f2c1cdee5f9e06105a01c052552ac0bafe3ca8c62b0ce1d7b5c4",
    "runs/timeout-only.stderr.log": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
    "runs/timeout-only.stdout.log": "c0faff6bd7c5b0212316946ae91742ce02c5d116506437cda69a07cbf095bc33",
    "verification-spec.json": "b08bcbe286378dae22d064f88b263fd667b85f735a9356fef6b3462cc48e414c",
    "verify.py": "e3ed969edb6fd4a5cebff0a8af69b948e85c5df52170c54da00054a47aeada7a"
  },
  "verification": "Actual isolated local subprocess execution; metadata/hash checks excluded from primary workload count.",
  "record_id": "295a40dc1c4d26c38af8f2ec8ea97d69e09f61e92a5da0d53842c1d9e3335172"
}
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