bundler: own the Bun.build transpiler with a Box so it drops when option setup fails

[robobun]

Problem

• Bun.build() placed a Transpiler in the per-build arena. When option validation fails (a define that is not valid JSON), src/bundler/BundleThread.rs:282 returned before the hand-written drop_in_place. The arena frees the bytes without Drop: each failed call leaks 23 KB.
• new Bun.Transpiler() moves its config into a Box and calls set_log to reseat the log pointer. set_log (src/bundler/transpiler.rs:152) skipped options.log, so configure_defines wrote the parse error into the moved-from stack slot. The error lost its message.

Fix

• create_and_configure_transpiler returns Box<Transpiler<'a>>. The box drops the transpiler on every path. Both hand-written drop_in_place blocks go away.
• The AST allocator is a stack local with an enter() guard, as in RuntimeTranspilerStore.
• set_log reseats all four log aliases, as wire_after_move does.
• Verified: test/bundler/bun-build-api.test.ts (leak delta 229 KB on main, 0 B fixed), test/bundler/transpiler/transpiler.test.js (define diagnostic).

Background

• A MimallocArena is a private mimalloc heap. Its Drop frees every block at once and runs no per-value Drop. A value placed with arena.alloc must release its own global-heap state by hand.
• init_and_run takes &'a mut Transpiler<'a>, a borrow for the arena lifetime. The borrow checker rejects it from a local that drops inside 'a, so one raw-pointer reborrow stays.
• Considered a scope guard around the raw pointer: it keeps the drop_in_place. Considered an arena-backed box type: nothing needs the transpiler in the arena, and a plain Box is the shape bundler: remove unsafe from bundle_v2, ParseTask, LinkerContext and the bundler thread pool #40438 adopts.

Downsides

• None found. One malloc and one free per Bun.build() call replace one arena placement.

Notes

• configure_linker stores the transpiler's own field addresses (resolve_queue, options, resolve_results) in linker, so the transpiler must sit at its final address before configure_bundler. The box provides that.
• Drop order in generate_in_new_thread: the transpiler box, then the AST scope guard, then the AST allocator, then the arena.
• src/runtime/cli/build_command.rs:121 documents the same &'a mut Transpiler<'a> constraint for the CLI path, which places its transpiler in the process-lifetime arena.
• The leak test runs with symbolize=0: symbolizing every leak stack in a debug binary costs seconds per process, and the test reads only the byte count. It compares a 1-iteration run against an 11-iteration run so the one-time residue (the -e script's own source map) cancels out. It asserts that both calls were rejected, that LSan ran, and that no other sanitizer report appeared.
• Probe on main: ASAN_OPTIONS=detect_leaks=1 bun-debug -e 'try { await Bun.build({ entrypoints: ["e.js"], define: { X: "{\"a\":" } }) } catch {}' reports 5451 bytes leaked in 202 allocations.
• The define diagnostic: new Bun.Transpiler({ define: { X: '{"a":' } }) now throws with Unexpected end of file and a position, instead of ParserError Failed to load define.
• Siblings not changed here: src/jsc/web_worker.rs:312 takes the temporary log's address before the log moves into its scope guard, the same class of bug. Report the resolve error when a Worker preload does not resolve #41496 fixes that path and adds a preload-specific message. src/jsc/VirtualMachine.rs:5315 and src/jsc/AsyncModule.rs:1102 swap the log pointers by hand instead of through set_log.
• Self-reviewed: 4 concerns raised, 3 addressed (plain Box instead of a new arena box type, the worker hunk left to Report the resolve error when a Worker preload does not resolve #41496, the test asserts its preconditions). Not addressed: converting the VirtualMachine.rs and AsyncModule.rs log swaps to set_log, which touches module loading and is out of scope.

---

no test proof · iteration 1 · Platform-specific test(s) that do not run on this machine. Deferring to CI, which covers all platforms: test/bundler/bun-build-api.test.ts

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📒 Files selected for processing (3)

• src/bundler/BundleThread.rs
• src/runtime/api/js_bundle_completion_task.rs
• test/bundler/bun-build-api.test.ts

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---

Walkthrough

Transpiler creation now returns an owned box. Thread setup uses a scoped stack allocator. The transpiler log option is synchronized, and tests cover malformed JSON define values and leak growth.

