node:net: honor readable/writable in the Socket constructor so a write-only adopted fd stays open

[robobun]

Problem

• new net.Socket({ fd, readable: false, writable: true }) destroys itself one tick after construction. 'end', 'finish', 'close' fire on their own, close(2) runs on the caller's fd, and later writes fail with EPIPE ("This socket has been ended by the other party"). Node wraps a piped stdout this way.
• Cause: the Socket constructor forced readable: true, writable: true into the Duplex (src/js/node/net.ts:1572) and the fd branch faked EOF with push(null). That 'end' makes allowHalfOpen: false end the writable side, and autoDestroy closes the fd.
• The forced writable also broke https-proxy-agent, which hands node:http a new net.Socket({ writable: false }): a refused CONNECT surfaced as ERR_SOCKET_CLOSED instead of the proxy's 407.

Fix

• Pass readable / writable to the Duplex as node does (net.js#L410-L419). A readable: false Duplex starts with endEmitted set, so no 'end' fires. tls.ts drops both flags like node's TLSSocket (wrap.js#L590-L600).
• A socket whose readable side has ended never starts native reads: afterConnect stops the handle, read() / resume() leave it stopped. Node reaches readStart() only from _read(), which an ended Readable never calls.
• Sync fd writes restart the idle timer like _writeGeneric. A regular-file fd now throws ERR_INVALID_FD_TYPE ("Unsupported fd type: FILE") like node's createHandle(). fd / readable / writable are read as own properties, the view the Duplex gets.
• Verified: 8 new tests in node-net.test.ts and proxy.test.ts, all red on 1.4.3; the fd tests adopt the write end of a FIFO. No new failures there or in 805 vendored net, tls and http files.

Background

• With allowHalfOpen: false (the net.Socket default) the stream layer ends the writable side once the readable side emits 'end', and autoDestroy follows.
• Given writable: true, Bun adopts pipe, tty and socketpair fds with a synchronous write(2) _write and no native handle. Its native sockets read as soon as they open. readStop() pauses one to emulate node's lazy readStart().

Notes

• Repro (needs mkfifo), bun 1.4.3 and main @4ff91937 vs node v26.3.0:

  const net = require('node:net'), fs = require('node:fs'), cp = require('node:child_process');
  const p = require('node:os').tmpdir() + '/m2-' + process.pid + '.fifo'; cp.execFileSync('mkfifo', [p]);
  const fd = fs.openSync(p, 'r+');
  const s = new net.Socket({ fd, readable: false, writable: true });
  for (const n of ['end', 'finish', 'close', 'error']) s.on(n, a => console.log('event', n, a && a.code || ''));
  setTimeout(() => {
    console.log('before late write: destroyed =', s.destroyed, 'fd open =', fs.existsSync('/proc/self/fd/' + fd));
    s.write('late\n', e => console.log('write callback:', e ? e.code : 'ok'));
  }, 100);
  setTimeout(() => { let got = ''; try { const b = Buffer.alloc(16); const rfd = fs.openSync(p, fs.constants.O_RDONLY | fs.constants.O_NONBLOCK); got = b.toString('latin1', 0, fs.readSync(rfd, b)); } catch (e) { got = 'read: ' + e.code; } console.log('bytes in the FIFO:', JSON.stringify(got)); fs.unlinkSync(p); process.exit(0); }, 300);

  node: before late write: destroyed = false fd open = true / write callback: ok / bytes in the FIFO: "late\n".
  bun before: event end / event finish / event close / destroyed = true fd open = false / write callback: EPIPE / bytes in the FIFO: "".
  bun after: identical to node. The same happened on Windows with a piped stdout (verified on 1.4.3).

• A write issued in the construction tick was still delivered (the teardown runs on process.nextTick), which is why the existing adoption test passed.

• Because the fd number freed by the self-close is recycled by the caller's next open(), a caller that still believed it owned the fd and later ran fs.closeSync(fd) closed an unrelated file. That goes away with the self-close.

• https-proxy-agent (7.0.6), proxy answers CONNECT with 407: node response 407, res end; bun 1.4.3 response 407, req error ERR_SOCKET_CLOSED, res end (with a 403 plus body the error wins and no response is delivered); bun after: same as node. node:http never writes a request to a socket that is not writable (_http_outgoing buffers it), which is what the fake { writable: false } socket relies on.

• Probes that now match node v26.3.0 event for event: net.connect({ port, readable: false }) against a server that sends a banner (no 'data', later writes succeed, destroying with the banner unread resets the peer like node); net.connect({ port, writable: false }) (ERR_STREAM_WRITE_AFTER_END on write); new net.Socket({ readable: false }) / { writable: false } flag values; new tls.TLSSocket(undefined, { readable: false, writable: false }) stays readable: true, writable: true; write-only FIFO whose reader went away (EPIPE, destroyed, fd closed); setTimeout() with steady writes on an adopted fd (no spurious timeout); options carrying fd on the prototype (ignored, like node's spread).

