error printer: never block on, or read from, a file that is not regular by robobun · Pull Request #42323 · oven-sh/bun

robobun · 2026-09-11T12:42:53Z

Problem

bun a.mjs never exits when a.mjs throws after its own file became a FIFO. The process stays in open(2) (kernel wait_for_partner). A //# sourceURL or an assigned e.stack that names a FIFO does the same. With a writer, it consumes the piped bytes and stays in read(2).
remap_zig_exception (src/jsc/VirtualMachine.rs) reads files by path with a blocking open and no file type check. The top frame's file goes through FetchFlags::PrintSource to cache::Fs::read_file_with_allocator. It also reads <file>.map (src/sourcemap/lib.rs:531, which a bare e.stack reaches), a source the map names (src/sourcemap/Mapping.rs:401), and the directory's package.json.

Fix

bun_sys::File::open_regular_at opens with O_NONBLOCK. An fstat of the descriptor then fails unless the file is regular (ENODEV, or EISDIR), before a byte is read. read_file_with_allocator takes a NonRegularFile policy, and only the PrintSource fetch passes Reject.
The two source map reads use File::read_regular_from. The printer's fetch no longer looks up package.json: it never used the result.
Correct because a module load is unchanged, and the printer already handles a failed read (error printer: remap frames when the original source is unavailable, and not twice after error.stack #38296, a deleted file): it remaps the frames and prints the lines JSC holds.
Verified: test/js/bun/util/inspect-error.test.js (seven new tests, each times out on 1.4.3). test/js/bun/sourcemap/, vm.test.ts, stack.test.ts, Node's test-compile-cache-*.

Background

A code frame is the source excerpt above a printed error. JSC keeps only a module's transpiled text, so the printer reads the original file again.
FetchFlags::PrintSource is the module loader's fetch with transpiling off. It returns the source text.
A read-only open(2) of a FIFO blocks until a writer opens it, unless O_NONBLOCK is set.

Notes

This is a fuzzer finding (fuzz ledger entry 33881, not a GitHub issue number). No user reported it. BUN_DISABLE_SOURCE_CODE_PREVIEW=1 avoids the code frame read, not the .map read.

Repro on 1.4.3:

cat > a.mjs <<'EOF'
import { unlinkSync } from "node:fs";
import { spawnSync } from "node:child_process";
unlinkSync(import.meta.path);
spawnSync("mkfifo", [import.meta.path]);
console.log("about to throw");
throw new Error("boom");
EOF
bun a.mjs   # prints "about to throw", then nothing, never exits

Opening and closing the write end from another shell (: > a.mjs) releases it: the report prints and bun exits 1. With this change it prints the error and exits 1 at once:

about to throw
2 | import { unlinkSync } from "node:fs";
3 | import { spawnSync } from "node:child_process";
4 | unlinkSync(import.meta.path);
5 | spawnSync("mkfifo", [import.meta.path]);
6 | throw Error("boom");
              ^
error: boom
      at /tmp/x/a.mjs:6:11

The excerpt is the transpiled text JSC holds (collect_source_lines), the same fallback a deleted file gets today. Node v26 prints the line it holds in memory and exits 1 for the replaced module and for the sourceURL case.

The new tests are in the source map remapping of the printed stack block, next to the deleted-file test. Each child blocks forever on 1.4.3:

the file of a transpiled module is a FIFO with no writer: Bun.inspect(e) returns and the uncaught error prints, frames remapped to swapped.ts:4, exit code 1;
the same with a writer that put 16 bytes in the pipe: the 16 bytes are still in the pipe after the child exits (before, the child consumed them and stayed in read);
node:vm code whose //# sourceURL names a FIFO;
a package.json that is a symlink to a FIFO, in a directory that only a sourceURL names;
a string assigned to e.stack whose frame names a FIFO (the route through ZigException.cpp, no code runs under that name);
main.js.map next to a // @bun file is a FIFO: e.stack and the uncaught print both return, frames unmapped;
a map whose sources[0] is a FIFO and sourcesContent is [null]: frames remapped to orig.ts.

An afterAll in the block kills any child that is still alive, so a regression cannot leave processes behind. Without it the hung children outlived the bun test run. It is afterAll and not afterEach because the tests are concurrent: afterEach runs while sibling tests are in flight and is not told which test it runs for, and onTestFinished throws in a concurrent test.

The package.json lookup: bun_bundler::options::get_loader_and_virtual_source has one caller, fetch_without_on_load_plugins, and that has one caller, the printer, with PrintSource. The lookup gave a module type that PrintSource does not use. In a directory the resolver had not seen, it read the directory, package.json and tsconfig.json. Removed: the LoaderResult::package_json field, the read_dir_info_package_json slot of VmLoaderCtx, its implementation in jsc_hooks.rs, and Loader::is_js_like, whose one caller was the lookup. One other consumer of the module type was on that path, the Node compile cache note for a module that failed to parse. It now runs for real loads only: a failed re-read by the printer is not a parse failure.

