Bun.write: after EAGAIN continue with the unwritten tail, the count and the open fd

[robobun]

Problem

• await Bun.write(Bun.stdout, text) can write the first bytes of text twice. fd 1 must be a non-blocking pipe with less room than text, and text must be shorter than 262,144 characters. The promise resolves with the full length.
• The first write(2) runs on the JS thread. On EAGAIN, write_string_to_file_fast and write_bytes_to_file_fast (src/runtime/webcore/Blob.rs:5202, :5273) drop their byte count and close the fd they opened. A WriteFile job then starts from byte 0.
• For a FIFO path, that close is the end of the data for a reader.

Fix

• After a partial write and EAGAIN, the attempt keeps its unwritten bytes, its count, and the fd it opened.
• A WriteFile job gets only those bytes, so nothing can write bytes 0..written again. It writes on that fd and resolves with the sum of both counts.
• Verified: "Bun.write to a full pipe" in test/js/bun/io/bun-write.test.js (4 tests, 3 fail on main).
• Self-reviewed: 12 concerns raised, 12 addressed.

Background

• Any use of process.stdout makes a pipe on fd 1 non-blocking, and Bun.stdout is the same open file.
• Considered an offset into the source (Bun.write: poll instead of spin when a nonblocking stdout pipe returns EAGAIN #35953): the job converts the string again. Considered an fstat before the attempt (Bun.write: deliver the whole payload to a FIFO instead of a torn prefix #36025): every small write pays it.

Downsides

• A write that gets no EAGAIN makes the same syscalls as on main (path 4.000, fd 1.000). For a string, write_file_internal runs 8 to 16 more instructions. .text grows by 2,560 bytes.
• After a partial write, the unwritten bytes are copied once, and a path keeps its fd until the rest is written. The job still repeats write(2) until the pipe has room, as on main.
• A write that gets EAGAIN before its first byte behaves as on main. The only writer of a FIFO path can then reject with ENXIO.

Notes

Repro with no setup

bun -e 'await Bun.write("/dev/stdout", Buffer.alloc(100000, "a"))' | (sleep 0.5; wc -c)

A path needs no setup, because the attempt opens it with O_NONBLOCK. main prints 165536 with a default pipe of 65,536 bytes (108192 on the machine I used, where a pipe holds 8,192 bytes). This PR prints 100000.

Who hits this. A script that prints with console.log or process.stdout.write and also calls Bun.write(Bun.stdout, ...), with stdout piped to a reader that is not fast enough. The same code serves Bun.stderr, Bun.file(fd), a fd number, BunFile.prototype.write, Bun.Image#write, a socket, and a path to a FIFO or to /dev/stdout. #43868 made Bun.spawn clear O_NONBLOCK on the stdio it gives to a child. That removes one way to get a non-blocking fd 1. A pipe that process.stdout touched in the same process is still non-blocking. There is no GitHub issue. The report is from a fuzz run.

The change

• Both fast functions take deferred: &mut Option<Deferred> in place of needs_async: &mut bool. Deferred::Whole means that nothing is written and nothing is open, so the async path does the whole write, as on main. That is the old ENOENT case, and also an EAGAIN before the first byte. Deferred::Rest(Resume { written, tail, fd }) is an EAGAIN after a part. fd is Some only when the attempt opened a path. It is a CloseOnDrop, so each early return closes it.
• An attempt that wrote nothing keeps no fd. In a burst of writes to one path that nobody awaits one by one, only a write that wrote a part holds a fd while it waits. The first revision of this PR kept a fd for each of them.
• write_file_after_would_block builds a WriteFile over a blob of tail and calls resume_from(written, fd).
• WriteFile::run takes adopted_fd as its opened_fd. If the job is released before it runs (its Worker exits), the field drops and the fd closes.
• WriteFile::then resolves with base_written + total_written.
• The attempt opens a path without O_TRUNC and truncates at the end. The job does the same for an adopted fd (truncate_on_finish), also when one of its writes fails: on main the job opened the path with O_TRUNC, so a failed job left no old bytes behind the new ones. The job preallocates from base_written. No test covers these lines: they need a regular file whose write(2) returns EAGAIN.
• CloseOnDrop::release is new in src/sys/lib.rs. It gives the fd back without a close.
• The fd stays open across the hand-off. Before, the attempt closed it and the job opened the path again. A reader of a FIFO saw the end of the data at that close, and the second open failed with ENXIO once the reader was gone.
• How the job waits does not change in this PR. A fd that the store knows as a pipe waits on the io thread, as before. Any other fd repeats write(2) on a pool thread, as before.
• The gate for the attempt counts characters for a string, so its UTF-8 bytes can be three times as many. The copy of the unwritten bytes is at most that long.

