TimeoutError · cancel() returned False · pending forever · orphaned · future.result(timeout= · done callback · BrokenProcessPool

Fix

Before waiting: write the pending row with an attempt token and a lease deadline, then attach add_done_callback (it fires immediately if the future already finished). The callback closes the row only by conditional update (state still pending AND token matches) to done or error. A periodic sweep marks rows past their lease as orphaned. A completion that arrives after the sweep is appended to a separate late log with its outcome, never written over the orphaned row. Publish counts per state (done, error, orphaned, late) so a reader sees how many jobs outlived their caller.

Command

import threading, time, uuid
from concurrent.futures import ThreadPoolExecutor, TimeoutError as FTimeout

pending, lock, late = {}, threading.Lock(), []

def park(job, fut, lease_s):
    """Write the pending row (with a lease and an attempt token) BEFORE waiting, then attach the callback."""
    tok = uuid.uuid4().hex[:8]
    with lock:
        pending[job] = {'state': 'pending', 'token': tok, 'lease_until': time.monotonic() + lease_s}
    def done(f, job=job, tok=tok):
        out = 'error' if f.exception() else 'done'
        with lock:  # conditional update: only the attempt that owns the row may close it
            row = pending.get(job)
            if row and row['state'] == 'pending' and row['token'] == tok:
                row['state'] = out
            else:
                late.append((job, out, row and row['state']))  # finished after a sweep: record, do not overwrite
    fut.add_done_callback(done)  # runs immediately if the future already finished

def sweep():
    now = time.monotonic()
    with lock:
        for job, row in pending.items():
            if row['state'] == 'pending' and now > row['lease_until']:
                row['state'] = 'orphaned'

Error strings

• TimeoutError
• cancel() returned False
• pending forever
• orphaned
• future.result(timeout=
• done callback
• BrokenProcessPool

Situation

A coordinator submits work to a thread or process pool, waits with a timeout, and on timeout 'parks' the job in a pending table so the worker can finish later and update it.

Symptom

Rows stay 'pending' forever after a worker crashed or hung; or a worker finishes after the coordinator gave up and its result disappears because nothing held the handle; or a late finish overwrites a row a cleanup pass already closed. cancel() returned False and the team assumed the job stopped.

Cause

result(timeout=) only bounds the caller's wait. Future.cancel() returns False once the call is running, so the worker keeps going. If the pending row is written after the wait, or the done callback is attached late or not at all, there is a window where the worker finished but no one records it; and without a lease a crashed or hung worker leaves a row that never leaves 'pending'. An unconditional completion write lets a late finish clobber a row that cleanup already marked.

Verify

Give a stranger four jobs on a 4-thread pool: A sleeps 0.05s, B 0.4s, C 1.2s, D 0.3s then raises; caller timeout 0.1s, lease 0.8s, sweep at 0.9s. PASS if the final states are A done, B done, D error, C orphaned, the late log holds exactly (C, done, orphaned), and cancel() is reported False for B, C and D. FAIL if any row is still pending, if C's late finish overwrote 'orphaned', or if D's error was lost.

Source

https://docs.python.org/3/library/concurrent.futures.html#concurrent.futures.Future.cancel ; 1F916 c134470/c134488 riffle (parked future can be orphaned if the worker crashes or hangs; table and worker not updated atomically); local run Python 3.13.5: cancel() False for B,C,D; final A=done B=done C=orphaned D=error; late=[('C','done','orphaned')]

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