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…ts past byte 0
new Response(image), new Request(url, { body: image }) and
fetch(url, { body: image }) encode inside the call. For an Image made
from Bun.file(path), encode_for_body replaces the source with the path
of the file and reads that. The offset and the size of a slice were
lost there, so the body was an encode of the whole file. The source
stayed the bare path, so later bodies and terminals decoded the whole
file, and so did a terminal that was pending across the call.
A source whose Blob starts past byte 0 now becomes a path source with
that window (Source::PathWindow). The path reader in PipelineTask::run
reads the window with one positioned read, after the same open, fstat
and regular-file guard, and the 256 MiB cap measures the window.
Only an offset proves a slice. A Blob that starts at byte 0 can be an
un-sliced Bun.file() with a cached size, so it is read to the end of
the file, as before. A window that starts at or past the end of the
file is read as the whole file, as before.
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Status
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Updated 6:36 PM PT - Oct 21st, 2026
❌ @robobun, your commit 67c7493 has 1 failures in
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked whether the raw InputPath::Window pointer can dangle: the only source.set() sites are the constructor, encode_for_body (replaces a Source::Blob, which never hands out a window pointer), and the blob-read resolver, which is guarded by matches!(source, Source::Blob(_)) and so drops its bytes instead of replacing a PathWindow a pool task may be reading. The Box therefore lives as long as the Image, and the SAFETY comments hold.
Extended reasoning...
The change adds a Source::PathWindow variant and InputPath enum in src/runtime/image/Image.rs so sliced Bun.file() image bodies read only their window via pread, plus a new test block in test/js/bun/image/image.test.ts. It touches unsafe raw-pointer borrows across a pool thread and size arithmetic on file metadata, but no auth, crypto, or injection surface. Three inline findings were posted (a 256 MiB test cost, the window-past-EOF whole-file fallback the author also flagged as an open design question, and the offset-0 slice gap), so a human decision is needed; the pointer-lifetime concern was ruled out from the source.set() call sites.
WalkthroughImage body encoding now preserves valid ChangesImage File Slice Support
Suggested reviewers: Priority: ⬇️ Low Merge Risk: ⚪ Minimal · up to
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…e-body-blob-window
The path reader dropped the window of a slice when the file ended at or before its offset, and read the whole file. One Image then changed between the slice and the first picture in the file when the file changed length. The reader now keeps the window. A window past the end is empty, so the body throws "unrecognised format", as image.bytes() does. slice(-n) on a Bun.file() whose size is not read yet counts back from the largest size of a Blob. No file has that offset. encode_for_body stores the bare path for it, so the body is the whole file, as before. The test of the 256 MiB cap no longer reads a window of exactly the cap. It asserts that a slice from byte 0 of a file over the cap is refused.
A regular file can report a size of 0 and have bytes (procfs). The length of a window came from that size, so a sliced body of such a file was empty. The reader now reads that file whole, as it does for a path source, and cuts the window from the bytes. New tests: a slice of fs.openAsBlob(), a slice of /proc/self/cmdline, a negative end index, and a window at the 256 MiB cap of a longer file. Two test.failing rows record a negative end index on a size that is not read yet, and structuredClone() of a slice(-n) Blob before the body.
…e-body-blob-window
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Actionable comments posted: 1
- 🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @test/js/bun/image/image.test.ts:
- Line 201: Change the “a slice of Bun.file() as a body” test suite from
describe to describe.concurrent so its isolated tests run concurrently,
preserving the existing test.each and test.failing entries.
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src/runtime/image/Image.rstest/js/bun/image/image.test.ts
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| // `fetch(url, { body: image })` encode inside the call. A Bun.file() source | ||
| // becomes a path source there, and a slice that starts past byte 0 keeps its | ||
| // window: the body, later bodies and every terminal read the slice. | ||
| describe("a slice of Bun.file() as a body", () => { |
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📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win
Run the slice-body tests concurrently.
Each test in this describe block creates its own tempDir. The tests share no file state. Several tests write files or spawn a subprocess, and they run one after another. Change the block to describe.concurrent. The test.each and test.failing entries keep working inside a concurrent describe.
As per coding guidelines: "Prefer concurrent tests over sequential tests: When multiple tests in the same file spawn processes or write files, make them concurrent with test.concurrent or describe.concurrent unless it's very difficult to make them concurrent."
♻️ Proposed change
--- "a/test/js/bun/image/image.test.ts"
+++ "b/test/js/bun/image/image.test.ts"
@@ -198,7 +198,7 @@
// `fetch(url, { body: image })` encode inside the call. A Bun.file() source
// becomes a path source there, and a slice that starts past byte 0 keeps its
// window: the body, later bodies and every terminal read the slice.
- describe("a slice of Bun.file() as a body", () => {
+ describe.concurrent("a slice of Bun.file() as a body", () => {
// Three pictures back to back. A slice of the file selects one of them.
const pictures = [gradientPng, cmykJpeg, cornersPng];
const [a, b, c] = pictures.map(picture => picture.length);📝 Committable suggestion
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Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| describe("a slice of Bun.file() as a body", () => { | |
| describe.concurrent("a slice of Bun.file() as a body", () => { |
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Review comment at @test/js/bun/image/image.test.ts at line 201:
Change the “a slice of Bun.file() as a body” test suite from describe to
describe.concurrent so its isolated tests run concurrently, preserving the
existing test.each and test.failing entries.