Changes

Transpiler cleanup and diagnostics

Layer / File(s)	Summary
Owned transpiler and scoped allocator src/runtime/api/js_bundle_completion_task.rs, src/bundler/BundleThread.rs, src/bundler/transpiler.rs	The creation method returns a boxed transpiler. Thread setup uses a stack-owned ASTMemoryAllocator with an entered scope and passes a raw-pointer reborrow to init_and_run. set_log also updates options.log.
Invalid define regression coverage test/bundler/transpiler/transpiler.test.js, test/bundler/bun-build-api.test.ts	Tests check malformed JSON diagnostics and source positions. An ASAN-only skipped test compares leak reports after 1 and 11 invalid-define failures.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to dbb4b

No actionable merge-blocking issue was found; the remaining platform-specific test validation can proceed in CI.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: owning the Bun.build transpiler with a Box so it is dropped when option setup fails.
Description check	✅ Passed	The description explains the problem, fix, verification results, design considerations, and limitations. It does not use the exact template headings, but it provides the required information in equiva…
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.

---

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[github-actions]

Found 1 issue this PR may fix:

1. Bun.build() leaks native memory per call (RSS unbounded, heapUsed flat) — kills long-lived watch/dev processes #34053 - Reports that every Bun.build() call leaks native memory (RSS grows unbounded while JS heap stays flat), killing long-lived watch/dev processes. This PR fixes the leak by properly releasing arena-allocated Transpiler/AST store on error paths.

If this is helpful, copy the block below into the PR description to auto-close this issue on merge.

Fixes #34053

🤖 Generated with Claude Code

[claude]

No issues found, but this adds unsafe { drop_in_place } teardown on new error paths in the arena-backed bundle-thread lifecycle — worth a human sanity-check on the ownership reasoning.

What was reviewed:

• Error-path teardown mirrors the existing success-path pop() + drop_in_place sequence; no double-drop (the caller's Err arm never sees tp).
• set_log now reseats options.log alongside the other three aliases, matching wire_after_move; audited all 8 callers (JSTranspiler, RuntimeTranspilerStore, web_worker) — each already expects the four log pointers to agree.
• Checked that Transpiler::init's own ? (before bump.alloc) is also covered by the new match arm in generate_in_new_thread.

Extended reasoning...

Overview

Three targeted error-path cleanups plus an ASAN-gated leak regression test:

• src/bundler/BundleThread.rs: replaces a ? with an explicit match so a failed create_and_configure_transpiler still runs ast_memory_store.pop() + drop_in_place(ast_memory_store) before returning — mirroring the teardown that already exists on the success path at the bottom of generate_in_new_thread.
• src/runtime/api/js_bundle_completion_task.rs: same shape one layer down — if configure_bundler fails after the transpiler is already bump.alloc'd, drop_in_place(tp) before returning Err.
• src/bundler/transpiler.rs: set_log now also reseats self.options.log, so all four aliased log pointers (self.log, options.log, linker.log, resolver.log) agree — matching what wire_after_move already does.
• test/bundler/bun-build-api.test.ts: new test.skipIf(!isASAN) spawns 1- vs 21-iteration failed-config loops under detect_leaks=1 and asserts the delta is under 4 KB.

Security risks

None. This is internal error-path resource release with no user-facing surface change.

Level of scrutiny

High. The changes are small and each is a symmetric mirror of an existing success-path teardown, but they involve unsafe { core::ptr::drop_in_place } on arena-allocated structs in a multi-threaded context, plus a raw-pointer aliasing change (options.log) that fans out to ~8 call sites. The SAFETY comments are thorough and I did not find a flaw in the reasoning:

• No double-drop: on configure_bundler Err, create_and_configure_transpiler drops tp and returns Err; the caller's Err arm only touches ast_memory_store, never the (unreturned) transpiler.
• The Transpiler::init(...)? early return (before bump.alloc) is also handled — nothing to drop there, and the caller's Err arm still restores the AST-alloc thread-local.
• set_log reseating options.log is strictly a consistency fix; every existing caller either restores the previous log via TranspilerStateGuard or passes a log that outlives the transpiler, and wire_after_move already reseats options.log the same way.

