[syrusakbary]

Summary

A V8 foreground task can post its successor while it runs (incremental marking, minor GC jobs). EdgeV8Platform::PumpPendingForegroundTasks drained the fallback queue until it was empty, so such a chain could keep one event-loop checkpoint busy, stalling the guest's event loop and streamed responses during GC. This ports the fix from #101 onto main: each checkpoint runs at most one ready fallback task, and reports when it ran one so the runtime takes another turn instead of waiting.

The fallback queue is the one used when no enqueue hook is bound. That is every V8 foreground task under the WASIX bridge, which attaches empty hooks. Native EdgeJS binds the hook, so it rarely reaches this path.

#101 was part 9 of the split of #84 and was closed with its stack. The production change is unchanged from #101 (author Christoph Herzog, kept as the first commit). In #85, the same change cut p95 for a streamed Astro dashboard on Edge from 1,109 ms to 84 ms.

Changes

• v8/src/edge_v8_platform.cc, .h: PumpPendingForegroundTasks runs one task (return v8::platform::PumpMessageLoop(...) instead of the while loop) and returns whether it ran one.
• v8/src/unofficial_napi.cc: when a task ran, unofficial_napi_event_loop_checkpoint reports host_tasks_admitted | pending_provider_work. EdgeJS reads host_tasks_admitted as "don't wait on libuv this turn". Without it, EdgeJS stops with "Provider reported pending work without admitting a host task" once its uv loop is idle.
• include/unofficial_napi.h: documents what the admitted bit means for a synchronous provider (new in this port).
• tests/src/v8_foreground_checkpoint_test.cc: a standalone process posts a four-task self-reposting chain and checks that each pump advances it by one. The test called EdgeV8Platform::Create(true), a signature from fix(v8): bound foreground work at each JavaScript checkpoint #101's stack; it now calls Create() (new in this port).

Testing

• make test-napi: 98/98 pass. test_35_promise and foreground_checkpoint_fairness pass 50 repeats.
• The fairness test fails with the old while loop (the first pump runs all four tasks) and passes with this change.
• EdgeJS main (18b0ce9e) built against napi main, with and without this change and Fix use-after-free when handing V8 foreground tasks to the enqueue hook #113 together, on macOS arm64:

EdgeJS check	napi main	with this change and #113
Native make test-only	1749/1749	1749/1749
WASIX make test-wasix-v8-only	1676/1676	1675/1676: one TLS test, which then passed 24/24 on both
test-intl, test-lang, test-wasix-v8-process	pass	pass
Astro SSR framework app (V8 WASIX)	pass	pass
GC-pressure streaming probe (V8 WASIX), 4 runs	3.9–4.1 s	3.5–3.7 s; FinalizationRegistry callbacks fire at the same rate

Notes for reviewers

• The admitted bit now also means "ran a provider task" for the V8 provider, in either checkpoint mode. The only EdgeJS code that reads it is its event loop, which uses it to skip the blocking libuv wait.
• test_35's ProviderEventLoopCheckpointProcessesMicrotasks expects no admitted bit from V8. Test fixtures bind no enqueue hook, so a V8 task that happens to be ready at that checkpoint would set it. It didn't flake in 50 repeats.
• Independent of Fix use-after-free when handing V8 foreground tasks to the enqueue hook #113, which changes the enqueue-hook path in PostTaskCommon. Either can land first.
• feat(napi): consolidate managed Edge V8 and metered WebAssembly #84 contains the same production change. If feat(napi): consolidate managed Edge V8 and metered WebAssembly #84 lands later, this part will already be on main.

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