watcher: free the owned path of evicted watchlist entries - #39197
watcher: free the owned path of evicted watchlist entries#39197robobun wants to merge 5 commits into
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MultiArrayList::swap_remove and ordered_remove copied the remaining rows over the removed one without running its destructor, so Watcher::flush_evictions leaked the Cow::Owned file_path of every evicted entry that had been added with CLONE_FILE_PATH (every module watched by --hot, plugin-loaded files, dev server directory watches, everything on Windows). Under --hot each save of a watched file evicts and re-adds its entry, leaking one path per save. Both removal functions now return the removed element, transferring ownership to the caller the same way pop does, and flush_evictions drops it. The only other caller (HTTP header entries, a Copy type) is unaffected.
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ChangesHot reload memory ownership
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Updated 8:58 PM PT - Aug 15th, 2026
✅ @robobun, your commit 4163a489ed7308cd605b419266aaeea7976263c0 passed in 🧪 To try this PR locally: bunx bun-pr 39197That installs a local version of the PR into your bun-39197 --bun |
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Status: reproduced and fixed; review follow-ups pushed.
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Found 1 issue this PR may fix:
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I reviewed this PR and didn't find any bugs. Because it changes the ownership contract of MultiArrayList::swap_remove/ordered_remove (a foundational unsafe SoA collection) and touches watcher memory lifetime, a human look would still be worthwhile.
Checked: gather runs before copy_rows_within/len -= 1, so the returned row is never re-visited by drop_elements (Miri test covers this, including the index == last self-swap). Verified the only other production caller (src/http/lib.rs header_entries.ordered_remove) already discards with let _ = on a Copy element type. Confirmed Fd has no Drop, so dropping the returned WatchItem cannot double-close the fd already closed in pass 1 of flush_evictions.
Extended reasoning...
Overview
The PR fixes a per-eviction leak of WatchItem.file_path in the --hot watcher by making MultiArrayList::swap_remove and ordered_remove return the removed row (mirroring pop), and having Watcher::flush_evictions explicitly drop it. It adds a Miri-checked unit test for the new ownership transfer, an LSan-gated integration test in test/cli/hot/watch-many-dirs.test.ts, and a doc comment on WatchItem.file_path. Three files touched; ~15 production lines.
Security risks
None. This is a leak fix on a filesystem-watcher path buffer; no untrusted input handling, auth, or crypto involved.
Level of scrutiny
High. multi_array_list.rs is an #[allow(unsafe_code)] module with hand-managed column ownership and an intentionally slab-only Drop; changing what a public removal method returns changes its ownership contract for every caller. REVIEW.md flags memory-ownership changes in native code as the most-blocked category. The mechanism here is straightforward — it reuses the exact gather-then-shrink pattern pop() already uses — but the semantic shift on a shared primitive warrants a maintainer confirming it matches their model of MultiArrayList ownership (particularly the interaction with bitwise .clone() aliasing that motivates the slab-only Drop).
Other factors
I audited all swap_remove/ordered_remove call sites: the only other MultiArrayList callers are the two header_entries.ordered_remove(i) sites in src/http/lib.rs, which already used let _ = before this PR and hold a Copy element type. Dropping the returned WatchItem is safe beyond file_path: Fd has no Drop impl (fd closure stays in pass 1), and the remaining fields are Copy. The new test guards against vacuous passes (asserts LSan actually ran and evictions were logged), awaits observable conditions rather than sleeping, and scopes its leak assertion to bun_watcher:: frames so unrelated LSan noise cannot flake it. The PR description documents the borrowed-pointer analysis for both on_file_update implementations; I did not independently re-trace those.
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… tests The --hot test now lets the child exit and relies on the exit-time leak check instead of an on-demand check filtered by symbol name, which does not depend on which frames survive inlining and needs no symbolization when clean. test/bake/dev/css.test.ts leaked the same way through the dev server's directory watches (css-13 and css-14 fail under LSan without the fix) and is clean with it, so it no longer opts out of LSan in CI.
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Actionable comments posted: 1
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Inline comments:
In `@src/collections/multi_array_list.rs`:
- Around line 1429-1436: Add a test assertion covering ordered_remove at the
final row, exercising the tail == 0 boundary for index == len - 1 in the
existing removal test alongside the swap_remove case. Verify the returned Foo
value and resulting column contents, using the established list/items symbols
and preserving the current variant coverage.
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src/collections/multi_array_list.rssrc/watcher/Watcher.rstest/cli/hot/watch-many-dirs.test.tstest/no-validate-leaksan.txt
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LGTM — the ownership transfer mirrors pop() exactly (gather before overwrite, then decrement len), and the Miri + LSan coverage proves it. The outstanding timeout nit on the new test is local-run-only and non-blocking.