That said, per the review guidelines, unsafe drop-ordering / arena-lifetime changes in a hot cross-thread path are the kind of change where a maintainer's eyes are cheap insurance.

Other factors

The test follows the file's established pattern for leak assertions (delta between two runs so at-exit residue cancels, ASAN-gated, subprocess-isolated, bunEnv spread). The 4 KB threshold sits well below the ~125 KB pre-fix delta the comment documents. The PR description also notes it obsoletes a suppression proposed in #35309.

[claude]

I didn't find any issues, but this touches memory-lifecycle code in the bundler thread (arena-allocated Transpiler teardown, drop_in_place, AST-allocator thread-local restore) and changes the RAII shape of generate_in_new_thread, so it's worth a human look.

What was reviewed:

• ASTMemoryAllocator::new(bump) already ignores its arg and calls default(), so the switch to stack-owned default() + enter() is semantically equivalent; Scope::drop restores the same thread-locals pop() did, and ASTMemoryAllocator::drop recycles the pooled mi_heap — now on every return path including the early ?.
• set_log now reseats options.log to match wire_after_move; verified against the JSTranspiler constructor path (config built on stack, moved into Box, then set_log → configure_defines), which explains both the leaked Msg and the lost diagnostic the new transpiler test asserts on.
• drop_in_place(tp) on the configure_bundler Err path: tp is the just-bump.alloc'd unique slot with no other references; the Ok path's drop_in_place in generate_in_new_thread is unchanged, so no double-drop.

Extended reasoning...

Overview

Three source changes plus two tests:

• src/bundler/BundleThread.rs: ast_memory_store moves from bump.alloc(ASTMemoryAllocator::new(bump)) + manual reset()/push()/pop()/drop_in_place to a stack-owned ASTMemoryAllocator::default() with an RAII _ast_scope = ast_memory_store.enter(). Drop order is reverse-declaration: _ast_scope (restores TLS via Scope::exit), then ast_memory_store (recycles pooled arena/state), then heap (bulk-free). This runs on both the success path and the early-? from create_and_configure_transpiler, which was the leak. The transpiler drop_in_place on the success path is unchanged; the comment was tightened to remove the AST-store half.
• src/runtime/api/js_bundle_completion_task.rs: create_and_configure_transpiler now drop_in_places the freshly-bump.alloc'd Transpiler when configure_bundler errs. On Err the caller never receives the pointer, so this is the only place that can release the embedded global-heap state.
• src/bundler/transpiler.rs: set_log additionally reseats self.options.log, matching wire_after_move. This fixes the new Bun.Transpiler case where options.log stayed pointing at the moved-from stack config.log, so configure_defines wrote its parse error there (leaking the Msg and losing the diagnostic). Checked all other set_log callers (RuntimeTranspilerStore, JSTranspiler transform paths, web_worker) — all pass a log that should also govern options.log, so the added assignment is correct everywhere.

Tests: an ASAN-gated leak regression that diffs LSan's leaked-byte count between 1 and 21 iterations (cancelling at-exit residue), and a transpiler test asserting the define-parse error surfaces the JSON lexer diagnostic + position instead of the generic ParserError fallback.

Security risks

None. Error-path resource cleanup and a stale-pointer write into a moved-from stack slot; no user-controlled data reaches any new sink.

Level of scrutiny

High. This is native memory-lifecycle code on the bundler thread with unsafe drop_in_place, arena bulk-free interaction, and thread-local install/restore. Getting the drop order or the drop_in_place reachability wrong would UAF or double-drop. The change simplifies toward RAII (which is the right direction per REVIEW.md's "arm a Drop/RAII guard before any fallible call"), but the interaction between stack-owned ASTMemoryAllocator, its Scope<'_> guard, the bump-allocated Transpiler, and the Arena bulk-free deserves a human sanity check.

Other factors

• ASTMemoryAllocator::new already delegates to default() (ignores the _fallback arg), so no allocator-routing change.
• The PR description's "Fix" bullet for BundleThread.rs describes an earlier commit (match + explicit pop()/drop_in_place); the final diff (from commit e720920) uses the cleaner stack-owned RAII approach instead. The description is slightly stale but the shipped code is the better shape.
• The transpiler error-message test is a real behavioral assertion (not just "doesn't leak"), which independently proves the options.log reseat.

