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watcher: open watched directories and files with O_CLOEXEC - #42703
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A --watch reload is an execve of the same binary. macOS has no close_range sweep before it, so the directory fd that the watcher opened without O_CLOEXEC survived into the new image and the watcher opened another one. One directory fd leaked per reload. Fixes #42700
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Included review availability: Your plan provides up to 10 included reviews per hour; 2 remain after this review. WalkthroughThe watcher now opens watched paths with ChangesWatcher close-on-exec handling
Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to The watcher’s changed directory and platform-specific file opens are configured not to survive reload execution, with no remaining concrete merge-blocking risk. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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LGTM — adding O_CLOEXEC to the watcher's open flags is the right fix for the per-reload fd leak, and the shared WATCH_OPEN_FLAGS constant carries it to every kqueue open site.
What was reviewed:
- Both
#[cfg]arms ofWATCH_OPEN_FLAGSand the fallbackopen_ainappend_directory_assume_capacity; grepped allWATCH_OPEN_FLAGSconsumers (jsc_hooks, bake dev_server, bundle_v2, Watcher.rs:839) — all inherit the flag. - Windows path:
bun_sys::O::CLOEXECis dropped by the uv flag mapping, so the non-macOS arm is a no-op there as the description states. - Test follows the file's existing conventions (
stdoutWaiter,watcheeafterEach cleanup,tempDir/bunEnv), polls/proc/<pid>/fdinfofor the flag bit rather than counting fds (whichclose_rangewould mask), and asserts the full{path: flag}map in one.toEqual. TheskipIf(!isLinux)has a WHY comment.
Extended reasoning...
Overview
The PR fixes a file-descriptor leak in bun --watch/--hot reloads by adding O_CLOEXEC to every fd the watcher opens. Three lines change in src/watcher/Watcher.rs: both #[cfg] arms of the WATCH_OPEN_FLAGS constant gain bun_sys::O::CLOEXEC, and the fallback bun_sys::open_a(file_path, 0, 0) in append_directory_assume_capacity now passes O_RDONLY | O_CLOEXEC. A new Linux-only test in test/cli/watch/watch.test.ts spawns a --watch process and reads /proc/<pid>/fdinfo to assert the O_CLOEXEC bit is set on the watched directory and both watched files.
Security risks
None. The change only adds a close-on-exec flag to fds that exist purely to receive kernel watch events. No child process or exec'd image needs these fds, and marking them CLOEXEC is strictly a resource-hygiene improvement. The test reads /proc for the test's own child process only.
Level of scrutiny
Low-to-moderate. This is a three-line native change to open flags with a well-understood mechanism (fds without O_CLOEXEC survive execve; macOS lacks the close_range(CLOSE_RANGE_CLOEXEC) sweep that Linux runs before reload). I confirmed the fix reaches every watcher open site: WATCH_OPEN_FLAGS is consumed at Watcher.rs:839, jsc_hooks.rs:3421, bake/dev_server/mod.rs:1384, and bundle_v2.rs:4710, so the constant change covers the whole class. The one direct open_a that bypassed the constant is fixed inline. On Windows the added flag is a no-op (dropped by uv::O::from_bun_o), so no behavior change there.
Other factors
The test is placed in the existing test/cli/watch/watch.test.ts (correct per repo conventions — this was never correct behavior, so not a test/regression/issue/ case), reuses the file's stdoutWaiter helper and watchee afterEach cleanup, uses tempDir/bunExe/bunEnv, and polls with a 10ms sleep inside a bounded loop (bounded by the test framework's default timeout, matching the file's other poll patterns). The skipIf(!isLinux) gate has a comment explaining the test reads /proc and why fd-counting on Linux would not catch the bug. The assertion is a single .toEqual on the sorted {path: flag} map, which fails clearly on the unfixed build ("/": 0). No CODEOWNERS cover the changed paths, and the timeline shows no outstanding reviewer objections.