Measurements (release builds, linux x64, main 2722608 against 2004a09)

• Syscalls per call on all threads, N=1000 against N=2000 awaited calls in one process: Bun.write(path, s) 4.000 (openat, write, ftruncate, close), Bun.write(Bun.file(path), s) 4.000, Bun.write(fd, s) 1.000, Bun.write(Bun.file(fd), s) 1.000, Bun.write(fd, new Blob([s])) 2.000. Equal on both builds. No fstat, fcntl, dup or poll on either.
• Instructions from the entry to the return of write_file_internal, warm, the same in 3 calls:

shape	main	this PR
Bun.write(path, string)	611	619
Bun.write(Bun.file(path), string)	651	664
Bun.write(fd, string)	453	464
Bun.write(Bun.file(fd), string)	495	511
Bun.write(path, Uint8Array)	385	387
Bun.write(fd, Uint8Array)	241	237

• Calls into the allocator inside write_file_internal, warm: 0 on both.
• size_of::<Job<WriteFile>>(), the one allocation of an async write: 560 to 576 bytes. Both are in the 640-byte size class of the allocator, so the block does not grow.
• .text: 58,158,389 to 58,160,949 bytes. write_file_internal 18,926 to 19,562. The two instances of write_string_to_file_fast 1,158 to 1,485 and 2,065 to 2,342. write_file_after_would_block is new, 1,152 bytes.
• Tools: a ptrace syscall counter, gdb single-step on the profile build, size, nm, a const assertion on the job size. The machine has no strace, perf or bloaty.

Tests. Four tests. A fixture process holds both ends of a FIFO. Before each write it fills the FIFO and takes at most 64 KiB out again, so the write stops in the middle on a host with any pipe size. A row counts only if the promise is pending before the first read. Each payload has its offset stamped every 4,096 bytes, so a byte that comes twice or not at all moves every later stamp. The fixture reads the FIFO to its end with cat, because a kqueue on macOS does not report the end of a FIFO to a reader (#40099).

• "writes every byte once, for each kind of destination", 18 cases: six ways to name the destination (Bun.write(fd), Bun.write(Bun.file(fd)), Bun.file(fd).write() and the same three with a path), with a Buffer, a latin1 string and a two-byte string, of 70,000 bytes, 262,143 bytes and the room plus 1, with room in the pipe and with none. On main 14 of 18 are wrong, for example resolved 70000, received 135536 of 70000 bytes, first difference at 65541 with a default pipe. The four that pass have no room in the pipe, so the attempt wrote nothing.
• "writes every byte once to Bun.stdout after process.stdout was used": a child with a pipe as stdout reads process.stdout.isTTY, then writes 100,000 bytes to Bun.stdout. main: received 165536 of 100000 bytes.
• "keeps the path it opened open until the write ends": Bun.write(fifoPath, ...) is the only writer of the FIFO, and both threads of the work pool are held, so the job cannot start. The pipe must then be empty with a write end still open. main: the write end is closed, and the job later fails with ENXIO.
• "keeps no fd for a write whose first try wrote nothing": 16 writes to the path of a FIFO with no room, with both threads of the work pool held. The number of fds on the FIFO must not change. This one passes on main. It fails on the first revision of this PR (16 fds).
• Release builds: main fails the first three tests, and this commit passes all four, with pipes of 8,192 bytes and of 65,536 bytes. With a pipe of 1 MiB (F_SETPIPE_SZ), this commit passes all four fixture modes. Debug build with ASAN: 4 of 4, and all of bun-write.test.js passes (90 tests).
• The tests set no timeout. On a debug build each one starts one or two debug bun processes, so on a loaded machine a local run can pass the default of 5 s. CI gives each test more.
• The eight tests of Bun.write: deliver the whole payload to a FIFO instead of a torn prefix #36025 ("larger than the pipe buffer": string, Uint8Array, Blob and Response, to a path and to a fd) pass on this commit. On main four of them pass (the Blob and Response ones).
• Not tested apart: Bun.Image#write. It enters through the same write_file_internal.
• macOS and Windows: not run. This commit is compiled on Linux only. cargo check for aarch64-apple-darwin, x86_64-unknown-freebsd and x86_64-pc-windows-msvc passed on the earlier draft, which held this change and the wait change together.