[robobun]

^{Updated 10:49 PM PT - Oct 1st, 2026}

❌ @robobun, your commit dbb4bb8 has 1 failures in Build #122692 (All Failures):

• test/js/bun/spawn/spawn.test.ts - code 1: stdout reader of an unref'd child and process lifetime > an idle reader stopped at the highwater … on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 35312

That installs a local version of the PR into your bun-35312 executable, so you can run:

bun-35312 --bun

[claude]

No bugs found, but this touches unsafe Rust memory management on the bundle thread (arena drop_in_place, RAII drop-order for ASTMemoryAllocator/Scope, aliased raw *mut Log pointers), so it's worth a maintainer confirming the drop-order and set_log reasoning.

What was reviewed:

• Drop order in generate_in_new_thread: _ast_scope → ast_memory_store → heap; Scope::exit() clears ast_pushed before ASTMemoryAllocator::drop's debug_assert — matches the RuntimeTranspilerStore pattern.
• No double-drop: on configure_bundler Err the transpiler is drop_in_place'd inside create_and_configure_transpiler; the caller's ? returns before its own drop_in_place.
• set_log now reseating options.log — audited all 9 callers (JSTranspiler, RuntimeTranspilerStore, web_worker); none rely on options.log diverging, and wire_after_move already sets all four.
• JSBundleCompletionTask is the only CompletionStruct impl, so no sibling create_and_configure_transpiler needs the same fix.

Extended reasoning...

Overview

Three targeted Rust changes plus two tests:

• src/bundler/BundleThread.rs: convert ASTMemoryAllocator from bump.alloc + manual push()/pop()/drop_in_place to a stack-owned value with an enter() RAII Scope guard, so early-return via ? still runs Scope::drop (restores AST-alloc thread-locals) and ASTMemoryAllocator::drop (recycles the pooled mi_heap). Trims the trailing drop_in_place block to just the transpiler.
• src/runtime/api/js_bundle_completion_task.rs: on configure_bundler Err, drop_in_place the already-bump.alloc'd Transpiler before propagating, since the arena bulk-free skips Drop.
• src/bundler/transpiler.rs: set_log also writes self.options.log so all four aliased log pointers agree (matching wire_after_move).
• Tests: an ASAN-gated leaked-byte-delta test (1 vs 21 iterations) and a non-gated test asserting the JSON parser diagnostic surfaces on invalid define.

Security risks

None. No user-facing input handling changes; the set_log fix moves a write from a moved-from stack slot to the correct heap slot, which is strictly a correctness/leak fix. No new attack surface.

Level of scrutiny

Medium-high. This is unsafe Rust in a hot path (Bun.build bundle thread) with manual drop_in_place, arena-vs-global-heap ownership, and thread-local restoration ordering. The reasoning in the PR description and SAFETY comments is thorough and I traced it end-to-end, but the REVIEW.md guidance flags memory-safety changes (RAII acquisition/release pairing, drop order, aliased raw pointers) as the most-blocked category — a maintainer should confirm the drop-order argument and that the options.log reseat has no unintended effect on the ~9 other set_log callers.

Other factors

• The RAII pattern exactly mirrors RuntimeTranspilerStore.rs:626-627 (let mut ast_memory_store = ...; let _ast_scope = ast_memory_store.enter();), so it's not novel.
• Verified Scope::drop → exit() clears ast_pushed before ASTMemoryAllocator::drop's debug_assert!(!self.ast_pushed) fires; Rust's reverse-declaration drop order guarantees _ast_scope drops first.
• Verified no double-drop: the Err from create_and_configure_transpiler is propagated by ? in generate_in_new_thread before that function's own drop_in_place(transpiler_ptr) line.
• JSBundleCompletionTask is the sole CompletionStruct implementer — no sibling sites to fix.
• The leak test uses the delta-between-two-runs technique (per REVIEW.md leak-test guidance) with a bound (4KB) well below the unfixed ~125KB delta; the diagnostic test is non-gated and asserts exact position/message.

