Bun.serve: route a missing file to error() for HEAD like for GET

[robobun]

Problem

• Bun.serve({ fetch: () => new Response(Bun.file('/nonexistent')) }) answers a GET with the error() handler (or the default 500), but answers a HEAD for the same URL with 200 and content-length: 0.
• Cause: do_render_head_response (src/runtime/server/RequestContext.rs) sizes a file-backed body with blob.resolve_size(). That goes through resolve_file_stat, which ignores a failed stat() by design ("the file may not exist yet"), leaves the size unknown, and resolve_size() then reports 0. The GET path (do_sendfile) opens the file and routes the error to run_error_handler.

Fix

• Split resolve_file_stat into stat_file_store() -> bun_sys::Result<()> (stats and records size, mode, seekability and mtime; the error carries the path or fd) and the tolerant wrapper that the .size getter and writers keep using.
• The HEAD arm calls stat_file_store() before resolve_size(). On error it calls run_error_handler(err.to_js(global)) and returns, before any status or header is written, so the error handler's Response (or the default 500) is rendered exactly as for GET.
• Verified: test/js/bun/http/bun-serve-file.test.ts ("a missing file returned from the fetch handler reaches error() for HEAD like for GET") fails on 1.4.3 (HEAD gives 200, content-length: 0, error() not called) and passes with the fix. The existing HEAD tests in that file and in bun-server.test.ts ("HEAD requests Bun always sets Content-Length to 0 for HEAD response #15355", empty files for GET and HEAD alike) pass.

Background

• Bun.serve does not run the GET body path for HEAD. on_response branches to do_render_head_response, which only computes the framing headers (content-length or transfer-encoding) from the body and ends the response without a body.
• Bun.file(path) is lazy: nothing touches the file system until the size or the bytes are needed. Blob::resolve_size() is the shared "make .size concrete" helper and must tolerate a missing file, because Bun.file(path).size on a path that does not exist yet is valid.
• run_error_handler calls the server's error(err) callback when set and renders the Response it returns; otherwise it writes the production 500 (or the dev error page in development).

Notes

• Only the missing-file (failed stat) case changes. A directory still takes the old path on HEAD (GET reports EISDIR from do_sendfile); that needs the directory check duplicated and was left out to keep this change small.
• The S3 arm of the same function already resolves the size asynchronously and maps a failed stat to content-length: 0; unchanged here.

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no test proof · iteration 1 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/http/bun-serve-file.test.ts

[coderabbitai]

Walkthrough

Changes

The blob implementation now centralizes file-stat operations and returns stat errors. HEAD response rendering handles missing file errors through run_error_handler. A regression test covers GET and HEAD responses for missing files.

File Blob Error Handling

Layer / File(s)	Summary
Centralize file statistics src/runtime/webcore/Blob.rs	stat_file_store performs file and descriptor statistics, updates store metadata, and returns stat errors. resolve_file_stat uses the helper.
Handle missing files during response rendering src/runtime/server/RequestContext.rs, test/js/bun/http/bun-serve-file.test.ts	HEAD responses stat unread file blobs before resolving size. Stat failures reach run_error_handler. The regression test validates matching GET and HEAD error responses and ENOENT reporting.

Suggested reviewers: jarred-sumner, dylan-conway, alii

Merge Risk: 🔵 Low · up to 37990

HEAD responses for missing Bun.file() values now use the configured error handler like GET responses, with regression coverage for both methods. Runtime behavior is covered, but the new test should be parameterized with describe.each() for independent method-level reporting.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: missing files in HEAD responses now route to the error handler, matching GET behavior.
Description check	✅ Passed	The description clearly explains the problem, root cause, implementation, behavior changes, verification, and known scope limits. It does not use the template headings exactly, but it provides the req…
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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[robobun]

^{Updated 3:17 AM PT - Sep 8th, 2026}

❌ @robobun, your commit 379907b has 1 failures in Build #112605 (All Failures):

• test/js/node/test/parallel/test-crypto-dh-leak.js - code 1 on 🐧 13 x64-asan

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🧪   To try this PR locally:

bunx bun-pr 41896

That installs a local version of the PR into your bun-41896 executable, so you can run:

bun-41896 --bun

[robobun]

Reproduced on 1.4.3-canary (f42e98025) with Bun.serve({ port: 0, fetch: () => new Response(Bun.file('/nonexistent-file')) }): GET returned 500 (ENOENT through the default error path), HEAD returned 200 with content-length: 0.

The new test in test/js/bun/http/bun-serve-file.test.ts fails on the stock binary (HEAD gives 200/content-length: 0 and error() is called once, for GET only) and passes on this branch; the rest of that file and the HEAD tests in bun-server.test.ts pass.

