Bun.file: read a regular file whose st_size is 0 instead of returning empty

[robobun]

Problem

• Since webcore: clone() keeps the Blob behind an unread native body stream instead of teeing it #42053, new Response(Bun.file("/proc/version")).clone() makes both copies and the Bun.file itself read "". A fresh Bun.file("/proc/version").text() returns the content.
• ReadFile::run_async_with_fd (src/runtime/webcore/blob/read_file.rs:757) returns an empty buffer when its fstat reports 0 bytes and the store's cached mode says regular file. A procfs file is a regular file with st_size 0 and real content. Only an earlier stat of the shared store sets that cached mode, and clone() now stats it through store_reads_repeatably.

Fix

• Drop the shortcut. A regular file whose fstat reports 0 bytes is read until EOF, like every other file.
• A truly empty file still reads as "": the first read() returns 0. A file with a nonzero st_size never reached the shortcut.
• Split out of Leave a body stream unsized when the file's st_size is 0 #41593 as requested there. Leave a body stream unsized when the file's st_size is 0 #41593 is stacked on this PR.
• Verified: body-clone.test.ts (clone(): both copies and the Bun.file) and body.test.ts (text() after the body getter stat'd the file) in test/js/web/fetch/, Linux only. They fail with src/ at main.

Background

• Every Blob over one Bun.file() shares a File store. resolve_file_stat caches st_size and the mode on it when .size, exists(), .body or clone() first needs them.
• ReadFile is the thread-pool reader behind .text(), .bytes(), .arrayBuffer() and .json(). It runs its own fstat, but the shortcut tested the store's cached mode.

Downsides

• .text() on an empty regular file whose store was stat'd earlier costs one read(): clone() then text() on both copies goes from 0.04 to 2.04 read syscalls. Nonzero files are unchanged (table in Notes).
• Still broken: f.size or await f.exists(), then f.text() on the same procfs Bun.file, reads "". Those copy size 0 onto the blob itself, so the read has a budget of 0 bytes.

Notes

read syscalls per operation, from /proc/self/io (syscr) over 200 iterations, debug builds of main (2838e1b) and of this branch. The fractions are noise from unrelated reads.

                                             main    this branch
Bun.file(empty).text()                       1.02    1.02
Bun.file(64 KiB).text()                      2.05    2.04
Bun.file(256 KiB).text()                     2.14    2.14
r.clone(), text() on both (empty file)       0.04    2.04
r.clone(), text() on both (64 KiB)           4.10    4.10
r.clone(), text() on both (256 KiB)          4.25    4.22

On main, /proc/self/status (about 1.5 KB):

                                        main     this branch
Bun.file(P).text()                      1534     1534
r.clone().text() / r.text()             0 / 0    1534 / 1534
f.text() after new Response(f).clone()  0        1534

Other suites run locally: body.test.ts, blob.test.ts, fs.test.ts, bun-file.test.ts, bun-file-exists.test.js.

Windows keeps its ReadFileUV shortcut: it tests the mode from the fstat it just ran, and NTFS regular files do not under-report their size.

An earlier revision of this PR also carried a rule for S3 body streams. A review showed it was partial at that layer (after Bun.inspect(request) the stream still asked for bytes=0-0), so it is out of this PR and tracked separately.

The stream half of the procfs problem (new Response(Bun.file(procfs)).body is an empty stream) stays in #41593, which is stacked on this PR.

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no test proof · iteration 8 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/web/fetch/body.test.ts, test/js/web/fetch/body-clone.test.ts

[robobun]

Status: reproduced on main (2838e1b). new Response(Bun.file('/proc/version')).clone() makes the clone, the original and the Bun.file read "". The fix removes the empty-file shortcut in ReadFile.

Tests, both Linux only, in test/js/web/fetch/: body-clone.test.ts ("Response over a procfs Bun.file(): both copies and the Bun.file() read it") and body.test.ts ("the body getter does not make a later text() on the same Bun.file() empty"). They fail with src/ at main and pass on this branch.