[robobun]

This also covers the leak that #41519 works around. That PR lists test/bundler/transpiler/define-deep-nesting-stack-overflow.test.ts in test/no-validate-leaksan.txt because a define value that fails to parse leaks the transpiler log (the options.log pointer left on the moved-from stack slot in the Bun.Transpiler constructor, and the missing teardown after configure_defines fails in the Bun.build path). Once this PR lands, that entry can be removed.

[github-actions]

If you need a paragraph-long comment to justify why the workaround is OK, the code is wrong — fix the code

[github-actions]

If you need a paragraph-long comment to justify why the workaround is OK, the code is wrong — fix the code

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
Review comments at @test/bundler/bun-build-api.test.ts:
- Line 1918: Remove the explicit 30_000 timeout from the test declaration in
bun-build-api.test.ts and rely on Bun’s default test timeout.

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Reviewing files that changed from the base of the PR and between e720920 and 534c722.

📒 Files selected for processing (5)

• src/bundler/BundleThread.rs
• src/bundler/transpiler.rs
• src/runtime/api/js_bundle_completion_task.rs
• test/bundler/bun-build-api.test.ts
• test/bundler/transpiler/transpiler.test.js

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[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[claude]

🟡 nit (optional): the new leak regression test silently passes with the fix reverted as soon as any leak: suppression matches a frame of the setup stack. Its only signal is the post-suppression SUMMARY byte count at test/bundler/bun-build-api.test.ts:1910, and the child opts into test/leaksan.supp at :1903; the PR text itself says #35309 adds leak:generate_in_new_thread, which would zero both runs. Fix: make the measurement independent of the shared suppressions file (the 1-vs-21 subtraction already cancels one-time residue, so drop suppressions= or use a test-local file) and assert LSan actually ran (verbosity=1 + "LeakSanitizer: checking for leaks", as test/cli/hot/watch-many-dirs.test.ts does). [also at: test/bundler/bun-build-api.test.ts:1903 - nit: the new leak test stops guarding the fix as soon as test/leaksan.supp gains any entry matching the bundle-thread stack (the PR itself names #35309 adding leak:generate_in_new_thread), because LSan's SUMMARY line only counts unsuppressed bytes.]

Why this was flagged

The child at test/bundler/bun-build-api.test.ts:1901-1908 is spawned with LSAN_OPTIONS=suppressions=test/leaksan.supp. LSan excludes any leak whose symbolized stack contains a frame matching a leak: substring and reports only the remainder on the SUMMARY line parsed at :1910; when nothing remains no SUMMARY line is printed and run returns 0 (:1911). The transpiler and AST-allocator leaks this test exists to catch are allocated under BundleThread::generate_in_new_thread (src/bundler/BundleThread.rs:282). The PR description states that #35309 adds a leak:generate_in_new_thread suppression to that file; once such a line lands both runs read 0, large - small is 0, and toBeLessThan(4000) at :1918 passes even if the Err-path drop_in_place at js_bundle_completion_task.rs:1297 and the stack-owned allocator at BundleThread.rs:278 are reverted. On the base branch there is no such test, so merging adds a safety net that can be neutralized without any assertion failing. Nothing in the test asserts LSan ran.

Verification: The child is spawned with suppressions=test/leaksan.supp (test/bundler/bun-build-api.test.ts:1903) and the sole measurement is the SUMMARY: AddressSanitizer: (\d+) byte(s) leaked match (:1910-1911). CheckForLeaks prints no SUMMARY line when unsuppressed_count == 0, so a suppressed leak contributes 0 to both runs and the delta assertion passes regardless of whether the Rust fix is present.

[claude]

Dropping the explicit suppressions= only helps when the test is run by hand. The child still inherits bunEnv.LSAN_OPTIONS (test/bundler/bun-build-api.test.ts:1905), and on the ASAN CI lane scripts/runner.node.ts:1098/1172/2205 set LSAN_OPTIONS=...:suppressions=${cwd}/test/leaksan.supp for the test process, so the spawned child still applies the shared file and a future leak:generate_in_new_thread entry would zero both runs there. To make the measurement actually independent, build LSAN_OPTIONS without inheriting bunEnv.LSAN_OPTIONS (e.g. LSAN_OPTIONS: "verbosity=1", optionally with malloc_context_size=30), or point suppressions= at a test-local empty file; the verbosity=1 + "checking for leaks" assertion part is fine.

