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fix(util): fail heap snapshots that Bun writes empty - #58967

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heap-snapshot-empty
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fix(util): fail heap snapshots that Bun writes empty#58967
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heap-snapshot-empty

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@Hona Hona commented Oct 25, 2026 •
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When the server's heap is large, Capture heaps writes a 0-byte .heapsnapshot and logs heap snapshot written with no error. This change makes HeapSnapshot.write detect the empty file, remove it, and fail with the reason.

  • Cause: a Bun bug. node:v8 writeHeapSnapshot writes the result of Bun.generateHeapSnapshot("v8"). When the V8-format output would exceed about 2 GiB, Bun returns an empty string and does not throw (BunObject.cpp, the "v8" branch skips the null check that the "jsc" branch has). Upstream: Stream V8 heap snapshots to a file so snapshots over 2 GiB can be written (today they silently come back empty) oven-sh/bun#46877.
  • The fix: after the write, stat the file. If it is empty, delete it and fail. The heap-dump endpoint already turns that failure into Failed to write heap snapshot: …, and the desktop debug manifest records it as the server's error entry.
  • Unchanged: snapshots under the limit, and the success response.
flowchart LR
  A[writeHeapSnapshot] --> B{file size}
  B -- "> 0" --> C[log bytes, return path]
  B -- "0" --> D[remove file] --> E["fail: Bun cannot write a V8 heap snapshot over about 2 GiB, and this heap is too large (N MiB used)"]
Loading
Heap in a Bun 1.4.4 process Before After
Small Snapshot written Snapshot written
1.45 GiB used (42M objects) 0-byte file, reported as written No file, clear error
Real server, 5.06 GB used 0-byte file after a 48.7 s pause Same pause, then a clear error
  • Not addressed here: the pause still happens, because the size is only known after Bun serializes the heap. The server's private memory also stays high after a failed attempt.

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Copilot AI balanced review requested due to automatic review settings October 25, 2026 12:07
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The fix looks correct to me, and I found nothing that should block a merge.

This machine can't build a heap large enough to hit the real Bun bug, so I tested by replacing node:v8's writeHeapSnapshot with a version that writes nothing. On the base, HeapSnapshot.write reported success and left a 0-byte .heapsnapshot. With this change it fails and removes the file. The error reaches the heap-dump endpoint's Failed to write heap snapshot: … message through cause.cause.message. A normal snapshot still writes and now logs its size. test/heap-dump.test.ts in packages/server passes. tsc --noEmit in packages/util passes; its typecheck script fails on the base too (Script not found "check").

Public API: unchanged. The heap-dump endpoint now returns its existing error where it used to report a false success.

@Hona Hona mentioned this pull request Oct 25, 2026
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