console: output goes behind what process.stdout/stderr have buffered, as in Node

docs/guides/write-file/stdout.mdx

@@ -18,6 +18,21 @@ Bun also exposes `stdout` as a `BunFile` with the `Bun.stdout` property. Pass it
 await Bun.write(Bun.stdout, "Lorem ipsum");
 ```
 
+For many small writes, `Bun.stdout.writer()` returns a buffered `FileSink` over the same destination.
+
+```ts
+const writer = Bun.stdout.writer();
+writer.write("Lorem ");
+writer.write("ipsum\n");
+writer.flush();
+```
+
+---
+
+`console.log`, `process.stdout.write()`, `Bun.write(Bun.stdout, ...)`, `Bun.stdout.writer()` and `console.write()` all share one output queue per thread, so their output comes out in the order the calls were made, however slowly the other end of a pipe reads. As in Node.js, `console.log` is a `write()` on `process.stdout` from the point of view of anything that replaces or wraps `process.stdout.write`.
+
+When the program exits — including through `process.exit()` or an uncaught exception — everything already written to `process.stdout` and `process.stderr` is flushed to the operating system first. (Node.js can truncate pending output to a slow pipe on `process.exit()`; Bun waits for it.)
+
 ---
 
 See [`Bun.write()`](/runtime/file-io#writing-files-bun-write).

src/bun_core/output.rs

@@ -2503,6 +2503,12 @@ impl ErrName for crate::CrateError {
 
 // ── ScopedDebugWriter ─────────────────────────────────────────────────────
 
+/// True while this thread is inside a `scoped_log!` write (debug builds).
+#[inline]
+pub fn is_inside_scoped_log() -> bool {
+    crate::env::IS_DEBUG && scoped_debug_writer::DISABLE_INSIDE_LOG.get() > 0
+}
+
 pub mod scoped_debug_writer {
     use super::*;
 

src/codegen/generate-jssink.ts

@@ -493,6 +493,7 @@ JSC_DEFINE_HOST_FUNCTION(${controller}__end, (JSC::JSGlobalObject * lexicalGloba
 }
 
 extern "C" JSC::EncodedJSValue ${name}__getInternalFd(void* sinkPtr);
+${name === "FileSink" ? `extern "C" bool FileSink__isStdio(void* sinkPtr);` : ""}
 
 // TODO: how to make this a property callback. then, we can expose this as a documented field
 // It should not be shipped as a function call.
@@ -532,7 +533,20 @@ JSC_DEFINE_HOST_FUNCTION(${name}__doClose, (JSC::JSGlobalObject * lexicalGlobalO
     if (ptr == nullptr) {
         return JSC::JSValue::encode(JSC::jsUndefined());
     }
-
+${
+  name === "FileSink"
+    ? `
+    // The per-thread stdio sink is shared (process.stdout, Bun.stdout.writer(),
+    // console.*): closing one handle to it flushes, it does not take the
+    // wrapper away from the others.
+    if (FileSink__isStdio(ptr)) {
+        FileSink__close(lexicalGlobalObject, ptr);
+        RETURN_IF_EXCEPTION(scope, {});
+        return JSC::JSValue::encode(JSC::jsUndefined());
+    }
+`
+    : ""
+}
     sink->detach();
     ${name}__close(lexicalGlobalObject, ptr);
     // detach() nulled m_sinkPtr so ~${className} won't finalize ptr; do the

src/io/PipeWriter.rs

@@ -73,9 +73,11 @@ pub trait PosixPipeWriter {
             FileType::File
         };
         match ft {
-            FileType::NonblockingPipe | FileType::File => {
-                self.try_write_with_write_fn(buf, sys::write)
-            }
+            FileType::NonblockingPipe => self.try_write_with_write_fn(buf, sys::write),
+            // No poll backs this fd, so an `EAGAIN` (someone else made the
+            // description non-blocking) could never be resumed; behave as the
+            // blocking fd we asked for.