[claude]

🟣 pre-existing, not blocking: pre-existing: Worker creation still has the same moved-from-stack-slot log bug this PR fixes for new Bun.Transpiler, so a failed entry-point resolve writes its Msg into dead stack memory. set_log now reseats all four aliases, but src/jsc/web_worker.rs:312 calls set_log(&raw mut temp_log) and then moves temp_log into scopeguard::guard at web_worker.rs:316. Fix: every caller must take the log pointer only after the Log reaches its final address; for the worker, build the guard first and pass &raw mut *restore to set_log, matching the reseat the constructor does at JSTranspiler.rs:1013.
A small fix can ride a push you are already making; otherwise a short reply is enough.

Why this was flagged

A parent thread calls new Worker(specifier) with a preload entry the resolver rejects; WebWorker__create at src/jsc/web_worker.rs:282 runs. Line 312 stores &raw mut temp_log into the parent transpiler's log, options.log (new in this diff), linker.log and resolver.log, then line 316 moves temp_log by value into scopeguard::guard, so those pointers now target a moved-from local. resolve_entry_point_specifier (web_worker.rs:1327) calls transpiler.resolve_entry_point, which on failure appends the error through self.log_mut() (src/bundler/transpiler.rs:525) into that dead slot, while the guard's own Log (read at web_worker.rs:1333) stays empty. The base already has it and the diff does not change it, only adds options.log to the stale alias set. The Msg pushed into the stale Vec is never freed, and the error event can carry the generic "Error resolving Worker entry point" instead of the resolver diagnostic. The existing restore guard at web_worker.rs:316-320 restores the pointer but cannot fix where the writes went.

Verification: Pre-existing: the base already fails the same way by the same route. Triggering condition: new Worker(entry, { preload: [...] }) with a preload specifier the resolver cannot resolve. web_worker.rs:312 set_log(&raw mut temp_log), then line 316 scopeguard::guard(temp_log, ...) moves the Log by value, so the pointers target the moved-from slot; on failure transpiler.rs:525 writes the Msg there, and web_worker.rs:1333 reads the guard-owned copy, which has zero msgs.

[github-actions]

If you need a paragraph-long comment to justify why the workaround is OK, the code is wrong — fix the code

[github-actions]

If you need a paragraph-long comment to justify why the workaround is OK, the code is wrong — fix the code

[github-actions]

If you need a paragraph-long comment to justify why the workaround is OK, the code is wrong — fix the code

[claude]

Beyond the inline findings, I also checked the Box refactor's drop order in generate_in_new_thread (the boxed transpiler drops before the AST scope guard, the allocator, and heap, so no Transpiler<'a> borrow outlives the arena on either the ? path or the success path), confirmed ASTMemoryAllocator::new(bump) was already identical to default() so the constructor swap is behavior-preserving, and confirmed JSBundleCompletionTask is the only CompletionStruct implementor, so no stale call site misses the new Box signature.

Extended reasoning...

The change moves Bun.build's bundle-thread Transpiler from arena placement to a Box and replaces manual reset/push/pop with an RAII scope guard in src/bundler/BundleThread.rs and src/runtime/api/js_bundle_completion_task.rs, plus a one-line set_log fix in src/bundler/transpiler.rs and two new tests. It touches no security-sensitive surface. The inline findings already signal a human look is needed (the ASAN-lane test failure is a CI blocker); this note only records the adjacent concerns that were examined and ruled out.

Findings marked 🟡 are optional suggestions and need no follow-up push.

Still open from earlier reviews (2):

• Unresolved: 2 minor or pre-existing.

[claude]

🔴 On the ASAN CI lane the new leak test fails every run: both child processes die with SIGABRT instead of exiting 1, so the toEqual at test/bundler/bun-build-api.test.ts:1918 fails before any leak delta is compared. The runner (scripts/runner.node.ts:2203) hands the test process ASAN_OPTIONS containing abort_on_error=1, which line 1903 keeps by spreading bunEnv.ASAN_OPTIONS; and symbolize=0 on that same line stops every leak: suppression from matching (src/runtime/bin_entry/mod.rs:86-91), so the detached Bundler thread's Arc is always reported. A reported leak makes LSan Die(), which aborts under abort_on_error=1. Fix: append abort_on_error=0 after the inherited options (last wins) so a residual leak exits 1 as line 1919 expects.

