fix(windows): handle null handle.data in libuv callbacks

[robobun]

Summary

• Add null checks to Windows libuv callbacks to prevent "cast causes pointer to be null" panics
• This is a recurring Windows-specific bug (Panic: cast causes pointer to be null bun has crashed :'( #7942, Bun crah report #15149, panic(main thread): cast causes pointer to be null #15104, Unexpected crash of our express server #15849, Bun randomly crashed while running an http server #13826) where libuv callbacks may be invoked with null handle.data due to race conditions during handle cleanup or event loop processing

Changes

The following callbacks now safely handle null handle.data:

PipeReader.zig:

• onStreamAlloc - returns empty buffer if null
• onStreamRead - returns early if null
• onPipeClose - returns early if null
• onTTYClose - returns early if null

PipeWriter.zig:

• onPipeClose - returns early if null
• onTTYClose - returns early if null

win_watcher.zig:

• uvClosedCallback - returns early if null

Root Cause

When the pipe/handle is being closed, the code sets pipe.data = pipe (self-pointer) before calling close. Due to race conditions in Windows libuv event processing, callbacks can be invoked:

1. After the handle's data pointer has been nullified
2. After the handle has been deallocated
3. During the transition period when data no longer points to the expected object

The bun.cast() function triggers Zig's runtime safety check when casting a null pointer, causing a panic.

Test plan

• Build succeeds (bun bd)
• Existing spawn tests pass on Linux
• Manual testing on Windows with standalone executables (requires Windows CI)

Fixes #26574

🤖 Generated with Claude Code

[robobun]

^{Updated 2:37 PM PT - Jan 29th, 2026}

❌ Your commit 373f0cac has 3 failures in Build #36131 (All Failures):

• test/js/node/worker_threads/worker_threads.test.ts - 1 failing on 🐧 25.04 x64
• test/bundler/compile-windows-metadata.test.ts - 1 failing on 🪟 2019 x64
• test/bundler/compile-windows-metadata.test.ts - 1 failing on 🪟 2019 x64-baseline
• test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🍎 13 aarch64

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🧪   To try this PR locally:

bunx bun-pr 26577

That installs a local version of the PR into your bun-26577 executable, so you can run:

bun-26577 --bun

[coderabbitai]

Walkthrough

This PR adds defensive null-safety checks to Windows-specific libuv handle callbacks in file watcher and pipe I/O modules. The changes guard against null dereferences in close operations and race conditions by validating handle.data pointers before casting or dereferencing them.

Changes

Cohort / File(s)	Summary
Windows File Watcher Callbacks src/bun.js/node/win_watcher.zig	UV close callback now safely derives PathWatcher instance from event data with null-check fallback to prevent null dereferences during cleanup races.
Windows Pipe I/O Callbacks src/io/PipeReader.zig, src/io/PipeWriter.zig	Added defensive null-handling for handle.data in stream read/alloc callbacks and pipe/TTY close handlers to prevent use-after-free scenarios during close races and handle state transitions.

Possibly related PRs

• fix(windows): prevent data loss in pipe reads after libuv 1.51.0 upgrade #23340: Modifies Windows PipeReader callbacks including onStreamRead with similar null-safety improvements to handle.data.
• Fix windows memory leak in error case for opening tty & pipe #23235: Reworks Windows pipe and TTY close/open lifecycles in PipeReader.zig, overlapping with this PR's pipe callback safety changes.

Suggested reviewers

• Jarred-Sumner
• pfgithub

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding null handle checks to Windows libuv callbacks to fix a critical crash issue.
Description check	✅ Passed	The description provides context, lists affected callbacks, explains the root cause, and includes a test plan, aligning well with the template requirements.
Linked Issues check	✅ Passed	All coding-related requirements from issue #26574 are met: null checks added to prevent panics, callbacks guard against null handle.data, and unsafe casts are guarded to prevent Zig runtime errors.
Out of Scope Changes check	✅ Passed	All changes are focused and in-scope: adding null-safety guards to Windows libuv callbacks in three specific files (PipeReader.zig, PipeWriter.zig, win_watcher.zig) to address the identified issue.

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