+            FileType::File => self.try_write_with_write_fn(buf, sys::write_retrying),
             FileType::Pipe => self.try_write_with_write_fn(buf, write_to_blocking_pipe),
             FileType::Socket => self.try_write_with_write_fn(buf, sys::send_non_block),
         }
@@ -600,6 +602,12 @@ pub struct PosixStreamingWriter<Parent: PosixStreamingWriterParent> {
     pub is_done: bool,
     pub(crate) closed_without_reporting: bool,
     pub force_sync: bool,
+    /// Sub-`CHUNK_SIZE` writes are coalesced and normally also arm the poll,
+    /// so the next writable event flushes them. An owner that flushes at end
+    /// of tick itself (FileSink's `AutoFlusher` on the stdio sink, whose poll
+    /// is deliberately unregistered while idle) turns this off to save the
+    /// registration syscall per buffered write.
+    pub poll_flushes_buffer: bool,
     /// Last reported `WriteStatus == Pending` (i.e. write(2) returned EAGAIN).
     backed_up: core::cell::Cell<bool>,
 }
@@ -613,6 +621,7 @@ impl<Parent: PosixStreamingWriterParent> Default for PosixStreamingWriter<Parent
             is_done: false,
             closed_without_reporting: false,
             force_sync: false,
+            poll_flushes_buffer: true,
             backed_up: core::cell::Cell::new(false),
         }
     }
@@ -697,7 +706,7 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
         self.handle.get_poll()
     }
 
-    pub(crate) fn get_fd(&self) -> Fd {
+    pub fn get_fd(&self) -> Fd {
         self.handle.get_fd()
     }
 
@@ -742,6 +751,16 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
         self.close();
     }
 
+    /// A write the parent issued on this writer's fd itself (bypassing
+    /// `write()`) failed terminally; tear down exactly as if `write()` had hit
+    /// the error.
+    pub fn fail(&mut self, err: sys::Error) {
+        if self.is_done || self.closed_without_reporting {
+            return;
+        }
+        self._on_error(err);
+    }
+
     fn _on_write(&mut self, written: usize, status: WriteStatus) {
         self.outgoing.wrote(written);
 
@@ -800,6 +819,15 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
     }
 
     pub fn write_utf16(&mut self, buf: &[u16]) -> WriteResult {
+        self.write_utf16_impl(buf, true)
+    }
+
+    /// See [`write_now`](Self::write_now).
+    pub fn write_utf16_now(&mut self, buf: &[u16]) -> WriteResult {
+        self.write_utf16_impl(buf, false)
+    }
+
+    fn write_utf16_impl(&mut self, buf: &[u16], may_buffer: bool) -> WriteResult {
         if self.is_done || self.closed_without_reporting {
             return WriteResult::Done(0);
         }
@@ -812,16 +840,25 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
 
         let buf_len = self.outgoing.size() - before_len;
 
-        self.maybe_write_newly_buffered_data(buf_len)
+        self.maybe_write_newly_buffered_data(buf_len, may_buffer)
     }
 
     pub fn write_latin1(&mut self, buf: &[u8]) -> WriteResult {
+        self.write_latin1_impl(buf, true)
+    }
+
+    /// See [`write_now`](Self::write_now).
