## Summary
Fixes#24593 - WebSocket segfault on Windows when publishing large
messages with `perMessageDeflate: true`.
Also fixes#21028 (duplicate issue).
Also closes#25457 (alternative PR).
**Root cause:**
On Windows, the C++ code was compiled against system zlib headers
(1.3.1) but linked against Bun's vendored Cloudflare zlib (1.2.8).
This version mismatch caused `deflateInit2()` to return
`Z_VERSION_ERROR` (-6), leaving the deflate stream in an invalid state.
All subsequent `deflate()` calls returned `Z_STREAM_ERROR` (-2),
producing zero output, which then caused an integer underflow when
subtracting the 4-byte trailer → segfault in memcpy.
**Fix:**
Add `${VENDOR_PATH}/zlib` to the C++ include paths in
`cmake/targets/BuildBun.cmake`. This ensures the vendored zlib headers
are found before system headers, maintaining header/library version
consistency.
This is a simpler alternative to #25457 which worked around the issue by
using libdeflate exclusively.
## Test plan
- [x] Added regression test `test/regression/issue/24593.test.ts` with 4
test cases:
- Large ~109KB JSON message publish (core reproduction)
- Multiple rapid publishes (buffer corruption)
- Broadcast to multiple subscribers
- Messages at CORK_BUFFER_SIZE boundary (16KB)
- [x] Tests pass on Windows (was crashing before fix)
- [x] Tests pass on macOS
🤖 Generated with [Claude Code](https://claude.com/claude-code)
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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