CI: the only red lane is test/js/node/test/parallel/test-crypto-dh-leak.js on x64-asan, which fails the same way on main and on unrelated PRs; everything else passed on retry. Nothing in the failure set touches this diff.

[claude]

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

Beyond the inline finding, I checked the .expect("file blob has a store") at RequestContext.rs:2666 — needs_to_read_file() (src/jsc/webcore_types.rs:420) only returns true when store is Some with Data::File, so the unwrap is a provable invariant. Also confirmed stat_file_store takes the same as_file_mut() path resolve_file_stat did, so an S3 store can't reach it (guarded by blob_is_s3 above and by needs_to_read_file()).

Extended reasoning...

The inline nit about resolve_file_stat now Box-allocating the path on every failed stat stands on its own. I separately verified two things a reviewer would likely check under REVIEW.md's ".unwrap() is for provable invariants only" and "fix at the layer that owns the invariant" rules: the .expect() on blob.store.get() is guarded by needs_to_read_file(), whose definition at src/jsc/webcore_types.rs:420-422 matches only Some(s) if matches!(s.data, store::Data::File(_)), so the store is guaranteed present and file-typed; and the new stat_file_store still enters via Store::data_mut(store).as_file_mut(), so it cannot be misapplied to an S3 store from the HEAD branch (the S3 arm is handled earlier and returns).

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@test/js/bun/http/bun-serve-file.test.ts`:
- Around line 878-887: Replace the loop over GET and HEAD in the HTTP method
test with describe.each() parameterization, creating an independent test case
for each method while preserving the existing request, response assertions, and
result collection behavior.

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Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
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Reviewing files that changed from the base of the PR and between b52d3e5 and 379907b.

📒 Files selected for processing (3)

• src/runtime/server/RequestContext.rs
• src/runtime/webcore/Blob.rs
• test/js/bun/http/bun-serve-file.test.ts

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[claude]

LGTM — the follow-up commits addressed the earlier allocation nit (path/fd context is now attached only at the RequestContext caller, so the tolerant resolve_file_stat wrapper stays allocation-free).

What was reviewed:

• stat_file_store refactor is behavior-preserving for existing callers: same fields written, same "may not exist yet" tolerance via let _ = in resolve_file_stat.
• HEAD error path mirrors do_sendfile's open/fstat error handling (RequestContext.rs:1803, 1818) — same with_path/with_fd decoration and run_error_handler call, before any bytes are written.
• .expect("file blob has a store") follows the established invariant pattern at fetch.rs:1424; needs_to_read_file() guarantees the store.
• Test uses port: 0, using cleanup, and asserts the full {status, headers, body} and {code, path} shape for both methods with .toEqual.

Extended reasoning...

Overview

The PR fixes an asymmetry between GET and HEAD in Bun.serve when the fetch handler returns new Response(Bun.file(missingPath)). GET already routed the open failure through run_error_handler; HEAD silently answered 200 with content-length: 0 because resolve_file_stat swallows stat errors by design. The fix extracts the stat-and-record logic into stat_file_store() -> bun_sys::Result<()> (deduplicating the previously copy-pasted path/fd arms), keeps resolve_file_stat as a tolerant wrapper for .size/.lastModified callers, and has the HEAD arm call stat_file_store directly so a failure reaches error() before render_metadata() writes anything.

Security risks

None. The change routes an already-observable ENOENT to the user's error() handler for HEAD the same way it already does for GET. No new input parsing, no auth/permission surface, no untrusted-length arithmetic. The error object carries the same code/path the GET path already exposes.

Level of scrutiny

Moderate — Bun.serve's response path is hot and correctness-critical, but this diff is small, additive, and copies the exact shape of the adjacent GET failure path (do_sendfile at RequestContext.rs:1796–1824). The Blob.rs refactor is a straight deduplication with identical field writes; the tolerant wrapper's semantics are unchanged. The .expect() on the store is guarded by needs_to_read_file() and matches the precedent at fetch.rs:1424, so it encodes a real invariant rather than a user-reachable panic.

Other factors

Since my earlier COMMENTED review, two commits landed that moved the with_path/with_fd decoration out of stat_file_store and into the one caller that surfaces the error, addressing the allocation-on-discarded-error nit. The coderabbit inline on the test was resolved by a non-author. No CODEOWNERS cover these files. The new test lives in the correct existing file, covers both methods, and asserts the strongest observable contract (status, header, body, error code, error path) with a single .toEqual per collection. The PR description honestly scopes out the directory (EISDIR) HEAD case as future work.

[robobun]

Same bug as #41585: HEAD for a file-backed Response sized the body with a bare stat instead of taking the GET path, which opens the file and routes the failure to error(). Consolidated there. #41585 sends the file body through do_sendfile for HEAD too, so it also covers the directory case and Range parity, and it now includes this PR's test. Closing in favor of #41585.