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[MG_Remote] (P6/dl): D5c - a bell that can wake itself cannot hear a death

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Measured while running the 78-entry spawn matrix, not predicted.

minecraft-1.21.4-fabric-iris-iterationrp-in-world kills the SERVER process
outright on this machine - llvmpipe cannot lower the shader pack's
%llvm.x86.vcvtps2ph.256 and aborts at call 159429 - and the client did NOT
take the SessionWait::ShutDown path, which already exists and is already
correct. It sat out the full 120 s and reported Fatal{BarrierTimeout,
"ResourceRespecify"}: the wrong diagnosis, at two minutes a case, for a
server that had been gone since the first second. The same trace fails in 3 s
under monolith.

The cause is structural and is the price of `sm`'s two-fd bell. The client
parks on clientBell[0] and rings ITSELF through clientBell[1], so it holds
both ends of that socketpair. EOF arrives only when every writer closes and
the client is one of them, so that descriptor can never hang up no matter
what happens to the peer.

`dl` therefore needs a witness descriptor whose far end only the server
holds, polled beside the park fd and never read from, whose POLLHUP/EOF
latches Dead(). The control socket already is exactly that, so no new
descriptor is needed.

This is NOT a defect in `sm`: holding both ends is what self-notification
requires, and without it the server's control pump could not wake its own
apply thread - the bug that cost this branch an afternoon. The mistake would
be assuming one descriptor can answer both questions.

Recorded rather than fixed: it turns no red case green, and the only case
that reaches it fails identically in all three arms for a reason that is not
MobileGL's. `dl` owns the fix and S2 already names the red-once for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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swung0x48
2026-09-21 12:42:03 -04:00
co-authored by Claude Opus 5
parent 1039502ee6
commit f6a0bd3024
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MobileGL/MG_Remote/CONTRACT-P6.md
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@@ -313,6 +313,26 @@ is bounded by `MOBILEGL_IPC_CONTROL_TIMEOUT_MS` (default 5000) and its expiry is
client consults the doorbell's death latch — dead latches device-lost, alive-but-silent is a named
diagnostic. Packages: `cp` (a), `dl` (b).
> **D5c — the client's self-bell CANNOT witness the server's death, and `sm` is why.** Measured, not
> predicted: `minecraft-1.21.4-fabric-iris-iterationrp-in-world` kills the *server* process outright
> on this machine (llvmpipe: `LLVM ERROR: Cannot select: intrinsic %llvm.x86.vcvtps2ph.256`, SIGABRT
> at call 159429), and the client did **not** take the `SessionWait::ShutDown` path that already
> exists and is already correct. It sat out the full 120 s and reported
> `Fatal{BarrierTimeout, "ResourceRespecify"}` — the wrong diagnosis, at 120 s a case, for a server
> that had been gone since the first second. Under monolith the same trace fails in 3 s.
>
> The cause is structural and is the price of the two-fd bell: the client parks on `clientBell[0]`
> (slot 5) and rings *itself* through `clientBell[1]` (slot 6), so it holds **both ends** of that
> socketpair. EOF arrives only when every writer closes, and the client is one of them — so that
> descriptor can never hang up, no matter what happens to the peer. A bell that can wake itself is
> a bell that cannot hear a death.
>
> **`dl` therefore needs a witness descriptor whose far end only the SERVER holds**, polled beside
> the park fd and never read from, whose `POLLHUP`/EOF latches `Dead()`. The control socket is
> already exactly that and needs no new descriptor. The fix does not belong to `sm`: holding both
> ends is required for self-notification and is not the defect — assuming one descriptor could
> answer both questions is.
---
## §6 The control plane (`cp`)