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Concurrency: exit of N inside a spawned worker does not stop the program — with the main thread blocked in recv it hangs forever; the doc never says what a worker's exit means

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
c
Domain
compilers, operating-systems

Research direction

Reproduce the probes in ~/src/wt/briefs/conc-review/probes-{A,B,C}/, then inspect builtin_exit at src/builtins.c:768-782 and builtin_recv at :4743-4745. Trace channel cleanup in handle_table_drain at :5282-5299 and review docs/SPEC.md §Concurrency and docs/CONCURRENCY.md. Done means the reproduced hang is resolved or the documented worker-exit semantics are enforced, with coverage for the blocked-recv and busy-loop cases.

Written by the indexing model from the issue text.

Description

area:concurrency bug

Found by the 2026-09-14 whole-runtime concurrency review (three independent reviewers, every finding below reproduced by execution on main @ a18deac unless marked reading). Probe files live in ~/src/wt/briefs/conc-review/probes-{A,B,C}/.

builtin_exit (src/builtins.c:768-782) sets the CALLING thread's flags and the state latch; nothing wakes the main thread blocked in builtin_recv's pthread_cond_wait (:4743-4745). Channels are broadcast-closed only in handle_table_drain (:5282-5299), which runs after main returns — never here.

Observed

c is channel of 1
define w() as:
    usleep of 20000
    exit of 5
h is spawn of w
v is recv of c
print of f"main woke: {v}"

Release: hung, killed at 20 s. Companion (main busy in a loop instead of recv): rc=3 at the end, but main printed its MARK_END AFTER the worker's exit — a worker's exit means "set the status, keep running". docs/SPEC.md §Concurrency and docs/CONCURRENCY.md ("decides its own status") never say this.

Fix direction

Either: a worker's exit closes+broadcasts every channel and sets a state flag every blocking builtin (recv, recv_timeout, thread_join, usleep) checks so main unwinds promptly; or document the real semantics ("exit from a worker records a status; the program ends when main returns") and make recv refuse to block forever when no thread can ever send (all other threads finished).

Dominant language
C
Stars
3
Forks
7
Avg merge
3h 56m
Merged PRs (30d)
102

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