#41487 Bun.write(path, Bun.file(fd)): trim the preallocated destination to the bytes copied

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.

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

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