Self-review (12 concerns: 5 changed this PR, 7 are answered by its scope)

• Addressed in this PR: the fd hand-over had no test (the third test is new). The tests passed without a blocked write on a host with a large pipe (they now leave at most 64 KiB of room and check that the promise is pending). "Under 256 KiB" was wrong for a string (the gate counts characters). An explicit 5 s timeout was stricter than CI's own (the tests now set none). The fixture waited for the end of a FIFO through a kqueue.
• Answered by the scope. An earlier draft also made every blocked write wait on the io thread, and it had four more changes for that wait. The review found that draft too wide: one hunk was a copy of Bun.write: reject with EPIPE when a FIFO's reader goes away mid-write (Linux) #37852, one rule was private state where Remove libuv on Windows #42819 orders jobs for each fd, one ReadFile change had no test, and one part fixed no report. Two cases of that draft waited with no end where main gets an error. None of that is in this PR.

After the first review round (2004a09)

• A write that wrote nothing before EAGAIN kept the fd it had opened. A burst of such writes to one path could use up the fds of the process, which main does not do. Such a write now keeps nothing.
• A job that failed after the hand-over left old bytes behind the new ones in a regular file. It now cuts the file, as the job on main did through O_TRUNC.
• Code comments are one line each.

Still open after this PR

• The only writer of a FIFO path, with no room for a first byte: the attempt closes the fd it opened, as on main, so a reader such as cat sees the end of the data, and the job's open then fails with ENXIO. Nothing is written twice or lost. The first revision of this PR kept the fd there, and that is what let a burst of writes use up the fds of the process.
• A job that continues a write preallocates from the bytes that are written. On an O_APPEND fd of a regular file that grows the file before the append. main does the same for every job (Bun.write(Bun.stdout, data) pads an appended file with NUL bytes when data is over 1024 bytes (fallocate on an O_APPEND fd) #44387), and a regular file does not return EAGAIN on the file systems I have.
• A job does not close a path fd that open(2) put on 0, 1 or 2 (a process that closed its stdin). main does the same for every write that takes the job. A separate fix is in progress.
• The job's wait. On main a job repeats write(2) on EAGAIN unless the store knows the fd is a pipe, and this PR keeps that. Bun.write: poll instead of spin when a nonblocking stdout pipe returns EAGAIN #35953 is the open PR for it. Branch robobun/0a88cec4/write-file-park-eagain has a version of that change on top of this PR. It is not a PR, so that Bun.write: poll instead of spin when a nonblocking stdout pipe returns EAGAIN #35953 stays the one place for it.
• Bun.write(Bun.stdout, Bun.file(path)) goes through CopyFile, which rejects with EAGAIN from sendfile after a part of the file.
• Concurrent writes to one fd that the caller does not await have no order. Remove libuv on Windows #42819 and console, process.stdout/stderr: one stdio sink per fd; console writes through the stream like Node #37128 work on that.

Credit and overlap

• Bun.write: poll instead of spin when a nonblocking stdout pipe returns EAGAIN #35953 carried the count of the attempt into the job. Bun.write: deliver the whole payload to a FIFO instead of a torn prefix #36025 handed the opened fd over. This PR takes both ideas without the cost that each had on a write that does not block (a dup for each async write, an fstat for each write).
• If this PR lands, Bun.write: deliver the whole payload to a FIFO instead of a torn prefix #36025 has nothing left to fix, and Bun.write: poll instead of spin when a nonblocking stdout pipe returns EAGAIN #35953 keeps its other half (the wait).
• Bun.write: do not send a small write to the thread pool after a stat of the Bun.file() handle #44056 changes the gate one line below let mut deferred in write_file_internal. FileSink, Bun.write: remove unsafe from FileSink.rs and blob/write_file.rs #40213, Blob, Bun.file: remove unsafe from Blob.rs, read_file.rs, copy_file.rs and Store.rs #40221 and Remove libuv on Windows #42819 rewrite parts of write_file.rs and Blob.rs, and Remove libuv on Windows #42819 still has both arms that this PR changes. The PR that lands second needs a rebase.