+    pub fn write_latin1_now(&mut self, buf: &[u8]) -> WriteResult {
+        self.write_latin1_impl(buf, false)
+    }
+
+    fn write_latin1_impl(&mut self, buf: &[u8], may_buffer: bool) -> WriteResult {
         if self.is_done || self.closed_without_reporting {
             return WriteResult::Done(0);
         }
 
         if bun_core::strings::is_all_ascii(buf) {
-            return self.write(buf);
+            return self.write_impl(buf, may_buffer);
         }
 
         let before_len = self.outgoing.size();
@@ -833,15 +870,17 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
 
         let buf_len = self.outgoing.size() - before_len;
 
-        self.maybe_write_newly_buffered_data(buf_len)
+        self.maybe_write_newly_buffered_data(buf_len, may_buffer)
     }
 
-    fn maybe_write_newly_buffered_data(&mut self, buf_len: usize) -> WriteResult {
+    fn maybe_write_newly_buffered_data(&mut self, buf_len: usize, may_buffer: bool) -> WriteResult {
         debug_assert!(!self.is_done);
 
-        if self.should_buffer(0) {
+        if may_buffer && self.should_buffer(0) {
             self.parent_on_write(buf_len, WriteStatus::Drained);
-            Self::register_poll(self);
+            if self.poll_flushes_buffer {
+                Self::register_poll(self);
+            }
 
             return WriteResult::Wrote(buf_len);
         }
@@ -889,11 +928,22 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
     }
 
     pub fn write(&mut self, buf: &[u8]) -> WriteResult {
+        self.write_impl(buf, true)
+    }
+
+    /// `write` that never coalesces: the syscall is attempted now (anything
+    /// already queued goes first), the remainder queued on EAGAIN / short
+    /// write. What a stream's `_write` wants, as opposed to a batching writer.
+    pub fn write_now(&mut self, buf: &[u8]) -> WriteResult {
+        self.write_impl(buf, false)
+    }
+
+    fn write_impl(&mut self, buf: &[u8], may_buffer: bool) -> WriteResult {
         if self.is_done || self.closed_without_reporting {
             return WriteResult::Done(0);
         }
 
-        if self.should_buffer(buf.len()) {
+        if may_buffer && self.should_buffer(buf.len()) {
             // this is streaming, but we buffer the data below `chunk_size` to
             // reduce the number of writes
             if self.outgoing.write(buf).is_err() {
@@ -903,7 +953,9 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
             // noop, but need this to have a chance
             // to register deferred tasks (onAutoFlush)
             self.parent_on_write(buf.len(), WriteStatus::Drained);
-            Self::register_poll(self);
+            if self.poll_flushes_buffer {
+                Self::register_poll(self);
+            }
 
             // it's buffered, but should be reported as written to
             // callers
@@ -977,6 +1029,9 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
                 self.outgoing.wrote(written);
                 if self.outgoing.is_empty() {
                     self.outgoing.reset();
+                } else {
+                    // Backed up: the writable event is what drains the rest.
+                    Self::register_poll(self);
                 }
             }
             WriteResult::Wrote(written) => {
@@ -1048,21 +1103,9 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
             return sys::Result::Ok(());
         }
 
+        let poll = self.ensure_poll(fd);
         // SAFETY: parent BACKREF set via set_parent; outlives this writer.
         let loop_ = unsafe { Parent::event_loop(self.parent()) };
-        let poll = match self.get_poll() {
-            Some(p) => p,
-            None => {
-                let p = FilePollRef::init(
-                    loop_,
-                    fd,
-                    Owner::new(Parent::POLL_OWNER_TAG, std::ptr::from_mut(self).cast()),
-                );
-                self.handle = PollOrFd::Poll(p);
-                p
-            }
-        };
-
         match poll.register_with_fd(loop_.loop_(), FilePollKind::Writable, fd) {
             sys::Result::Err(err) => {
                 return sys::Result::Err(err);
@@ -1072,6 +1115,47 @@ impl<Parent: PosixStreamingWriterParent> PosixStreamingWriter<Parent> {
 
         sys::Result::Ok(())
     }
+
+    /// `start` without registering the poll with the loop: the `FilePoll`
+    /// exists (so `FileType` and keep-alive work) but the kernel isn't asked
+    /// for writability until the first backpressure (`register_poll`). For a
+    /// long-lived writer whose fd is rarely full — an idle `EVFILT_WRITE` /
+    /// `EPOLLOUT` registration on a pipe makes every `write(2)` *and* the
+    /// reader's `read(2)` pay for knote/wakeup bookkeeping. Pair with
+    /// [`unregister_poll`](Self::unregister_poll) once drained.