After applying the fix, consider running `coderabbit review --agent` for local
review. Visit https://docs.coderabbit.ai/cli?utm_source=ghpr
Source: Coding guidelines
Problem
new Response(img)onBun.file(path).slice(n).image()encodes the whole file from byte 0.Requestandfetch()bodies do the same.img.bytes()andimg.metadata()also decode the whole file.Image::encode_for_body(src/runtime/image/Image.rs:1187). It swaps the image'sSource::BlobforSource::Path, which has no offset or size.Fix
Source::PathWindow(path, offset, size). This stored value is the fix.PipelineTask::run, the reader of path sources, reads that window with one positioned read. A body then equalsimg.blob().test/js/bun/image/image.test.ts(29 new tests, 24 fail without the fix). Other image suites and clippy: Notes.Background
encode_for_bodyreads aBun.file()by its path.Bun.file()also caches a size (CallingBunFile.existsmakesBun.writewrite nothing #4930). So only an offset proves a slice.Downsides
slice(0, n), andslice(-n)on an unread size.img.bytes()does. So a slice at or past the end of the file throwsunrecognised format, where main gave the whole file. A slice also ends at a stale cached size (Notes).Notes
Origin
new Response(await img.blob())gives the slice, if the image was not a body before.What changes for a caller
"The whole file" is the first picture in the file. The window of a sliced image is the offset and the size of its Blob at the first body.
slice(n)orslice(n, m), window inside the filebytes(),metadata(),widthon that imagebytes()ormetadata()that is pending across the bodyfs.openAsBlob(path)Bun.file()whose size was readslice(n, n)unrecognised format, asimg.bytes()rejectsunrecognised format, asimg.bytes()rejectsinput exceeds maxPixels limitBun.file()with a stale cached sizeslice(n, -m)on a size that is not read yetnto the end of the file, asimg.bytes()readsslice(-n)on a size that is not read yetslice(0, n),slice(0, 0)slice(0, n)of a file over 256 MiBinput exceeds maxPixels limitBun.file(), path string, bufferThe 256 MiB cap of a path read now measures the window. A window over the cap is refused before the read.
By the
slice()algorithm of the File API, a slice at or past the end of a Blob is empty, and a negative index counts back from the size. Each row that this PR changes moves toward that.A window at or past the end of the file
len = min(size, file_size - offset), and 0 when the file ends at or before the offset.unrecognised format, asawait img.bytes()rejects for that slice on main.slice(n)withnat or past the size of the file. Main gave the first picture of the file. It is now a throw.One image,
img = Bun.file(p).slice(a).image(), over a file of[PNG 16x16, a bytes][JPEG 64x64][PNG 4x3]:new Response(img)abytesunrecognised formatA regular file can report a size of 0 and have bytes (procfs). The reader reads such a file whole, as it does for a path source, and cuts the window from the bytes. A Linux test does this with
/proc/self/cmdline.A negative index on a size that is not read yet
slice()counts a negative index back from the size of the Blob. For aBun.file()whose size is not read yet, that size is the largest size a Blob can have (2^52 - 1).slice(-n)then has an offset near 2^52, which is not a place in the file.encode_for_bodystoresSource::Pathfor an offset above half of that size, so the body is the whole file, as on main. That is correct whennis at or over the size of the file. Whennis smaller, the body must be the lastnbytes: atest.failingrow records that.img.bytes(). On main,await img.bytes()of that slice depends on the file system. On ext4 the seek fails, the read ignores that, and it returns the firstnbytes of the file. On tmpfs the seek works and the read is empty. I did not run macOS or Windows.slice(n, -m)keeps its start and loses its end: the window runs fromnto the end of the file, for the body as forimg.bytes(). A secondtest.failingrow records that.structuredClone()of the Blob does this, and it leaves aslice(-n)Blob empty at the end of the file, on main too. Its body then throwsunrecognised format, where main gave the whole file. A thirdtest.failingrow records that.slice()count a negative index from the real end of the file. Whichever of that PR and this one lands second must look again at&& offset <= MAX_SIZE / 2inencode_for_bodyand at the threetest.failingrows. Without that bound, a positive start of 2^51 or more on a size that is not read yet changes from the whole file to a throw.A stale cached size
Bun.file()caches its size when.sizeorexists()reads it. A laterslice(n)of that handle ends at the cached size.img.bytes()does. Main encoded the file from byte 0.img.blob()gives for those bytes: a throw, or a cut picture with no error where the decoder accepts missing pixel data.decode failed. A JPEG or GIF that is cut inside its pixel data encodes with no error. On macOS and Windows the system decoders read BMP and GIF first. I did not run those.Bun.file()with a stale size is read to the end of the file, as on main. The test "an un-sliced Bun.file() with a stale size is still served whole" pins this.Not fixed here