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Updated 11:25 PM PT - Sep 15th, 2026
❌ @robobun, your commit 4bf2929 has 1 failures in 🧪 To try this PR locally: bunx bun-pr 42703That installs a local version of the PR into your bun-42703 --bun |
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@robobun fix conflicts |
…h-dir-fd-cloexec # Conflicts: # test/cli/watch/watch.test.ts
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Merged main into the branch in 10c8f15. The conflict was in the import lines of test/cli/watch/watch.test.ts. All 14 tests in that file pass with the debug build. |
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Actionable comments posted: 1
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Inline comments:
In `@test/cli/watch/watch.test.ts`:
- Around line 186-243: Extend the O_CLOEXEC watcher coverage to macOS and
FreeBSD so it exercises add_file_by_path_slow and detects missing close-on-exec
flags there, while preserving the Linux-specific handling. In the watcher opens
test, retain every matching descriptor rather than storing one flags value per
pathname, then inspect all descriptors associated with each expected path.
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test/cli/watch/watch.test.ts
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The O_CLOEXEC check reads /proc and runs on Linux only. The leak itself is on macOS, so a second test counts the numbered fds on the project directory after two reloads through lsof. The /proc check now keeps every fd per path instead of one.
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Pushed 4bf2929. The review asked for coverage of the kqueue open path. A new test counts the fds on the project directory after two --watch reloads, through lsof on macOS and /proc on Linux. On macOS that reproduces the leak from the issue without the fix. The /proc flag check now keeps every fd per path. All 15 tests in test/cli/watch/watch.test.ts pass with the debug build. |
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CI on 4bf2929: 180 of 181 jobs pass. The one red lane (debian 13 x64-asan) fails on test/js/bun/http/serve-pending-promise-abort-leak.test.ts, which also fails on main and does not touch the watcher. The new watch tests pass on every lane, including the macOS reload count. Ready for review. |
2 participants
Problem
bun --watchleaks one fd on the working directory per reload on macOS.lsofshows the count grow by one per save and never drop. Fixesbun --watchleaks one directory fd on the cwd per reload (macOS/kqueue) #42700.append_directory_assume_capacityinsrc/watcher/Watcher.rs:607. It opens the directory with flags0, so the fd has noO_CLOEXEC. A reload is anexecveof the same binary (reload_process,src/bun_core/util.rs). On Linux,on_before_reload_process_posixrunsclose_range(CLOSE_RANGE_CLOEXEC)first, which hides the leak. macOS has no such sweep, so the old fd survives into the new image and the watcher opens a second one.Fix
O_RDONLY | O_CLOEXECto theopen_acall that opens the watched directory.O_CLOEXECtoWATCH_OPEN_FLAGS. That constant is the flag set for every other fd the watcher opens on kqueue platforms (Watcher.rs:838,src/runtime/jsc_hooks.rs:3421,src/runtime/bake/dev_server/mod.rs:1384,src/bundler/bundle_v2.rs:4710). The watch fds exist only to receive kernel events. No child or exec'd image needs them.test/cli/watch/watch.test.ts. One new test counts the fds on the project directory after two reloads (macOS throughlsof, Linux through/proc). Another readsO_CLOEXECfrom/proc/<pid>/fdinfofor every fd the watcher holds and fails on main with"/": [0]. Also all oftest/cli/watch/andtest/cli/hot/.cargo check -p bun_watcherforaarch64-apple-darwinandx86_64-pc-windows-msvc.Background
EVFILT_VNODEevents. On Linux, inotify works by path, but the watcher still opens the directory.O_CLOEXECmarks an fd so that the kernel closes it onexecve. Without it, an fd passes to the new program image.close_range(3, ~0, CLOSE_RANGE_CLOEXEC)call marks every fd above 2 as close-on-exec before a reload. It is gated onOS(LINUX) || OS(FREEBSD)insrc/jsc/bindings/c-bindings.cpp:301, so macOS depends on each open site to set the flag.Notes
close_rangesweep./proc/<pid>/fdinfoshows the missing flag: the directory fd hasflags: 0100000(O_LARGEFILEonly), while the file fds and the inotify fd have02000000(O_CLOEXEC). That matches the report that only the cwd handle is duplicated.bun_sys::O::CLOEXECis a placeholder value thatuv::O::from_bun_odrops, so the flag change is a no-op there./proc. The reload count test is the fail-before proof on macOS: 3 directory fds after two reloads without the fix, 1 with it. On Linux it passes both ways because of theclose_rangesweep.--hot).no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/cli/watch/watch.test.ts