+    pub fn start_lazy(&mut self, fd: Fd, is_pollable: bool) -> sys::Result<()> {
+        if !is_pollable {
+            return self.start(fd, false);
+        }
+        let _ = self.ensure_poll(fd);
+        sys::Result::Ok(())
+    }
+
+    /// Undo `register_poll` (keeps the `FilePoll`). No-op if not registered.
+    pub fn unregister_poll(&mut self) {
+        let Some(poll) = self.get_poll() else { return };
+        if !poll.is_registered() {
+            return;
+        }
+        // SAFETY: parent BACKREF set via set_parent; outlives this writer.
+        let loop_ = unsafe { Parent::loop_(self.parent()) }.cast();
+        let _ = poll.unregister(loop_, false);
+    }
+
+    fn ensure_poll(&mut self, fd: Fd) -> FilePollRef {
+        if let Some(p) = self.get_poll() {
+            return p;
+        }
+        // SAFETY: parent BACKREF set via set_parent; outlives this writer.
+        let loop_ = unsafe { Parent::event_loop(self.parent()) };
+        let p = FilePollRef::init(
+            loop_,
+            fd,
+            Owner::new(Parent::POLL_OWNER_TAG, std::ptr::from_mut(self).cast()),
+        );
+        self.handle = PollOrFd::Poll(p);
+        p
+    }
 }
 
 impl<Parent: PosixStreamingWriterParent> Drop for PosixStreamingWriter<Parent> {
@@ -2457,6 +2541,21 @@ impl<Parent: WindowsStreamingWriterParent> WindowsStreamingWriter<Parent> {
         self.write_internal_u8(buffer, WriteKind::Bytes)
     }
 
+    /// Windows never coalesces (`uv_write` / `WriteFile` per call), so these
+    /// are the plain writes; see the posix `write_now`.
+    #[inline]
+    pub fn write_now(&mut self, buffer: &[u8]) -> WriteResult {
+        self.write(buffer)
+    }
+    #[inline]
+    pub fn write_latin1_now(&mut self, buffer: &[u8]) -> WriteResult {
+        self.write_latin1(buffer)
+    }
+    #[inline]
+    pub fn write_utf16_now(&mut self, buf: &[u16]) -> WriteResult {
+        self.write_utf16(buf)
+    }
+
     pub fn flush(&mut self) -> WriteResult {
         if self.is_done {
             return WriteResult::Done(0);

src/io/lib.rs

@@ -482,8 +482,10 @@ pub mod pipes;
 #[cfg(windows)]
 #[path = "source.rs"]
 pub mod source;
+pub mod stdio_lock;
 #[path = "write.rs"]
 pub mod write;
+pub use stdio_lock::StdioLock;
 
 // ── re-exports for higher tiers ─────────────────────────────────────────────
 // Byte-level `Write` trait + helpers. Downstream
@@ -1890,6 +1892,10 @@ impl FilePollRef {
         self.inner().flags.insert(f);
     }
     #[inline]
+    pub fn clear_flag(self, f: FilePollFlag) {
+        self.inner().flags.remove(f);
+    }
+    #[inline]
     pub(crate) fn file_type(self) -> crate::pipes::FileType {
         #[cfg(not(windows))]
         {

src/io/openForWriting.rs

@@ -147,7 +147,10 @@ where
                     // this.force_sync = true;
                     // this.writer.force_sync = true;
                     on_force_sync_or_isa_tty(ctx);
-                } else if !is_nonblocking {
+                } else if !is_nonblocking && *pollable {
+                    // O_NONBLOCK is meaningless on regular files / char devices
+                    // and lives on the shared open file description, so only
+                    // set it where it buys us EAGAIN semantics.
                     let flags = match bun_sys::get_fcntl_flags(fd) {
                         Ok(flags) => flags,
                         Err(err) => {