Same-class sites left alone, on purpose:

The other four callers of read_file_with_allocator (package.json, tsconfig.json, the bundler's parse task, the CSS build) and the other four ParseOptions sites pass NonRegularFile::Read. They read right after the resolver found the path, the resolver skips FIFO directory entries, and the arena reader avoids fstat for files under 16 KB on purpose. An import of a symlink that points at a FIFO still blocks in open, as cat would.
File::read_from keeps its behavior for its other callers. Read whole files only when they are regular files #39734 (open, has conflicts) changes read_from itself after a review of each caller. open_regular_at and ensure_regular here are that PR's helper, with the same names, flags and errnos, so it can drop its copy on rebase. The same open, fstat, ISREG sequence is inline in RuntimeTranspilerCache.rs (Verify a transpiler cache entry before any field of it is used #39717), env_loader.rs (dotenv: skip .env entries that are not regular files #40711) and Image.rs. Moving those onto the helper is a follow-up.
No size cap. A regular file reads in bounded time, and the loader has no cap for the same file.
On Windows there is no O_NONBLOCK (it would make the handle overlapped). The fstat check still applies there.

Related open PRs:

error: do not read a frame's source URL from disk unless the loader loaded it #41477 stops the disk read for a frame URL the loader did not load. With it the sourceURL, package.json and e.stack tests pass on their own. A module the loader did load stays trusted there and is still read back, so the replaced-module tests need this change.
Module loading: remove unsafe from ModuleLoader, AsyncModule and RuntimeTranspilerStore #40383 rewrites ModuleLoader, AsyncModule and RuntimeTranspilerStore. The new ParseOptions field is one line in each of four files it touches.

Self-reviewed: the first version gated only the code frame read, with an inline copy of the check and a bool parameter. The review showed that the .map, sources[i] and package.json reads still blocked on that build (each exit 124 under timeout), so they are in this PR. The bool became the two-value NonRegularFile enum, and the check moved into the bun_sys helper.

Other checks: 500 rejected reads in one process leave the fd count unchanged (10 before, 10 after). cargo clippy on bun_sys, bun_resolver, bun_sourcemap, bun_bundler. cargo check of those for x86_64-pc-windows-msvc and aarch64-apple-darwin. inspect-error-leak.test.js and error-gc-test.test.js fail only by their time limits on the ASAN debug build (the leak test's RSS assertion passes, #39401 tracks its sizing).

no test proof · iteration 2 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/util/inspect-error.test.js

The error printer reads files by path when it prints an error: the top frame's file for the code frame, the <file>.map sidecar, and an original source that a source map names. Each was a blocking open(2) with no file type check. When one of those paths named a FIFO, the process stayed in open() forever and never printed the error or exited. With a writer on the FIFO it took the bytes in the pipe and stayed in read(). The <file>.map read is also reached by a bare error.stack. - bun_sys::File gets open_regular_at, ensure_regular and read_regular_from: O_NONBLOCK on unix, then an fstat of the descriptor that fails with EISDIR or ENODEV unless the file is regular. - cache::Fs::read_file_with_allocator takes a NonRegularFile policy. Only the PrintSource fetch passes Reject, so a module load keeps its single open and no fstat. - The two source map reads use read_regular_from. - The printer's fetch no longer looks up the directory's package.json. It never used the result, and the lookup scans a directory that the frame's URL chooses.

robobun · 2026-09-11T12:43:14Z

Status: ready for review. The new tests pass on every platform. The two red jobs in CI are not from this diff.

How I reproduced it, on the released 1.4.3 and on main:

cat > a.mjs <<'EOF2'
import { unlinkSync } from "node:fs";
import { spawnSync } from "node:child_process";
unlinkSync(import.meta.path);
spawnSync("mkfifo", [import.meta.path]);
console.log("about to throw");
throw new Error("boom");
EOF2
timeout 5 bun a.mjs; echo $?   # "about to throw", then nothing, 124

The same timeout probe gives 124 for a FIFO at main.js.map, at a map's sources[0], at a //# sourceURL, in an assigned e.stack, and for a package.json symlink to a FIFO in a directory a sourceURL names. With this branch each prints the error and exits at once.

USE_SYSTEM_BUN=1 bun test test/js/bun/util/inspect-error.test.js: the 7 new tests time out, 29 pass. bun bd test on this branch: 36 pass.

