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Status at 704fa30, six commits on main 9f70da0. This PR holds the Reproduced on 1.4.3 (canary 367d939) with a 26-byte file and
The two new blocks in The stdin half ( |
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WalkthroughChangesFile-backed Blob slices now preserve source offsets and effective sizes during ChangesFile View Boundaries
Suggested reviewers: Merge Risk: 🟡 Moderate · up to The change correctly handles many sliced-file cases, but some non-seekable slices can still copy the wrong range, and concurrent file growth on Windows can exceed the requested bound. These correctness issues should be fixed before merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@src/runtime/webcore/blob/copy_file.rs`:
- Line 897: Update fcopyfile_serves_view at
src/runtime/webcore/blob/copy_file.rs lines 897-897 to select fcopyfile only
when source_size == MAX_SIZE, and update the FreeBSD path at lines 793-795 to
use do_read_write_loop_capped(self.max_length) for finite source views instead
of the uncapped NodeFS helper; retain whole-source paths for unbounded views.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/runtime/webcore/blob/copy_file.rs— pre-existing: the FreeBSD path-destination arm still callsNodeFS::copy_file_using_read_write_loop, which drains the source to EOF and ignoresmax_length; a bounded non-regular source such asBun.file("/dev/zero").slice(0, 10)still never settles on FreeBSD (as on main), and a regular-file slice copies from the seek offset to EOF and only trims viaftruncate. Fix: gate the FreeBSD Path branch the same way the macOS branch is gated — fall through todo_read_write_loop_capped(self.max_length)whensource_offset != 0or!is_regular && source_size != MAX_SIZE, so every POSIX arm honours the source view.Extended reasoning...
The PR adds
self.max_length = self.max_length.min(self.source_size)for non-regular sources and seeks regular sources tosource_offset, then updates the Linux fallback and the macOSfcopyfilegate to respect the cap, but leaves the#[cfg(target_os = "freebsd")]block untouched. That block, when the destination is a path, callsnode_fs::NodeFS::copy_file_using_read_write_loop(..., stat_size=0, ...), which the PR itself notes "treats its length as a stat hint and drains the source to EOF past it". For/dev/zero.slice(0, N):is_regularis false,max_lengthis set to N, but the loop reads /dev/zero forever — the promise never resolves, exactly as on the base branch. For a large regular-file slice, the loop copies fromsource_offsetall the way to EOF and only afterwards ftruncates tomax_length, so a 10-byte slice of a 10 GB file writes ~10 GB before trimming. REVIEW.md requires sibling platform arms to receive the same class fix; the macOS arm got thefcopyfile_serves_viewgate + capped loop but FreeBSD did not.Verification: pre-existing — the FreeBSD
Path-destination arm at src/runtime/webcore/blob/copy_file.rs:928-958 is byte-identical to base and still callsnode_fs::NodeFS::copy_file_using_read_write_loop(..., stat_size=0, ...). That helper (src/runtime/node/node_fs.rs:4572-4653) withstat_size=0skips the sized loop (remain=0) and falls into theif !broke { loop { ... } }block whose comment reads…
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Inline comments:
In `@src/runtime/webcore/Blob.rs`:
- Around line 2291-2293: Update the Blob slice handling around the offset/size
early return to preserve whether the slice was explicitly bounded, rather than
inferring a whole-file view solely from offset 0 and a size equal to MAX_SIZE or
file.max_size. Ensure later file growth keeps the original slice range during
copying, and add Bun.write and spawn regressions covering this sequence.
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Review round, addressed in be768c3:
Not changed: |
On macOS a clone has the mode of the source. truncate(2) fails on a clone of a read-only file, so the destination held the whole file. The copy now removes that clone and copies the bytes of the window. The capped copy loop and NodeFS::copy_file_using_read_write_loop are one loop now. The loop reads until EOF or until the cap.
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All three findings of this review are addressed in e5bdfd6. Each thread has the details.
Not run on macOS or Windows. The change compiles for all 12 targets. |
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The e5bdfd6 push addresses what I raised last round: the clonefile branch now checks truncate(2) and unlinks the clone before falling back to do_open_file, the two read/write loops are one function in node_fs.rs, and the socket test asserts the exact bytes. Beyond the inline findings, I also checked whether file.size/exists() caching the stat size makes a later file.slice(n) copy stop at the cached end rather than the file's current end — that is the documented lazy-size behavior of slice() on a file Blob and predates this PR, so it is not a regression here.
