io(windows): keep the buffer of a file read started during chunk delivery

[robobun]

Problem

• On Windows, reading a file through Readable.fromWeb(Bun.file(path).stream()) segfaults: panic(main thread): Segmentation fault at address 0xFFFFFFFFFFFFFFFF, crashing inside mimalloc under WindowsBufferedReader::get_read_buffer_with_stable_memory_address. A debug build asserts earlier: uv_read_cb: buf is not in buffer! (src/io/PipeReader.rs:1836). A 1 MiB file is enough.
• The cause is in WindowsBufferedReader::on_read_chunk (src/io/PipeReader.rs:1161). It takes _buffer, re-enters JS to deliver the chunk, then puts the old buffer back whenever _buffer.is_empty(). During that dispatch, native-readable.ts pauses the reader (push() over the highWaterMark) and the next _read unpauses it. start_reading then issues a new uv_fs_read whose iov points into a freshly reserved _buffer. The restore replaces that Vec and frees the allocation libuv is still writing into.

Fix

• Record whether HAS_INFLIGHT_READ was set before the dispatch. The flag turning on across the dispatch means a nested read started. In that case on_read_chunk keeps the new buffer and leaves the flag set.
• The flag is now only cleared when it was set at entry (the pipe and tty path, where the delivered read owns it). On the file path it was already clear at entry, so behavior without a nested read is unchanged.
• Correct because the invariant is: while a libuv read is in flight, _buffer must stay the allocation the iov points into. The nested read owns _buffer from the moment start_reading reserves it.
• Verified: new test in test/js/node/stream/node-stream.test.js. On Windows x64 it fails on the current release (child segfaults) and passes with the fix. Full node-stream.test.js, spawn-streaming-stdout/stdin, and web/streams/streams.test.js pass on Windows and Linux debug builds.

Background

• WindowsBufferedReader serves every libuv read from the spare capacity of its _buffer Vec. For files it issues uv_fs_read with an iov pointer into that capacity and commits the length when the completion callback fires.
• on_read_chunk detaches _buffer before re-entering JS so the dispatch cannot observe a half-committed buffer, and restores it afterwards to reuse the allocation.
• The bug is Windows-only: the POSIX reader reads synchronously on the event loop and holds no cross-callback buffer pointer. The gate's fail-before run on Linux cannot show the failure. The fail-before proof above is from Windows x64 (release canary 1b88ad3 and a debug build of main at 7c9a51e).

Notes

Reproduction (issue #39890 script, trimmed): write a 1 MiB file, read it back with for await over Readable.fromWeb(Bun.file(p).stream()). The empty-file read in the issue script is not required.

Debug trace (BUN_DEBUG_FileReader=1) of the crash on main:

[filereader] onReadChunk() = 65536 (progress)   <- dispatch, _buffer taken
[filereader] setFlowing(false) was=true         <- push() returned false -> pause
[filereader] setFlowing(true) was=false         <- _read -> unpause -> new uv_fs_read into fresh _buffer
[filereader] onPull(16384) = pending
panic: uv_read_cb: buf is not in buffer!

When the dispatch returns, the old restore path replaced _buffer (dropping the new 64 KiB allocation) and cleared HAS_INFLIGHT_READ. The in-flight read then wrote into freed memory. In release builds the corruption surfaces later inside mimalloc, matching the bun.report trace in the issue (mi_page_malloc_zero under Vec::reserve in get_read_buffer_with_stable_memory_address).

The flag handling also matters for has_pending_read(): with the nested read's flag preserved, drain() no longer steals _buffer out from under the in-flight read.

Pipe and tty streams are unaffected by the nested-read case: uv_read_start does not call the alloc callback synchronously, so no buffer is reserved during the dispatch. For them had_inflight_read is true and the clear behaves exactly as before.

Runs: repro 10/10 clean on a Windows x64 debug build with the fix. Fail-before at test level: bun --expose-internals test ... -t 39890 with the system canary fails (child exits 3), passes with bun bd test.

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no test proof · iteration 1 · Platform-specific test(s) that do not run on this machine. Deferring to CI, which covers all platforms: test/js/node/stream/node-stream.test.js

[coderabbitai]

Walkthrough

Changes

The Windows PipeReader now preserves nested read buffers and in-flight state during reentrant callbacks. A regression test validates a 1 MiB file round trip through Readable.fromWeb(Bun.file().stream()).