Checked: swap_remove/ordered_remove gather the row before it's overwritten/shifted, so the returned T is the sole owner and drop_elements/Drop (which walk 0..len) can't reach it again — no double-free. Verified the only other production MultiArrayList caller of these (header_entries.ordered_remove in src/http/lib.rs) uses HeaderEntry: Copy, so the () → T return-type change is a no-op there. The new --hot test drains stderr concurrently with the stdout loop and asserts evictions happened, LSan ran, and reported nothing — non-vacuous. Removing css.test.ts from the LSan exclusion list strengthens CI rather than weakening it.
Extended reasoning...
Overview
Fixes a per-eviction memory leak in the file watcher: MultiArrayList::swap_remove and ordered_remove previously overwrote the removed row without transferring ownership out, so WatchItem.file_path: Cow<'static, [u8]> was leaked on every --hot/dev-server eviction of a heap-owned path. The fix makes both methods return the removed T (exactly as pop() already does via gather → decrement → ManuallyDrop::into_inner), and flush_evictions now drop()s the returned WatchItem. Four files touched: the collection (~10 production LOC + unit tests), the watcher (2 lines), a new LSan integration test, and one line removed from no-validate-leaksan.txt.
Security risks
None. No user-controlled input parsing, no auth/crypto/permissions. The change is memory-ownership bookkeeping in a struct-of-arrays list.
Level of scrutiny
Medium-high: this touches ownership semantics in an unsafe-adjacent collection type used widely across the codebase. However, the change is mechanically identical to the existing pop() implementation (both call Slice::gather for a bitwise copy, then remove the row from the list's view). I verified the last-row edge cases: swap_remove(len-1) gathers, then copy_rows_within(last, last, 1) memmoves the row onto itself (defined for overlapping), then decrements; ordered_remove(len-1) gathers, then copy_rows_within(_, _, 0) early-returns. Both are covered by the extended ordered_remove_memmove test and the new remove_returns_owned_element test, which the description says pass under Miri. The () → T signature change is source-compatible for all call shapes (foo(); and let _ = foo();); I confirmed the only other production MultiArrayList caller (src/http/lib.rs header_entries.ordered_remove, via HeaderEntryList = MultiArrayList<HeaderEntry> in src/http_types/ETag.rs:188) uses a #[derive(Copy)] element type, so dropping the return value is a no-op there.
Other factors
All prior review feedback is addressed: six comment-cop flags on verbose comments were cut to one-liners in 723d0ac/96d34fb5, and CodeRabbit's ordered_remove last-row coverage request landed in 4163a48. My earlier inline nit about the missing per-test timeout on the new LSan test remains open, but it's explicitly a local-run-only flake risk (CI's runner passes --timeout with an ASAN multiplier, and the author reports ~0.5s wall time over 5 runs), so it doesn't block. The new test is well-constructed per REVIEW.md: awaits observable conditions (RELOAD n lines), wires early exit to reject, drains stdout and stderr concurrently, asserts the eviction log so a non-evicting cycle can't pass vacuously, asserts LSan ran before asserting no leaks, and checks {exitCode: 0, signalCode: null}. Removing test/bake/dev/css.test.ts from no-validate-leaksan.txt un-suppresses a CI safety net rather than weakening one.