• { fd, writable: true } with readable unspecified still goes through the push(null) EOF emulation and so still tears itself down on a pipe. Node reads from the fd in that case (or destroys with read ENOTCONN for an O_WRONLY pipe). Matching that needs read support for adopted pipe fds (net: native Pipe handle, Socket({fd}) via createHandle #29141 / net: Pipe writeBuffer/shutdown via StreamingWriter #29142), so it is left as is.

• Not addressed, separate root causes: a write() larger than a blocking pipe's free space blocks inside write(2) on the sync path (node:net: queue what EAGAIN leaves of a write to an adopted fd, cancel it on destroy() #41835 touches the same function for the non-blocking case); a bare { fd } creates no handle, so destroy() never closes the fd and connect() ignores it (node:net: adopt connected socket fds in new net.Socket({ fd }) #35341 covers connected socket fds, net: native Pipe handle, Socket({fd}) via createHandle #29141 pipes); a bare { fd } (no writable: true) is still not fstat'ed, so a regular-file or TTY fd there does not throw the way node's does; a TTY or other character device given writable: true is still adopted with sync writes where node throws Unsupported fd type: TTY / FILE; connect({ fd }) (Bun-internal, used by child_process extra stdio) does not apply the never-read rule for readable: false.

• node:net: adopt connected socket fds in new net.Socket({ fd }) #35341 (open) changes the same constructor lines, scoped to sockets built with an fd. Whichever lands second is a small rebase.

• Self-reviewed: the review agreed the change should exist in this shape. The execution concerns it raised (a readable: false client erroring on peer data, the idle timer on the sync write path, inherited option properties) are addressed above.

---

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/node/net/node-net.test.ts

[robobun]

Status: ready for review.

Reproduced on bun 1.4.3 and main @4ff91937 (Linux, and Windows with a piped stdout) with the FIFO script in the PR notes: new net.Socket({ fd, readable: false, writable: true }) emits end / finish / close on its own, closes the fd, and a write 100 ms later fails with EPIPE. Node v26.3.0 keeps the socket open and delivers the write. With this branch the script's output matches node line for line.

Verification on the debug (ASAN) build:

• test/js/node/net/node-net.test.ts: the fd adoption tests now adopt the write end of a FIFO (POSIX) and a regular-file fd is rejected with ERR_INVALID_FD_TYPE like node, per review. 8 new or reworked tests, red on 1.4.3, green here.
• test/js/bun/http/proxy.test.ts: new https-proxy-agent refused-CONNECT test, red on 1.4.3 (req error ERR_SOCKET_CLOSED), green here. 71/71 in the file.
• 805 vendored test-net-*, test-tls-*, test-https-*, test-http-*, test-stream-duplex* and stdio files: no new failures versus main.

CI (build 113977 at 1012038): 180/181 jobs passed. The one red test, test/bake/deinitialization.test.ts (segfault on Windows x64), does not touch this diff and is red on main's build 113974 and other PRs at the same time. Everything else that retried went green.

Open question for @cirospaciari in the thread above: node also rejects TTY / character-device fds here (Unsupported fd type: TTY / FILE); this branch still adopts them. Happy to reject them too if wanted.

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Walkthrough

Changes

Socket and TLS behavior now preserves caller stream options, manages adopted file descriptors, refreshes write timers, and avoids restarting ended readable handles. Tests cover duplex flags, descriptor lifecycle, and refused proxy CONNECT responses.

Socket and TLS semantics

Layer / File(s)	Summary
Socket construction and descriptor adoption src/js/node/net.ts, test/js/node/net/node-net.test.ts	Socket construction validates normalized options. Adopted descriptors preserve explicit stream behavior, reject regular files, and remain usable until explicit closure.
Readable lifecycle and write timers src/js/node/net.ts, test/js/node/net/node-net.test.ts	Ended readable handles are not restarted. Synchronous descriptor writes refresh socket timeout timers. Connection completion treats unreadable sockets as paused.
Socket and TLS duplex options src/js/node/tls.ts, test/js/node/net/node-net.test.ts	TLSSocket omits supplied stream flags when it constructs the underlying NetSocket. Tests verify Node-compatible duplex flags.

Proxy CONNECT response handling

Layer / File(s)	Summary
Refused CONNECT response coverage test/js/bun/http/proxy.test.ts	The regression test verifies that a 407 Proxy Authentication Required response and Proxy-Authenticate header reach the HTTP response stream without a request-level socket error.

Suggested reviewers: cirospaciari, jarred-sumner

Priority: ➖ Normal

Merge Risk: 🟡 Moderate · up to 10120

The updated socket/file-descriptor adoption logic in net.ts has two edge cases that diverge from Node.js: passing a UDP/datagram file descriptor can be silently accepted and written to synchronously instead of being rejected, and passing a descriptor with both readable and writable disabled leaves that descriptor unmanaged by the socket. Both are edge-case option combinations rather than the common adoption path, but they should be fixed before merge to avoid unexpected behavior or descriptor leaks for callers using these less common option combinations.

