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heap-snapshot: keep node ids stable across v8 snapshots #34085

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heap-snapshot: keep node ids stable across v8 snapshots #34085

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1 change: 0 additions & 1 deletion src/jsc/bindings/BunHeapProfiler.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -940,7 +940,6 @@ WTF::String generateHeapSnapshotV8(JSC::VM& vm)
{
vm.ensureHeapProfiler();
auto& heapProfiler = *vm.heapProfiler();
heapProfiler.clearSnapshots();

JSC::BunV8HeapSnapshotBuilder builder(heapProfiler);
return builder.json();
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1 change: 0 additions & 1 deletion src/jsc/bindings/BunObject.cpp
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Expand Up @@ -776,9 +776,8 @@
JSC_DEFINE_HOST_FUNCTION(functionGenerateHeapSnapshot, (JSC::JSGlobalObject * globalObject, JSC::CallFrame* callFrame))
{
auto& vm = JSC::getVM(globalObject);
vm.ensureHeapProfiler();
auto& heapProfiler = *vm.heapProfiler();

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Claude / Claude Code Review

HeapProfiler snapshot chain grows unboundedly after removing clearSnapshots() 

Removing `clearSnapshots()` means `HeapProfiler::m_snapshots` now grows by one entry per snapshot for the life of the VM, and every subsequent GC's `removeDeadHeapSnapshotNodes` (plus `sweepCell`/`shrinkToFit`/`nodeForCell`) recurses the full `m_previous` chain — so per-GC overhead and container memory scale O(N) with snapshots taken. The PR description's "delta storage bounds growth" only covers node storage, not the chain itself. Retaining the chain is required for id stability so this shouldn
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heapProfiler.clearSnapshots();

Bun__Feature__heap_snapshot += 1;

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1 change: 0 additions & 1 deletion src/jsc/bindings/webcore/JSWorker.cpp
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Expand Up @@ -741,7 +741,6 @@ static inline JSC::EncodedJSValue jsWorkerPrototypeFunction_getHeapSnapshotBody(
auto& vm = workerCtx.vm();
vm.ensureHeapProfiler();
auto& heapProfiler = *vm.heapProfiler();
heapProfiler.clearSnapshots();
JSC::BunV8HeapSnapshotBuilder builder(heapProfiler);
String snapshot = builder.json();

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45 changes: 45 additions & 0 deletions test/js/bun/util/v8-heap-snapshot-fixture.ts
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5 changes: 5 additions & 0 deletions test/js/bun/util/v8-heap-snapshot.test.ts
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Expand Up @@ -89,6 +89,11 @@ test("v8.writeHeapSnapshot() with path", async () => {
expect(await runFixture("write-path", { args: [path] })).toEqual({ returnedPath: path, ...structure });
});

test("v8 heap snapshot node ids are stable across snapshots", async () => {
const { id1, id2, found, stable } = await runFixture("id-stability");
expect({ found, stable, id1, id2 }).toEqual({ found: true, stable: true, id1, id2: id1 });
});

test("v8 heap snapshot labels Web Streams internal edges", async () => {
const edges = await runFixture("stream-edges");
// Every WriteBarrier member reported by analyzeHeap shows up as a named
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