---

no test proof · iteration 3 · 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]

Status: fix pushed (2004a09), waiting on CI.

Reproduced how: this command prints 165536 on a release build of main (2722608) and 100000 with this change.

bun -e 'await Bun.write("/dev/stdout", Buffer.alloc(100000, "a"))' | (sleep 0.5; wc -c)

The new tests in test/js/bun/io/bun-write.test.js ("Bun.write to a full pipe") fail on that build of main and pass with this change, on a release build and on a debug build with ASAN.

PR: #44389

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• src/runtime/webcore/Blob.rs
• src/runtime/webcore/blob/write_file.rs
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Walkthrough

On non-Windows, Bun.write now continues partial synchronous writes asynchronously after EAGAIN. The continuation preserves the written count and open descriptor. New FIFO tests cover multiple destination forms, blocked work-pool threads, and Bun.stdout.

Changes

Bun.write blocked-pipe continuation

Layer / File(s)	Summary
Capture deferred synchronous writes src/runtime/webcore/Blob.rs	The string and byte fast paths distinguish missing destinations from partial writes that return EAGAIN. They retain the written count, remaining bytes, and open descriptor for continuation.
Continue writes in the async writer src/runtime/webcore/blob/write_file.rs, src/sys/lib.rs	WriteFile adopts the descriptor and prior byte count, writes the remaining bytes, reports the combined count, and truncates on completion when enabled. CloseOnDrop::release transfers the descriptor without closing it.
Exercise blocked FIFO writes test/js/bun/io/bun-write-blocked-pipe-fixture.js, test/js/bun/io/bun-write.test.js	The fixture and tests cover writes to full FIFOs through multiple destination forms, Bun.stdout, and blocked work-pool scenarios. They check write results, received bytes, and descriptor state.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 2004a

Bun.write now resumes after a partial write that hits EAGAIN, so it no longer duplicates data or emits incomplete output. No concrete merge-blocking risk was found. macOS and Windows behavior was not run.

🚥 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 fix: continuing after EAGAIN with the unwritten tail, written count, and open file descriptor.
Description check	✅ Passed	The description provides detailed problem, fix, verification, test results, limitations, and performance information. It does not use the exact template headings, but it covers the required content th…

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Review comments at @test/js/bun/io/bun-write.test.js:
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• src/runtime/webcore/Blob.rs
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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.

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

• 🟡 src/runtime/webcore/Blob.rs — Daemons or servers that closed fd 0 (or 1/2) and then write to a FIFO path with Bun.write leave the FIFO write end open forever, so the reader never sees end-of-file. bun_sys::open at Blob.rs:5221 returns the lowest free fd, which is 0, 1 or 2 in that process; on EAGAIN that fd is adopted by the job. When the job finishes, do_close at Blob.rs:6760-6763 refuses to close any fd whose stdio_tag() is Some, so the adopted FIFO fd is never closed. Fix: the close decision for an fd the job opened or adopted must key on ownership (we opened it) rather than on its number; keep the stdio guard only for fds supplied by the user.

  Why this was flagged

  Trigger: a process that has closed stdin (fs.closeSync(0)) calls Bun.write('/path/to/fifo', data) while the FIFO is full. open(2) at Blob.rs:5221-5227 returns fd 0. The sync loop gets EAGAIN at Blob.rs:5274 and stores close.take() into Resume.fd (Blob.rs:5278), so the CloseOnDrop that would have closed it unconditionally is disarmed. The job adopts it at write_file.rs:400-402 and later calls do_close(is_allowed_to_close()) at write_file.rs:435; Blob.rs:6762 skips the close because Fd(0).stdio_tag() is Some. The FIFO's write end stays open for the process lifetime, so a reader blocked on read(2) never gets EOF and a cat/consumer hangs. The base branch's job reopened the path after the sync close and hit the same guard, so the base leaked too, but the PR newly disarms the unconditional CloseOnDrop on this path and adds nothing to make the adopted fd's close depend on ownership. Remedy: track that the fd was opened by Bun.write and close on that, not on stdio_tag.