PipeReader reentrant read handling

Layer / File(s)	Summary
Preserve nested read state src/io/PipeReader.rs	on_read_chunk tracks the original in-flight read and preserves the buffer and flag when a callback starts a nested read.
Validate web stream delivery test/js/node/stream/node-stream.test.js	The test harness imports tempDir. A subprocess test verifies a 1 MiB file round trip through Readable.fromWeb(Bun.file().stream()) and checks successful exit.

Suggested reviewers: jarred-sumner, dylan-conway, cirospaciari

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the Windows buffer-preservation fix for file reads during chunk delivery.
Description check	✅ Passed	The description explains the problem, fix, verification steps, regression test, and platform-specific behavior in sufficient detail.

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[coderabbitai]

Actionable comments posted: 1

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In `@test/js/node/stream/node-stream.test.js`:
- Around line 502-506: Update the subprocess test’s Promise.all around
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[robobun]

CI status: both runs (builds 102458 and 102473) are red only on lanes unrelated to this diff. The repeat failure is test/js/bun/http/bun-server.test.ts on Windows x64, which also fails on main. The rest are known flaky tests that passed alone or on retry. The new regression test passes on every lane in both builds.

[robobun]

^{Updated 1:10 AM PT - Aug 21st, 2026}

❌ @robobun, your commit 6cbcb49 has 1 failures in Build #102473 (All Failures):

• test/js/bun/http/bun-server.test.ts - code 1 on 🪟 2019 x64

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🧪   To try this PR locally:

bunx bun-pr 39897

That installs a local version of the PR into your bun-39897 executable, so you can run:

bun-39897 --bun

[claude]

I reviewed this PR and didn't find any bugs — the earlier stderr-drain nit has been addressed. Because this is a memory-safety fix in the Windows libuv buffer-ownership path (re-entrancy across on_read_chunk → start_reading), a maintainer look would still be worthwhile.

Checked: moving the HAS_INFLIGHT_READ clear before the dispatch is a no-op for pipe/tty (uv_read_start never allocs synchronously, so the flag stays clear across the dispatch and the buffer restore behaves as before); for files the added !HAS_INFLIGHT_READ guard prevents dropping the nested read's Vec, and the on_file_read epilogue's can_start() gate prevents a double-issue. has_pending_read() now correctly reports true across the dispatch when a nested file read is in flight.

Extended reasoning...

Overview

The PR fixes a Windows-only use-after-free in WindowsBufferedReader::on_read_chunk (src/io/PipeReader.rs). The old code took _buffer, dispatched the chunk into JS, then unconditionally restored the old Vec and cleared HAS_INFLIGHT_READ — even when the JS re-entry had triggered pause()/unpause() → start_reading(), which reserves a fresh _buffer and hands its spare-capacity pointer to uv_fs_read. Restoring the old Vec dropped that allocation while libuv still held a pointer into it. The fix clears the flag before the dispatch and gates the buffer restore on !HAS_INFLIGHT_READ. A regression test round-trips a 1 MiB file through Readable.fromWeb(Bun.file().stream()) in a subprocess.

Security risks

This is a heap-UAF fix in native I/O code. The change tightens an invariant (the in-flight read's iov allocation must not be freed) rather than relaxing one, so it reduces risk. No new attack surface; no untrusted-input parsing changes.

Level of scrutiny

High. This is unsafe Rust managing a raw buffer pointer held by libuv across re-entrant JS callbacks — the exact category REVIEW.md flags as most-blocked. The reasoning is subtle: correctness depends on which callers of on_read_chunk had HAS_INFLIGHT_READ set at entry (pipe/tty: yes, via on_stream_alloc; file: no, on_file_read clears it first), whether uv_read_start can synchronously invoke the alloc callback (it cannot, so pipes never hit the nested case), and whether on_file_read's post-dispatch epilogue can double-issue a read when a nested one is already in flight (can_start() on the file state prevents it). I traced these and they hold, but this is Windows-only code I cannot execute, and the interaction with has_pending_read()/drain() mentioned in the PR notes is a second-order effect worth a maintainer's confirmation.

Other factors

• All prior review threads (stderr draining, comment-cop) are resolved; the test now drains stderr and asserts { stdout, stderr }.
• The PR description includes a debug trace demonstrating the exact re-entry sequence and fail-before/pass-after runs on Windows x64.
• The diff is ~10 lines of native code plus one test; scope is tight and well-argued.
• The test uses bunEnv (which sets BUN_DEBUG_QUIET_LOGS=1), so the stderr: "" assertion should be stable — matching the neighboring tests in the same file that assert the same way.