Extended reasoning...
Two new confirmed findings are posted inline (a Windows abort on a crafted deserialized Blob offset, and the pre-existing same-path truncation); the body records only the prior-feedback follow-through and one concern examined and ruled out this run.
The offset of a structured clone record comes from bytes that the caller controls. A file store of unknown size had no bound for it, so the Windows copy loop could not cast the offset to i64.
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Both findings of this review have an answer in their threads.
Not run on Windows or macOS. |
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I reviewed the three commits on this head and found no new bugs; 1852be9 addresses the deserialize-offset point from the previous round (MAX_SIZE is 2^52-1, so the clamped offset fits the i64 cast in apply_source_view). Because this is a user-visible behavior change to Bun.write with platform-gated copy paths (clonefile/fcopyfile/FreeBSD/libuv) that cannot be exercised on this Linux checkout, a human look would still be worthwhile.
What was reviewed:
copy_lengthand the POSIXrun_asyncwindow handling: seek gated on regular files, non-regular sources capped bysource_size,max_length == 0early-close on both arms.- The rewritten
copy_file_using_read_write_loop_capped: single loop bounded bymax_len/EOF replaces the two-phase loop;read_write_fallbackstill ftruncates a Bun-opened dest tototal. - Windows
ReadWriteLoop:remaining -= ncannot underflow sincewantis clamped toremaining;prepare_read_write_loopnow opens the source first and closes it before the mkdirp re-entry. size_is_explicitpropagation through dupe,slice(), structured clone (serialize writes MAX_SIZE when not explicit), FileReader and both ReadableStream directions.
Extended reasoning...
The diff (709 insertions, 184 deletions across 7 files) adds a size_is_explicit flag to Blob, a file_window() accessor, source-window parameters on both CopyFile and CopyFileWindows, reroutes windowed copies away from the whole-file fast paths on macOS/FreeBSD/Windows, and collapses two read/write loops into one capped loop in node_fs.rs. It touches structured-clone deserialization of untrusted bytes (offset now clamped to MAX_SIZE before use) but no auth, injection or credential surface. The third commit resolves the Windows expect("int cast") abort flagged in the prior run, and no CODEOWNERS entry covers these paths. Deferring rather than approving because the change alters observable Bun.write semantics for sliced sources, overlaps several open PRs the author names, and its macOS/FreeBSD/Windows branches are only type-checked here, not run.
The flag has a meaning only when max_size is set, so the two are one value now. The test for offsets from deserialize() runs one child for the three offsets.
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Thank you. Two facts for the person who looks at this next.
Two questions are open for a maintainer. The thread of the report has both: a 1.4.x release for this change, and which design for the cached size stays (the flag in this PR, or draft #43910). |
A window takes the read and write loop on Windows, and the loop opened the destination with O_TRUNC. For a window of the destination itself that emptied the file before the first read. uv_fs_copyfile does nothing for one file, so main left the file whole. The loop now opens the destination without O_TRUNC. For one file it moves the window to the start of the file and cuts the rest. For two files it empties the destination first, as O_TRUNC did.
…ory source first fstat fails on the write-only handle of the destination, so the check for one file never matched and the loop emptied the file. The loop now compares the stat of the destination path with the fstat of the source, as uv_fs_copyfile does, and opens the destination with O_TRUNC only for two files. A window of a directory now rejects with EISDIR before the destination is opened.