CI on 94c7624 (build 114285, 179 jobs passed, 2 failed):

test/js/bun/util/inspect-error.test.js passes on Linux (glibc, musl, ASAN), Windows x64 and aarch64, and macOS aarch64 and x64 (36 of 36 on both macOS x64 hosts). The source map, node:vm and compile cache suites pass too.
Red job 1: test/js/bun/http/serve-pending-promise-abort-leak.test.ts on debian 13 x64-asan. This PR does not touch the HTTP server or streams, and the same test fails on main. It is reported for main-break triage.
Red job 2: one of the two macOS x64 shards, on host darwin-x64-rye. The host lost its checkout directory in the middle of the job at 13:27 UTC (bun-profile: No such file or directory, then the runner died in os.userInfo()). Four other Intel hosts failed the same way in the same minute in builds 114287 and 114288. The other macOS x64 shard passed.
The other entries passed on retry (bun-lock, bun-install-registry, bun-patch, public-hoist-pattern, process-stdin) or passed alone (color.test.ts, and shell/exec.test.ts, expect-assertions.test.ts, napi/uv.test.ts, where children exit 17 where 1 is expected). That last group also shows on the builds of two other branches that contain the WebKit upgrade (Upgrade WebKit to cf1b36ec8703 #42319), 114269 and 114279, and on none without it.

coderabbitai · 2026-09-11T12:56:27Z

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Walkthrough

The change adds regular-file read policies and propagates them through parsing and transpilation. It removes package JSON module-type lookup, changes sourcemap reads, and adds FIFO-based error-inspection regression tests.

Changes

Regular-file read enforcement

Layer / File(s)	Summary
Filesystem regular-file policy `src/sys/file.rs`, `src/resolver/lib.rs`	Adds regular-file opening, validation, and reading helpers. Resolver reads now apply `NonRegularFile::Read` or `Reject`.
Parser read-policy propagation `src/bundler/...`, `src/jsc/...`, `src/runtime/...`, `src/resolver/package_json.rs`, `src/resolver/resolver.rs`	Adds `ParseOptions.non_regular_file` and passes explicit policies through parser, transpiler, CSS, package JSON, and tsconfig reads.
Package metadata path removal `src/bundler/lib.rs`, `src/bundler/options.rs`, `src/jsc/VirtualMachine.rs`, `src/runtime/jsc_hooks.rs`, `src/ast/loader.rs`	Removes package JSON metadata from loader results and VM-loader dispatch. The fetch path uses `ModuleType::Unknown`. The `Loader::is_js_like` predicate is removed.
Sourcemap and FIFO regression coverage `src/sourcemap/...`, `test/js/bun/util/inspect-error.test.js`	Uses regular-file reads for sourcemaps and source fallbacks. Adds FIFO regression cases, process cleanup, and updated snapshots.

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to cb18c

The change is mergeable with awareness that a failing FIFO regression can leave its child running until group cleanup. Per-test child ownership would improve cleanup; no concrete ordinary module-loading regression was established.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly and concisely describes the primary change: preventing the error printer from blocking on or reading non-regular files.
Description check	✅ Passed	The description clearly explains the problem, implementation, scope, regression tests, verification results, and known unrelated failures. It does not use the template headings exactly, but it provide…

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In `@src/runtime/jsc_hooks.rs`:
- Around line 2607-2613: Replace the direct args.flags ==
FetchFlags::PrintSource check in the non_regular_file selection with the
existing disable_transpiling boolean, preserving Reject for the non-transpiling
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src/resolver/lib.rs
src/resolver/package_json.rs
src/resolver/resolver.rs
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It is the same condition as the PrintSource comparison today, and it is the boolean the neighboring ParseOptions fields already use.

robobun · 2026-09-11T13:01:26Z

^{Updated 6:45 AM PT - Sep 11th, 2026}

❌ @robobun, your commit 94c7624 has 1 failures in Build #114285 (All Failures):

test/js/bun/http/serve-pending-promise-abort-leak.test.ts - code 1 on 🐧 13 x64-asan

🧪 To try this PR locally:

bunx bun-pr 42323

That installs a local version of the PR into your bun-42323 executable, so you can run:

bun-42323 --bun

The compile cache note goes back to its place, now under the same condition as the watcher registration above it.

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Inline comments:
Review comments at @test/js/bun/util/inspect-error.test.js:
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src/resolver/lib.rs
src/resolver/resolver.rs
src/runtime/api/JSTranspiler.rs
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claude

Code review found no issues

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Its one caller was the package.json lookup in get_loader_and_virtual_source, which this branch removes.

robobun assigned Jarred-Sumner Sep 11, 2026

github-actions Bot added the claude label Sep 11, 2026

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Comment thread src/runtime/jsc_hooks.rs Outdated

Select the non-regular file policy from disable_transpilying

03fd5dd

It is the same condition as the PrintSource comparison today, and it is the boolean the neighboring ParseOptions fields already use.