Bun.write(path, Bun.file(fd)): trim the preallocated destination to the bytes copied

[robobun]

Problem

• Bun.write(path, Bun.file(fd)) on Linux leaves the destination at the source's full st_size when the fd is positioned past 0. The copied bytes are followed by a zero-filled tail. Example: a 4 MiB file read to 3 MiB, then copied, resolves 1048576 but dst is 4 MiB.
• The cause is in src/runtime/webcore/blob/copy_file.rs. run_async preallocates the destination to st_size with fallocate (line 809). do_copy_file_range then copies from the fd's current position and stops at EOF. Nothing trims the destination. The strace is fallocate(dst, st_size), copy_file_range = 1048576, copy_file_range = 0, close.

Fix

• After the copy_file_range / sendfile / splice loop, if Bun opened the destination (a path, so O_TRUNC) and total_written < max_length, ftruncate the destination to total_written.
• This is the same reconciliation the read/write fallback already does in read_write_fallback (ftruncate(dest_fd, *total)). The kernel-copy path did not have it.
• A destination given as an fd is not preallocated and is not touched. An unknown-size source (FIFO, char device) skips the check.
• Verified: test/js/bun/io/bun-write.test.js (new test, fails on bun 1.4.3 with size: 2162688 instead of 65536). Also test/js/web/fetch/blob-write.test.ts.

Background

• Bun.write(dst, Bun.file(src)) runs CopyFile on the work pool. When the source is a regular file it fstats it, clamps max_length to st_size, and on Linux preallocates a path destination above 2 MiB so the copy does not fragment.
• copy_file_range(2) with null offsets uses and advances each fd's file position. A user fd that was already read from starts the copy in the middle of the file.

Notes

Repro under bun 1.4.3:

const fd = fs.openSync(src, "r"); // src is 4 MiB
fs.readSync(fd, Buffer.alloc(3 << 20), 0, 3 << 20, null);
await Bun.write(dst, Bun.file(fd)); // resolves 1048576, dst is 4194304 bytes

The should work when copyFileRange is not available > on large files test in the same file is slow under the debug ASAN build (a 256 MiB synchronous fill) and timed out once while the file ran concurrently. It passed on the next run and is not touched by this change.

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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/io/bun-write.test.js

[robobun]

^{Updated 1:30 AM PT - Sep 6th, 2026}

❌ @robobun, your commit 0d65b4e has 1 failures in Build #110795 (All Failures):

• test/napi/node-napi-tests/test/js-native-api/test_object/do.test.ts - stalled in the parallel batch on 🐧 13 x64

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bunx bun-pr 41487

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bun-41487 --bun

[coderabbitai]

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Walkthrough

Changes

Linux and Android bounded path-based copies now truncate destinations after short writes. A Linux-only Bun.write test verifies positioned source descriptors do not leave preallocated zeros.

Bounded file copy

Layer / File(s)	Summary
Short-write destination truncation src/runtime/webcore/blob/copy_file.rs	Linux and Android bounded copies truncate path-based destinations to total_written when fewer bytes are written than requested.
Positioned descriptor regression test test/js/bun/io/bun-write.test.js	The test detects Linux and verifies that a source descriptor positioned past 2 MiB writes only the remaining tail bytes and sets the destination size correctly.

Suggested reviewers: dylan-conway, jarred-sumner

Merge Risk: ⚪ Minimal · up to 9afb9

Path-based bounded copies on Linux and Android now remove stale trailing bytes when a positioned source has fewer remaining bytes than requested. The change preserves excluded destination and source cases, with no current merge-blocking risk identified.

🚥 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 main change: trimming a preallocated destination to the bytes copied by Bun.write.
Description check	✅ Passed	The description explains the problem, root cause, fix, scope, and verification. It does not use the exact template headings, but it provides the required information and is sufficiently complete.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/runtime/webcore/blob/copy_file.rs`:
- Line 509: Update the comparison in the relevant write loop to compare
total_written directly with self.max_length, removing the redundant u64::from
conversion while preserving the existing boundary condition.