Fixes #4930
Problem
Bun.write(dest, Bun.file(p).slice(a, b))copies the whole source.Bun.write("key.bin", Bun.file("/dev/urandom").slice(0, 16))writes until the disk is full, then rejects withENOSPC.write_file_with_source_destination(src/runtime/webcore/Blob.rs:4416) givesCopyFileonly the destination's offset and size, which.sizeorexists()can have cached (CallingBunFile.existsmakesBun.writewrite nothing #4930).Fix
CopyFiletakes the source window. It seeks a regular source to the window's start, and every copy path stops at the window's end.Blob::size_is_explicitmarks a size thatslice()set.BlobExt::file_windowreads it for the source and the destination.test/js/bun/io/bun-write.test.js(35 cases fail on 1.4.3). Self-reviewed twice (Notes).Background
Blobis a view (offset,size) onto a refcounted store.slice()changes only the view.sizeisMAX_SIZEuntil something stats the file.slice(5, 10)then copies bytes 0 to 4.Downsides
Bun.file()toBun.writenow gets only the window.Bun.file(p).slice(-5)of a file that nothing has stat'd copies nothing (main: the whole file). Bun.file().slice(): count a negative index back from the file's real end #41257 fixesslice(). Merge it first, or with this PR.Bun.write(path, Bun.file(small))8,607 to 8,657 instructions,slice()1,098 to 1,106.bun-linux-x64.zipgrows by 9,864 bytes.Notes
Scope change (2026-09-29). Until 8206895 this PR also changed spawn stdin: it read the whole window of a sliced file into the parent with
preadbefore the child started. A pre-merge check measured +537 MB of RSS and 616 ms on the JS thread for a 512 MiB window, and a 3 GiB window in a 2 GB container killed the parent. A design review ranked that buffer last of six designs. Its pick is a handle that Bun opens before the child exists, and a pump that sends one chunk of 64 KiB per writable event. That work is draft #44301, one commit on this branch. A later self-review said that a person must choose between the buffer and the chunk sender, so #44301 waits for that choice.src/runtime/api/bun/spawn/stdio.rsandtest/js/bun/spawn/spawn.test.tsare as on main in this PR.What changed in this head against 8206895.
Bun.file(fd).slice(0, 5)on a descriptor at byte 4 copied bytes 4 to 8. A test coversslice(0, 5)andslice(0, size).new Response(file.stream())of a file whose.sizewas read is no window, so the copy takes the whole file. A test covers it.uv_fs_copyfileskips the window block. A window never takesuv_fs_copyfile.What changed in e5bdfd6.
truncate(2)fails on a clone of a read-only file. The copy now checks the result. If it fails, the copy removes the clone and copies the bytes of the window. A test covers a source with mode 0444.NodeFS::copy_file_using_read_write_loopare one loop innode_fs.rs. It reads until EOF or until the cap. The net change is 51 lines less.What changed in 1852be9.
deserialize()bounds the offset and the size of a file record byMAX_SIZE. The offset comes from bytes that the caller controls, and a file store of unknown size had no bound for it. On Windows the copy loop casts the offset toi64, so an offset abovei64::MAXstopped the process. A new case covers offsets of 2^52 + 123, 2^63 + 5 and 2^64 - 1.What changed in d36bbf6.
FileReaderkeeps the flag insidemax_size(Option<MaxSize>), because the flag has a meaning only whenmax_sizeis set. Themordantcheck asked for that.What changed in 457b6e9 and 704fa30. A window of the destination itself on Windows (
Bun.write(p, Bun.file(p).slice(10, 20))). A window takes the read and write loop there, and the loop opened the destination withO_TRUNC, so the file was empty before the first read. On mainuv_fs_copyfiledoes nothing for one file, so that call left the file whole. The loop now compares thestatof the destination path with thefstatof the source (volume and file index, asuv_fs_copyfiledoes). For one file it opens the destination withoutO_TRUNC, moves the window to the start of the file and cuts the rest. For two files nothing changes. The first form of this fix (457b6e9) calledfstaton the write-only handle of the destination. That call fails on Windows, so CI showed the file empty (Buildkite 121911), and 704fa30 replaced it. A window of a directory now rejects withEISDIRbefore the destination is opened. Five cases cover both on Windows only. I did not run them by hand. They pass on both Windows lanes in CI (Buildkite 121926).The same file on POSIX, with #43388. On POSIX main empties a file that is copied onto itself (#20462), and this PR does not change that. #43388 fixes it. At its head 20ea576,