Why this was flagged

The ASAN lane runs this file through scripts/runner.node.ts:2200-2204, which sets ASAN_OPTIONS="allow_user_segv_handler=1:disable_coredump=0:detect_leaks=1:abort_on_error=1". test/harness.ts:66 copies process.env into bunEnv, so test/bundler/bun-build-api.test.ts:1903 produces a child ASAN_OPTIONS ending in "...abort_on_error=1:detect_leaks=1:symbolize=0". With symbolize=0 LSan cannot match function-name suppressions (src/runtime/bin_entry/mod.rs:86-91 says exactly this), so the always-present detached "Bundler" thread Arc leak (src/bundler/BundleThread.rs:148-156) is reported in both runs. When leaks are reported LSan calls Die(), and with abort_on_error=1 Die() calls Abort() rather than exiting with exitcode 1. Bun's crash handler re-raises SIGABRT with SIG_DFL (src/crash_handler/lib.rs:2927-2977), so proc.signalCode is "SIGABRT" and exitCode is null, and the expectation {exitCode: 1, signalCode: null, summaries: 1} at lines 1918-1922 fails on every CI run. The base branch has no such test, so merging adds a test that is red on the ASAN lane and never exercises the leak delta at line 1928.

Verification: scripts/runner.node.ts:2201-2205 sets abort_on_error=1 in ASAN_OPTIONS for every test file not in test/no-validate-leaksan.txt. test/bundler/bun-build-api.test.ts:1903 builds the child's ASAN_OPTIONS from bunEnv.ASAN_OPTIONS plus symbolize=0, so suppressions cannot match (src/runtime/bin_entry/mod.rs:86-91), Die() calls Abort(), and the assertion at :1918-1922 requiring signalCode: null fails.

[claude]

🟡 nit (optional): maintainers get SAFETY comments on the shared Log pointer that name the wrong owner and lifetime, so the invariant they should state cannot be checked. src/bundler/transpiler.rs:156-157 credits ThreadPool::Worker::create with passing an arena-allocated Log that outlives the transpiler, but no such call exists; every real caller passes a stack or field Log and restores it. Fix: restate each SAFETY block with the pointer's real provenance and the mechanism that bounds its use (guard restore, owner waiting on completion), which covers the 2 sites listed. Same pattern at 2 sites (src/bundler/transpiler.rs:157, src/bundler/BundleThread.rs:304).

Why this was flagged

set_log (src/bundler/transpiler.rs:152) is called from src/runtime/api/JSTranspiler.rs:1297, 1483 and 1653 with &raw mut log where log is a local dropped at method end; nothing in src/bundler/ThreadPool.rs calls it (workers use wire_after_move, ThreadPool.rs:722). The SAFETY text at transpiler.rs:156-157 therefore names a nonexistent caller and a lifetime claim ("outlives this Transpiler") that is false for every actual caller; the real bound is TranspilerStateGuard restoring the pointer at JSTranspiler.rs:1139. At src/bundler/BundleThread.rs:302-304 the SAFETY text says transpiler.log is "the arena-allocated *mut Log set up by configure_bundler; valid for the lifetime of heap"; it is actually &raw mut self.log from src/runtime/api/js_bundle_completion_task.rs:1279, and it is valid because the owner keeps the task alive until complete_on_bundle_thread at BundleThread.rs:309, not because of heap. No runtime misbehaviour today; the base carries the same text, but this PR rewrites the sibling SAFETY comments in both functions without correcting these, so the unsafe blocks remain justified by claims that do not match the code.

Verification: /home/claude/bun/src/bundler/transpiler.rs:156-157 reads "SAFETY: caller (ThreadPool::Worker::create) passes the per-worker arena-allocated Log, which outlives this Transpiler<'a>", and the diff adds self.options.log = log; at line 154. ThreadPool::Worker::create (/home/claude/bun/src/bundler/ThreadPool.rs:685) never calls set_log; the real callers are all stack/field logs with restore guards. No runtime behavior is affected.