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[claude]

LGTM — the follow-up commits look good.

What was reviewed:

• The trim mirrors read_write_fallback (line 979) exactly: same let _ = bun_sys::ftruncate(...) best-effort pattern and same i64::try_from(...).expect("int cast"), which is unreachable here since total_written < max_length and max_length is derived from st_size (off_t).
• Guard is sound: Path destinations are opened O_TRUNC so truncating to total_written cannot lose data even below the 2 MiB preallocate threshold; fd destinations and unknown-size sources are correctly excluded; error paths return before the trim.
• Test threshold (2 MiB + 64 KiB) clears PREALLOCATE_LENGTH = 2048 * 1024 with the > comparison at line 817; tempDir, Buffer.alloc(n, fill), combined-object toEqual, and try/finally fd cleanup all match harness conventions.

Extended reasoning...

Overview

This PR fixes a Linux-only bug where Bun.write(path, Bun.file(fd)) left trailing zero bytes in the destination when the source fd's position was already advanced. The Linux copy path preallocates the destination to the source's st_size via fallocate, but copy_file_range from a positioned fd copies fewer bytes than st_size, leaving the preallocated tail. The fix adds a post-copy ftruncate(dest_fd, total_written) in do_copy_file_range_linux when the size was known, fewer bytes were copied than max_length, and Bun opened the destination (Path, hence O_TRUNC). A Linux-gated test in bun-write.test.js exercises the exact scenario at the preallocate threshold.

Security risks

None. This is an output-size reconciliation on a file Bun itself opened O_TRUNC. No user-controlled paths, sizes, or offsets flow into new trust decisions; the ftruncate target is derived from the loop's own accumulated write count. There is no injection, auth, or data-exposure surface here.

Level of scrutiny

Low-to-moderate. The change is 11 lines of native code plus a test. Crucially, it is not novel: it copies the exact reconciliation read_write_fallback already performs at line 979 of the same file, with the same discarded-result pattern (let _ =) used at four other ftruncate/preallocate_file sites in this file, and the same .expect("int cast") idiom used throughout. The .expect is provably unreachable because the guard requires total_written < max_length and max_length is clamped to st_size (an off_t). REVIEW.md's concern about discarded I/O results is satisfied by precedent — this is a best-effort trim of an over-allocation, and failure leaves the file in the same state the bug produced, not a worse one.

Other factors

Two follow-up commits since the initial push addressed earlier inline feedback (dropped a redundant u64::from, shortened the comment to one line). The coderabbitai thread was resolved by a non-author. No CODEOWNERS entry covers these paths. The test's 2 MiB + 64 KiB fixture correctly clears the > PREALLOCATE_LENGTH (2048 * 1024) guard, uses tempDir/isLinux from harness, Buffer.alloc(n, fill), combined-object .toEqual, and cleans up the fd in finally. The bug is Linux-specific because PREALLOCATE_SUPPORTED gates fallocate to Linux/Android only; the macOS fcopyfile path already has its own post-copy ftruncate. Exit reason was dry_streak.

[robobun]

Status: the fix and its test are unchanged since 9afb964. Two CI runs (110729, 110795) both pass test/js/bun/io/bun-write.test.js on every lane. The remaining red lanes are unrelated flaky tests (bun-install-registry, bun-patch, fetch-leak, compile-node-compile-cache, a napi test_object stall) that pass on retry or alone. Ready for review.

[robobun]

Closing: this is covered by #44425.

• Keep the file size when preallocating a write #44425 makes preallocate_file pass FALLOC_FL_KEEP_SIZE, so a preallocation never sets the file size. copy_file_range from a positioned source fd then leaves the destination at the bytes copied, with no trim. That is the one function every preallocation goes through, so it is a better place for the fix than this call site.
• Verified: the test from this pull request fails on bun 1.4.3 and passes with a debug build of Keep the file size when preallocating a write #44425 (d4167ac) that has no change to copy_file.rs. The test is kept in 5276f89 for Keep the file size when preallocating a write #44425.

If #44425 does not land, reopen this pull request.