This PR is a partial fix. The image body is 1 of 6 consumers on main that take the path of a sliced
Bun.file()and read the whole file. I ran each row on main.Bun.write(dest, slice),Bun.write(dest, new Response(slice))src/runtime/webcore/Blob.rs:4401Bun.spawnandBun.spawnSyncstdinsrc/runtime/api/bun/spawn/stdio.rs:253,:610cat < ${slice}src/runtime/shell/shell_body.rs:349key,cert,caread_from_blob,src/runtime/socket/SSLConfig.rs:94ReadFilehasoffsetandmax_sizeimport()andnew Worker()of ablob:URLget_loader_and_virtual_source,src/runtime/jsc_hooks.rs:3995In this arm, two window shapes still read the whole file: a slice from byte 0, and
slice(-n)on a size that is not read yet. The test for a slice is0 < offset <= MAX_SIZE / 2. It stays as it is: the better test is at the Blob level, and two open PRs propose one.file_window()accessor. Draft Bun.file: do not use a cached stat size as the byte budget of a read or a copy #43910 stops the use of a cached size as a byte budget. A maintainer has to pick one.offset > 0half of the test becomes amatchonfile_window(). Thetest.failingrow forslice(0, n)and thefromStartassertion of the cap test then change.Source::PathWindowdo not change in either case.Measurements
Release builds (linux-x64) of main
71bc8185c8and of this change on it, from one recipe. After that, main963452075ewas merged in. No commit between the two touches the image code.Input: 64 B -> 64 B.Job<PipelineTask>allocation: 352 B -> 352 B (const assert, gdb disassembly ofImage::schedule).Bun.file()body: 658 -> 690 instructions fromencode_for_bodyto the close of the file, 6 -> 6 file syscalls, 26 -> 26 allocator entries.PipelineTask::runto the close of the file): 410 -> 442 instructions.openat,fstat,pread64,close), 247 -> 90 bytes read, 28 -> 29 allocator entries (the 32-byteBox<PathWindow>).openat,fstat,close), 247 -> 0 bytes read.slice(-4)on a size that is not read yet: 6 -> 6 file syscalls..text: 107,319,777 -> 107,320,465 B (+688).PipelineTask::run: 28,765 -> 22,959 B.encode_for_body: 2,222 -> 2,222 B. Newread_window: 397 B.Image/Blobwrappers after 100 slice-image cycles with a body: 1 / 1 -> 1 / 1.The 32 instructions of a path read:
read_to_endis a call and no longer inline inrun(+15),run(+10),InputPath::file(+6), the window length (+2), one less elsewhere.The instruction counts are from gdb
stepi, two runs each, equal. The syscall counts are from gdbcatch syscall. An allocator entry is a call of an out-of-line mimalloc entry, the same value in at least 6 of 7 runs. Sizes are fromsizeandnm -S. The container has nostrace,valgrind,bloatyorperfbinary.Other open PRs in this file
metadata()on a path string reads a head of the file) conflicts in one region ofImage.rs: the cap and read statements ofPipelineTask::run. It also conflicts in theharnessimport line of the test file.read_window(..).map(HeadRead::Whole), or with the whole read and the cut for a file with no size. Ametadata()call with no window reads withread_head. Each other call reads withread_to_end.ea6ddc833d, before the arm for a file with no size was there.image.test.tsgave 133 pass and 0 fail.metadata()call toread_headreads the head of the whole file for a window. I built it too: 4 tests of this PR fail. One is "after a body, later bodies and terminals read the slice".read_head. Itssize == 0branch reads the whole file.fs.openAsBlobpins the file) conflicts in one region, theSource::Blobarm ofencode_for_body. Its arm for a pinned file has to read the window too. The test "a slice of fs.openAsBlob() as a body is the slice" fails if it does not.Inputas an enum of its sources) conflicts in 3 more regions.InputPath::Windowbecomes one more variant there.clone()andstats()) has no conflict, but itsmatchon the source indo_cloneneeds an arm forSource::PathWindow.History of this branch
encode_for_bodyand did not write the source.Bun.file(). A body with a stale cached size threw. The image kept its Blob alive after a body.Tests
bun bd test test/js/bun/image/image.test.ts(debug build with ASAN): 132 pass, 5 skip, 0 fail.src/of main, 24 of the 29 new tests fail. The 5 that pass there: the un-sliced stale-size test, thetest.failingrows forslice(0, n),slice(-n)andslice(n, -m), and the test ofslice(-n)withnover the size.image-kernels.test.ts: 37 pass.image-vs-sharp.test.ts: 29 pass.image-adversarial.test.ts: 98 pass, 2 fail. Two tests exceed their 5 s timeout on the debug build in my container: "decode/encode cycles plateau" and "doesn't crash when the ICCP tag of a WebP input is rewritten while a pipeline is pending". They do the same with thesrc/of main. They read no file.cargo clippy -p bun_runtimeis clean.bun run rust:check-all: 12 targets ok.no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/image/image.test.ts