  Verification: A process has closed fd 0/1/2 and calls Bun.write(fifoPath, data) while the FIFO is full. bun_sys::open at Blob.rs:5221-5227 does not call move_above_stdio and returns fd 0. Blob.rs:5274-5279 disarms the guard with close.take(), write_file.rs:400-402 adopts it, and do_close at Blob.rs:6760-6763 skips stdio-numbered fds, so the reader never gets EOF. On base CloseOnDrop closed fd 0 unconditionally.

• 🟣 src/runtime/webcore/blob/write_file.rs — Callers writing to a FIFO path or /dev/stdout whose reader is slow now pin one work-pool thread at 100% CPU until the reader drains the tail. The resumed job built by write_file_after_would_block carries a Path store, so run_with_fd sets could_block = false (write_file.rs:454-473) and do_write spins on continue for every EAGAIN (write_file.rs:360). Fix: a job resumed after EAGAIN must park on the io loop instead of spinning, e.g. have resume_from record that EAGAIN was observed and have run_with_fd keep could_block = true in that case. Pre-existing for persistent readers; on the base a one-shot reader made the reopen fail with ENXIO instead, so the spin is now the normal outcome for every FIFO-path write that fills the pipe. [also at: src/runtime/webcore/blob/write_file.rs:338 - pre-existing, now reached more often: after the fast path hits EAGAIN, the resumed job busy-spins a pool thread on write(2) until the reader drains the pipe.]

  Why this was flagged

  Bun.write(fifoPath, data) or Bun.write("/dev/stdout", data) with data under 256 KiB, through write_file_internal (Blob.rs:4700), gets EAGAIN once the pipe is full and goes to write_file_after_would_block (Blob.rs:5182), which makes a WriteFile whose file_blob store has a Path pathlike and an adopted fd. On the pool thread run_with_fd computes could_block only from file.pathlike.is_fd() and file.seekable (write_file.rs:454-473), so for a Path, or a raw fd whose store was built by Store::init_file with no stat, could_block is false. do_write then hits Err(err) if err.get_errno() == io::RETRY && !self.could_block => continue (write_file.rs:360) and re-issues write(2) in a tight loop with no yield until the reader has taken enough bytes. On the base branch the fast path closed its fd on EAGAIN, a reader that exits on EOF (cat, shell pipelines) then went away and the job's reopen failed with ENXIO, so the spin was reached only with a persistent reader or /dev/stdout.

  Verification: Pre-existing. In src/runtime/webcore/blob/write_file.rs:454-473 a Path store yields could_block = false, and do_write at write_file.rs:360 does continue on io::RETRY, so the pool thread busy-spins on write(2) until the reader drains the pipe. On the base, the same EAGAIN fell through to the async path at Blob.rs:4773-4782, which enters the identical run_with_fd/do_write loop with could_block = false.

• 🟣 src/runtime/webcore/blob/write_file.rs — Scripts that issue two un-awaited Bun.write calls to the same non-blocking fd (Bun.stdout after process.stdout use) with a slow reader get the second promise rejected with an epoll_ctl EEXIST error instead of its byte count. Both resumed jobs reach wait_for_writable at write_file.rs:480 and each registers the same fd in the one io-thread epoll via register_for_epoll; the second one's EPOLL_CTL_ADD at io/lib.rs:1729-1744 fails with EEXIST and io/lib.rs:1009-1011 routes it to on_io_error, which sets errno and finishes the job. Fix: a second writer on an fd already parked on the io loop must queue behind the first (share one Poll per fd or serialize jobs per fd) rather than register a second epoll entry.

  Why this was flagged

  Trigger: two Bun.write(Bun.stdout, data) calls in one tick, fd 1 a full non-blocking pipe, Linux. Each call's sync attempt gets EAGAIN and schedules a WriteFile through write_file_after_would_block (Blob.rs:4717-4724). For the stdout store, could_block is true (write_file.rs:459-462) and is_writable reports NotReady, so both jobs call wait_for_writable (write_file.rs:480) and schedule on IoRequestLoop. The io thread handles Action::Writable at io/lib.rs:1001-1013 by calling register_for_epoll on each job's own fresh Poll; neither has WasEverRegistered so both issue EPOLL_CTL_ADD on the same fd to the same epoll (io/lib.rs:1724-1740). The kernel returns EEXIST for the second; the Err goes to WriteFile::on_io_error (write_file.rs:233-248), which stores errno and system_error, and the promise rejects. The base takes the same path after its own EAGAIN, so this is pre-existing in mechanism, but the PR's stated contract that every byte of each Bun.write to Bun.stdout is written once is not met for the concurrent case and no test covers it.