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Heads-up: #39293 rewrites the "Watcher Thread" block of |
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Problem
bun --hot:Direct leak of 111 byte(s) in 3 object(s) allocated from: ... <bun_watcher::watcher_impl::Watcher>::append_file_assume_capacity::<true> src/watcher/Watcher.rs:543(full report below).Watcher::flush_evictions(src/watcher/Watcher.rs:441) removes entries withMultiArrayList::swap_remove, andswap_remove(src/collections/multi_array_list.rs:994) only copies the last row over the removed one. The removed row'sWatchItem.file_path: Cow<'static, [u8]>is never dropped, and the list's ownDropis slab-only by design, so nothing else frees it either.CLONE_FILE_PATH = true: the modules--hotwatches from the runtime transpiler (src/jsc/RuntimeTranspilerStore.rs:939,:967,src/runtime/jsc_hooks.rs:3603,:3717), the entrypoint (add_file_by_path_slow), plugin-loaded files inBundleV2, dev server directory watches (add_directory::<true>), and every entry on Windows.--hot, saving a watched file raises a directory event that evicts the file's entry (src/jsc/hot_reloader.rs:1218) and the reload re-adds it with a new copy, so a long-running session leaks one path per save.Borrowedentries were never affected.bun --hotalways leaks memory #11083 (--hotRSS growth per reload). Its loop hits this leak too, but only for a few dozen bytes per reload, so the growth reported there is mostly something else.Fix
MultiArrayList::swap_removeandordered_removereturn the removed element, transferring ownership to the caller exactly aspopalready does.flush_evictionsdrops the returnedWatchItem; dropping frees anOwnedpath and is a no-op for aBorrowedone.lenshrinks, so the list never refers to it again: neitherdrop_elementsnorDrop(both of which only cover rows still in the list) can free it a second time. The newremove_returns_owned_elementunit test checks this under Miri.on_file_updateimplementations (src/jsc/hot_reloader.rs,src/runtime/bake/DevServer.rs) read thefile_pathcolumn only before their deferredflush_evictionsand copy whatever they keep (StringSet/StringArrayHashMap/StringHashMapkeys are owned), the Windows event scan indexes live rows only, andsrc/runtime/bake/dev_server/mod.rs:1439already documents that the watcher owns the copy until eviction runs.src/http/lib.rs(header_entries, aCopyelement type), which compiles and behaves unchanged.set()intentionally keeps overwriting without dropping:append_assume_capacityuses it on slots that hold no element.Watcherdropped while it still has live entries leaks them too. That is the teardown path watcher: wake the watcher thread on shutdown so torn-down dev servers release it #30644 covers withimpl Drop for Watcher; this PR only changes eviction.--hot/append_filecoverage: new test intest/cli/hot/watch-many-dirs.test.ts("evicting watchlist entries does not leak their paths", Linux + ASAN builds only). It runsbun --hotwith LSan enabled, saves an import three times, lets the child exit, and asserts that the reloader logged the evictions (so the cycle cannot stop evicting and pass vacuously), that the exit-time leak check ran, and that it reported nothing and the exit code is 0. On the unfixed build it fails with exactly one report block, the one quoted below; with the fix it passes (5 of 5 runs, about 0.5 s each).append_directorycoverage:test/bake/dev/css.test.tsis removed fromtest/no-validate-leaksan.txt, so CI's ASAN lane now applies its exit-time leak check to that file. Without the fix, css-13 ("changing html file with link tag works") and css-14 ("css import before create") fail there withDirect leak of 30 byte(s)fromappend_directory_assume_capacity::<true>viaDirectoryWatchStore::insert(src/runtime/bake/dev_server/mod.rs:1447); with the fix all 15 cases pass under the same environment. The file was excluded in the same batch as the rest of the "Watcher Thread" block, with no css-specific reason. The other entries in that block are left alone:bundle.test.tsstill has an unrelated leak (bundler: free the ServerComponentParseTask after it generates its file #38004), and the rest are not affected by this change.cargo test -p bun_collections,bun run rust:miri -p bun_collections,bun bd test test/cli/hot/(17 pass),test/bake/dev/bundle.test.ts(dev server file andDirectoryWatchStoreevictions, 21 pass),test/bake/dev/hot.test.ts,test/bake/dev/incremental-graph-edge-deletion.test.ts,test/js/web/fetch/fetch-redirect.test.ts(theordered_removecaller, 30 pass);cargo clippy -p bun_collections -p bun_watcheris clean.Background
MultiArrayList<T>is a struct-of-arrays list: each field ofTlives in its own column, so a row is never a singleTin memory. Removing a row is a byte copy per column, and the list'sDropfrees only the backing slab (bitwise clones of a list can share columns, see the comment on theDropimpl), so element destructors run only when a caller asks for them:pop,drop_elements, and now the two*_removefunctions.WatchItemper watched file or directory in such a list.file_pathis aCow: callers whose path string is interned for the life of the process store a borrow (CLONE_FILE_PATH = false); callers holding a transient buffer store a heap copy (true).remove_at_indexonly records an index inevict_list;flush_evictions, run on the watcher thread at the end of eachon_file_updatebatch, closes the entries' fds and thenswap_removes the rows, largest index first so the remaining recorded indices stay valid.__asan_default_optionsturns it off; CI's ASAN lane turns it back on for every test process not listed intest/no-validate-leaksan.txt(together withBUN_DESTRUCT_VM_ON_EXIT=1, which tears the VM down first so allocations still referenced from JS are not reported). The new--hottest sets the same variables itself so it also works under a plainbun bd test.LSan report from the unfixed build (3 saves of lib/dep.js)
One 37-byte object per save (the length of the temp dir path of
lib/dep.js); five saves give five objects.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/hot/watch-many-dirs.test.ts