Split out of #41593 as requested there. #41593 is stacked on this PR. An S3 rule that was here for a few hours is out again: it was partial at that layer and is tracked separately.

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Walkthrough

The POSIX read path no longer exits early when a regular file has a resolved size of zero. Linux-only tests check body reads and response cloning with /proc/version.

Changes

Procfs response reads

Layer / File(s)	Summary
Read zero-size files and verify procfs body access src/runtime/webcore/blob/read_file.rs, test/js/web/fetch/body-clone.test.ts, test/js/web/fetch/body.test.ts	Regular-file reads with a resolved size of zero continue through buffer setup and the read loop. Linux-only tests check /proc/version body access and response cloning.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 4709d

The procfs regression tests now check for nonempty content, and no remaining issue is established that should block merging.

🚥 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 identifies the main change: reading regular files whose reported st_size is zero instead of returning an empty result.
Description check	✅ Passed	The description explains the problem, fix, verification, scope, limitations, and performance impact. It does not use the template headings exactly, but it provides the required change summary and veri…

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Inline comments:
In `@test/js/web/fetch/body.test.ts`:
- Line 362: In the test around `Bun.file(path).text()`, assert that `expected`
is non-empty before comparing it with the other read, so two empty reads cannot
pass the equality check.

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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.

Beyond the inline findings, two things were checked and ruled out: the truly-empty regular-file case after the deleted shortcut (the fall-through allocates a 16-byte buffer, the first read() returns 0, read_eof is set and on_finish runs, so "" is still the result), and the wasm_streaming.rs call-site switch (it cannot observe the S3 branch, since new Response(s3File) builds a 302 with a Null body in Response.rs, so that hunk is inert rather than wrong).

Extended reasoning...

The diff removes the size==0/regular-file short circuit in ReadFile (src/runtime/webcore/blob/read_file.rs), adds Blob::resolve_size_for_stream that skips resolve_size for S3 blobs, and rewires three stream-construction call sites in Body.rs and wasm_streaming.rs, plus two new tests. It touches no auth, crypto or injection surface. Five confirmed inline findings (procfs reads still empty after a prior stat, S3 range still 0-0 when size resolved elsewhere, Windows twin left with the shortcut, EAGAIN spin in do_read, non-seekable device stream truncation) already signal that a human should review, so this body only records what else was examined.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🟣 src/runtime/webcore/blob/read_file.rs — pre-existing, left by this partial fix: on Windows, a user reading a regular file whose fstat says 0 bytes but that has content still gets "", on every read, with or without a prior stat. ReadFileUV::on_file_initial_stat keeps the shortcut at read_file.rs:1263 that the POSIX twin just deleted, so files exposed through \\wsl$\<distro>\proc\... (9P passes the Linux st_size of 0 through) or WinFsp/Dokan mounts that report 0 are never read. Fix: drop this shortcut too, or gate it on something stronger than st_size == 0, so both platforms read a 0-byte regular file to EOF; the PR note that NTFS regular files do not under-report only covers NTFS, not every filesystem a Win32 path can name.

  Why this was flagged

  Input: await Bun.file("\\\\wsl$\\Ubuntu\\proc\\version").text() (or any file on a user-mode filesystem that reports st_size 0 with content) on Windows, entering through Blob::do_read_file at Blob.rs:444, which starts ReadFileUV. on_file_initial_stat at read_file.rs:1245 leaves…

  Verification: pre-existing; acknowledged in diff: the PR description states "Windows keeps its ReadFileUV shortcut: it tests the mode from the fstat it just ran, and NTFS regular files do not under-report their size" — that bound holds for NTFS but does not cover non-NTFS/user-mode filesystems on Windows. Triggering condition: on Windows, Bun.file(...).text()/bytes()/arrayBuffer() on a file whose fstat…

• 🟣 src/runtime/webcore/blob/read_file.rs — A Bun.file(path).text() on a regular file whose read() returns EAGAIN now spins a work-pool thread forever and never settles, where the base short-circuited many of those reads. do_read computes result once at read_file.rs:550-555, then continue at read_file.rs:569 re-matches the same stale Err(EAGAIN) without re-issuing read(), so the loop never exits. This PR removes the size==0 shortcut at read_file.rs:755, so every stat'd zero-st_size regular file now reaches this loop; files are opened with O_NONBLOCK (Blob.rs:6469), and FUSE/network filesystems can return EAGAIN for regular files. Fix: re-issue the read() inside the loop (or return retry=true) so an EAGAIN on a could_block=false fd cannot spin.