  Verification: Pre-existing. The base branch already fails the same way by the same route. Every WriteFile owns its own io_poll (write_file.rs:108); a never-registered Poll uses EPOLL_CTL_ADD (io/lib.rs:1724-1730), and epoll_ctl returns EEXIST when the same fd is already in that epoll instance. The error is routed to on_io_error (io/lib.rs:1009-1011 -> write_file.rs:233-248), and then (:378-380) rejects the promise.

[robobun]

^{Updated 1:29 PM PT - Oct 1st, 2026}

✅ @robobun, your commit 2004a0996339a02a1c6c22635582882bf15c2e33 passed in Build #122507! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 44389

That installs a local version of the PR into your bun-44389 executable, so you can run:

bun-44389 --bun

[claude]

I re-reviewed after 2004a09 and found no bugs; both points from my earlier pass (truncating on every finish path, preallocating from base_written, and the per-test timeout) are addressed in that commit. Because this hands an open fd and a partial byte count from the JS thread to a pool job in the Bun.write fast path, a human look is still worthwhile.

What was reviewed:

• Ownership of the adopted fd on every exit: close.take() only in the Rest arm, adopted_fd: Option<CloseOnDrop> closes if the job is released unrun or cancelled before run, and once released into opened_fd the existing do_close(is_allowed_to_close()) closes it for path destinations (fd destinations never carry a Some).
• get_fd returns early when opened_fd is already set, so the resumed job neither reopens the path (no O_TRUNC re-truncation, no second FIFO open) nor runs mkdirp; preallocate_file and the final ftruncate both use base_written + total_written.
• Blob refcounts in write_file_after_would_block: find_or_create_file_from_path/dupe()/Blob::init each yield a +1 view that WriteFile owns and drops in then, matching the borrowed_view() (alias of dupe) pattern on the existing path.
• The Whole arm (ENOENT, or EAGAIN with 0 bytes written) is byte-for-byte the base-branch behaviour, so nothing outside the partial-write case changes; the test's UV_THREADPOOL_SIZE knob is read in src/bun_core/util.rs.

Extended reasoning...

The change touches src/runtime/webcore/Blob.rs (the non-Windows Bun.write sync fast path and a new write_file_after_would_block helper), src/runtime/webcore/blob/write_file.rs (three new WriteFile fields plus resume_from and truncate-on-finish), a small CloseOnDrop::release in src/sys/lib.rs, and a new FIFO fixture with four tests in test/js/bun/io/bun-write.test.js. It touches no auth, crypto, or injection surface; the sensitive part is fd ownership moving from the JS thread into a work-pool job, and I traced every exit (unrun, cancelled, failed, finished) to exactly one close. The bug hunt ran dry with no findings and the previous run's two inline comments were addressed by the latest commit, but the handoff logic is non-trivial enough that it should not merge on an automated approval alone; no CODEOWNERS entry covers these files.

[robobun]

Follow-up on the review findings, at 2004a09.

In this push:

• A job that got its fd from the first attempt now cuts the file on every finish, also after a failed write, and preallocates from the bytes that are written.
• The tests set no timeout.
• An attempt that wrote nothing before EAGAIN keeps no fd. It leaves the whole write to the async path, as main does. Before this push, a burst of writes to one path held one fd each while they waited.

Not in this PR, because main does the same:

• A job never closes a path fd that open(2) put on 0, 1 or 2. On main, fs.closeSync(0) and then await Bun.write(path, new Blob(["x"])) leaves fd 0 open on the file. This PR does not touch do_close. A separate fix is in progress.
• A job repeats write(2) until the pipe has room. Bun.write: poll instead of spin when a nonblocking stdout pipe returns EAGAIN #35953 is the open PR for that.
• Two writes that wait on Bun.stdout at the same time: the second rejects with EEXIST from epoll_ctl. Remove libuv on Windows #42819 orders the jobs of one fd.