do_open_filestats the destination path before each open. When the path names the regular file of the source, it opens nothing, andrun_asynctakes a same-file arm that does nothing. That is wrong for a window of the file: the result must be the window. The PR that merges second must add the case to that arm. For a window that is not the whole file, the arm must open the destination withoutO_TRUNC, seek the source to the window, run the capped loop (the reads stay ahead of the writes) andftruncatethe file to the bytes copied. A whole file stays a no-op. The rowa slice of the file as the source gives the same result as for another destinationof #43388 must fail on a merge of the two PRs until that case exists. The session of #43388 merged the two branches in a scratch tree (two conflicts, both sides stay) and did not build that merge. The four same-file cases of this PR, which run on Windows only today, then run on POSIX too.Merge order. #41257 makes
slice()count a negative index back from the file's real end. Without it,slice(-5)of a file that nothing has stat'd has an offset nearMAX_SIZE, so this PR copies nothing. A pre-merge check got the last 5 bytes with both PRs, in 4 of 4 cases. The two merge clean in both orders.Measurements. The pre-merge check ran a release build of 8206895 against main a4f1429 (instructions per call, one pinned core, 15 rounds):
Bun.write(path, Bun.file(small))8,607 to 8,657 (+0.6 %),.sizeof a known handle 62 to 64,slice()of any Blob 1,098 to 1,106. Its storm of 12,000 random operations gave 0 wrong bytes and 0 ASan reports. Whole files take the same syscalls as on main.Counts from
/proc/self/iofor a 256 MiB source (release 1.4.3 against a debug build of this PR):A window that starts past byte 0, or a window of a caller's descriptor, adds one
lseek. An unsliced copy adds no syscall.straceis not on this machine.Binary size (zip sizes from the CI artifact list, base build 121139 against build 121190 of 8206895, which still had the stdin half):
bun-linux-x64.zip37,920,840 to 37,930,704 (+9,864),bun-linux-aarch64.zip+11,696,bun-windows-x64.zip+14,439,bun-darwin-aarch64.zip+81. The artifacts cannot be downloaded from this machine, sosizewas not run on the binaries.The small cut. Main already takes a length from the destination:
Bun.write(Bun.file(dst).slice(0, 5), Bun.file(src).slice(5, 10))writesabcdeon 1.4.3. So a cut that passes only the source's length stops the copy of/dev/urandom, and it makesslice(5, 10)of a regular file copy the wrong 5 bytes.Probe on a 26-byte file
a..z,f = Bun.file(p):Sources that are not regular files. A socket, a pipe or a character device has no position.
Bun.writecopies the window's size from where the stream is. The two character device tests run underulimit -fandulimit -t, so a build without the cap cannot fill a disk or run without end.Replaces older PRs. #31515 (
Bun.writewindow, its review asked to seek only regular files and to test a source that cannot seek), #37877 (the destination's cached size, itssize_is_explicitflag and its 17 test cases) and #33715 (the byte count, which #42066 fixed on main since). They are closed with a comment that points here. #31931 (stdin) is replaced by the stack on this PR.Overlap with open PRs. A maintainer can pick one shape.
Blob.rs.resolve_sizeand.sizechange, plus a stat on everyexists(). This PR carries only that hunk, because the window check needs it.read_write_fallbackchange, for destination windows.copy_file.rs.Not changed here.
Bun.write(Bun.file(dst).slice(5, 10), src)) resolves 0 and leaves the destination empty, as on main. The destination offset is applied on no copy path. It is reported as a separate bug.Bun.write(p, Bun.file(p))empties the file (Bun.write on itself causes content to be empty #20462). Bun.write: compare the destination with the source before the truncating open #43388 is open for it. See "The same file on POSIX" above.f.slice(20, 100).sizeis 80. The size of a file Blob that nothing has stat'd is lazy.Bun.file(fd).slice(0, 5).text()on a descriptor that is not at byte 0 reads from the descriptor's position. The read path seeks only for an offset above 0.Bun.serveanswers 206 withContent-Rangefor a sliced file.docs/runtime/http/routing.mdxdocuments that.cat < ${f.slice(1, 3)}reads the whole file.Reviews. Two self-reviews and the bot reviews found these, all fixed and covered by tests: a failed stat cached a size of 0, so
exists()on a missing file and a laterBun.writecopied nothing. A size comparison stood in for the flag and was wrong both ways. On macOS and FreeBSD a window took a copy path that reads the whole source. Not taken: the merge order against #40221, and a 1.4.x release for this change. A maintainer decides those.Test runs on this head.
Bun.writeblocks pass (46 cases) when they run alone. On 1.4.3 the same blocks fail 35 cases.cp.test.tsand thecopyFilecases offs.test.tspass. A 256 MiB copy through the fallback takes 36 to 39readcalls and 33writecalls.structured-clone-blob-file,blob,blob-write,bun-file-exists,bun-file, and the stream cases ofbody.bun-write.test.jspassed on a Windows x64 debug build.bun run rust:check-allfor Windows, macOS, FreeBSD and Linux: no errors, and no warning in the changed files.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/io/bun-write.test.js