  Why this was flagged

  Trigger: a Bun.file() on a filesystem whose regular-file read() can return EAGAIN under O_NONBLOCK (FUSE daemons pass errno through; the fd is opened with OPENER_FLAGS = O::NONBLOCK | O::CLOEXEC at src/runtime/webcore/Blob.rs:6469). resolve_size_and_last_modified sets could_block = false for S_ISREG at read_file.rs:724. do_read evaluates result once at read_file.rs:550-555 and then loops at :557; the e == io::RETRY arm with !self.could_block executes continue at :569, which re-enters match &result on the identical error value. No syscall is re-issued, no counter advances, and state is not consulted, so the work-pool thread busy-loops forever and the .text()/.bytes()/.json() promise never settles. The dismissal argued regular files never return EAGAIN, but the diff itself widens the population reaching this loop: before, a regular file with st_size 0 whose store had been stat'd (clone(), .size, exists()) returned an empty buffer at the deleted read_file.rs:755 block without calling read(); now every such read goes through do_read. Remedy: move the read()/recv()…

  Verification: pre-existing. Mechanism verified: in /home/claude/bun/src/runtime/webcore/blob/read_file.rs do_read (lines 550-555) evaluates let result = ... bun_sys::read(self.opened_fd, buf) exactly once, OUTSIDE the loop at line 557; the e if e == io::RETRY arm (line 565, io::RETRY = E::EAGAIN at src/io/lib.rs:1773) does if !self.could_block { continue; } at line 569, which re-enters `match…

[robobun]

^{Updated 8:44 PM PT - Sep 23rd, 2026}

✅ @robobun, your commit 4709d6593a4265045dc34253cc595e732344cd7f passed in Build #120177! 🎉

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[claude]

I reviewed the new push and found no bugs in the code, but a human should confirm the PR's intended scope: the title and description say this PR adds Blob::resolve_size_for_stream for S3 bodies (Body.rs, wasm_streaming.rs) plus an S3 Range test in body.test.ts, and none of that is in the diff — the branch only removes the 9-line ReadFile shortcut and adds the two procfs tests.

What was reviewed:

• The fall-through after removing the shortcut in run_async_with_fd: size stays 0 for a regular file with st_size 0, so it reserves 16 bytes and do_read_loop reads through the 64 KB stack buffer to EOF; a genuinely empty file takes one read() returning 0 and finishes with an empty buffer, and an explicit slice(0, 0) still yields an empty target from remaining_buffer.
• The Windows ReadFileUV twin keeps its shortcut, which is fine since it tests the mode from its own fstat and there is no procfs there.
• Both tests fail on the pre-fix code for the stated reason (clone() and .body both reach resolve_file_stat, which caches the regular-file mode that the shortcut keyed on) and assert the non-empty precondition before relying on it.

Extended reasoning...

The diff deletes a 9-line early exit in src/runtime/webcore/blob/read_file.rs (non-Windows ReadFile) that returned "" when the cached store mode said regular file and fstat reported st_size 0, and adds two Linux-only tests over /proc/version in test/js/web/fetch/body-clone.test.ts and body.test.ts. It touches no security-sensitive surface. The code change is small and behavior-preserving for non-empty and non-regular files, and the read loop handles the zero-reservation case correctly. Deferring rather than approving because the PR title and description describe an S3 stream-size change and an S3 test that are absent from this branch, so the intended scope needs a human check; the previously noted remaining gap (.size/exists() before text() on the same procfs Bun.file) is acknowledged by the author and was already raised.

Still open from earlier reviews (2):

• Unresolved: 2 minor or pre-existing.