🚥 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 change: honoring readable and writable options in the node:net Socket constructor to keep write-only adopted file descriptors open.
Description check	✅ Passed	The description is detailed and covers the problem, implementation, verification, tests, compatibility behavior, and known limitations. It does not use the template headings exactly, but it provides t…

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[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@test/js/node/net/node-net.test.ts`:
- Line 1383: Update the spawned program used by the test to use a module-scope
static import of node:net instead of require("node:net"), while preserving the
test’s existing behavior and ensuring the import is declared at the program’s
top level.
- Line 1444: Update the expectation in the affected test to construct the
20-character expected value with Buffer.alloc(20, "x").toString() instead of
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[robobun]

Addressed the automated review notes: the spawned test program uses a module import, the repeated string comes from Buffer.alloc (b8aa672), and the new comments in net.ts / tls.ts are one line each (cccae6a). No behavior change. Waiting on CI.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[cirospaciari]

Node v26.3.0 throws ERR_INVALID_FD_TYPE ("Unsupported fd type: FILE") for new net.Socket({ fd }) on a regular file, so the three new tests that adopt an fs.openSync(path, "w") fd assert Bun-only behavior.
Please adopt a pipe fd in those tests (Node accepts pipes, TTYs and sockets) and make a file fd throw ERR_INVALID_FD_TYPE like Node.

[robobun]

^{Updated 3:50 PM PT - Sep 10th, 2026}

❌ @robobun, your commit 1012038 has 1 failures in Build #113977 (All Failures):

• test/bake/deinitialization.test.ts - pid 2856 segmentation fault at address 0x0 on 🪟 2019 x64

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⚠️ Outside diff range comments (1)

src/js/node/net.ts (1)

1568-1660: 🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Adopt explicit file descriptors when both stream sides are disabled. For { fd, readable: false, writable: false }, Duplex.$call disables both sides, then the constructor skips fd handling because it only enters that branch when opts.writable === true. The socket returns without an _handle, and the supplied fd is not managed. Node creates the fd handle and skips only read startup for readable: false. Handle explicit fd ownership independently from synchronous-write setup.

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In `@src/js/node/net.ts` around lines 1568 - 1660, Update the fd-handling logic in
the Socket constructor so explicit descriptors are adopted even when
opts.writable is false, including { fd, readable: false, writable: false }.
Separate fd handle ownership/adoption from the synchronous-write setup,
preserving fd validation and only configuring fdSyncWrite/fdSyncWritev for
writable descriptors while skipping read startup when readable is false.

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Inline comments:
In `@src/js/node/net.ts`:
- Line 1651: Update the descriptor classification around kSyncWriteFd so
datagram sockets are rejected before fdSyncWrite or any synchronous write
assignment. Distinguish stream sockets from datagram sockets, throw
ERR_INVALID_FD_TYPE for datagrams, preserve existing FIFO, character-device, and
stream-socket behavior, and add a regression test covering a UDP descriptor.

---

Outside diff comments:
In `@src/js/node/net.ts`:
- Around line 1568-1660: Update the fd-handling logic in the Socket constructor
so explicit descriptors are adopted even when opts.writable is false, including
{ fd, readable: false, writable: false }. Separate fd handle ownership/adoption
from the synchronous-write setup, preserving fd validation and only configuring
fdSyncWrite/fdSyncWritev for writable descriptors while skipping read startup
when readable is false.

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[robobun]

@cirospaciari done in 1012038:

• new net.Socket({ fd, writable: true }) over a regular file now throws ERR_INVALID_FD_TYPE ("Unsupported fd type: FILE"), matching node's createHandle(). New test: "rejects a regular-file fd with ERR_INVALID_FD_TYPE, like node".
• The fd adoption tests (your original sync-write test, the readable: false lifecycle test and the idle-timer test) now adopt the write end of a FIFO created with mkfifo and read the bytes back from the non-blocking read end. They are skipIf(isWindows); the piped-stdout child test still covers the write-only path on Windows.
• The inherited-options test keeps a file fd on purpose: node ignores an fd that is not an own property, and with this change an own file fd would throw, so construction not throwing is the proof that the inherited one was ignored.

One thing to flag rather than change silently: node v26.3.0 also rejects a TTY here. new net.Socket({ fd, readable: false, writable: true }) with a pty on fd 1 throws ERR_INVALID_FD_TYPE: Unsupported fd type: TTY (node wraps TTY stdio in tty.WriteStream instead), and /dev/null reports as FILE. This branch still adopts character devices with sync writes as before. Say the word and I will reject those too in this PR.

Not changed here (both predate this PR and belong with the native fd-handle work in #35341 / #29141): a datagram socket fd given writable: true still lands on the sync write path (telling SOCK_DGRAM from SOCK_STREAM needs getsockopt(SO_TYPE), which this JS shim does not have), and { fd, readable: false, writable: false } still creates no handle.

[claude]

Code review found no issues

No high-confidence issues detected in this change.