Implement synchronous writes for stdout/stderr in Bun socket implementation

src/bun.js/api/bun/socket.zig

@@ -1432,7 +1432,9 @@ fn NewSocket(comptime ssl: bool) type {
             owned_protos: bool = true,
             is_paused: bool = false,
             allow_half_open: bool = false,
-            _: u7 = 0,
+            /// When created from stdout/stderr fd, force synchronous writes
+            is_stdio_sync: bool = false,
+            _: u6 = 0,
         };
 
         pub fn hasPendingActivity(this: *This) callconv(.C) bool {
@@ -1491,6 +1493,15 @@ fn NewSocket(comptime ssl: bool) type {
                     );
                 },
                 .fd => |f| {
+                    // Check if this is stdout or stderr
+                    if (f.stdioTag()) |tag| {
+                        switch (tag) {
+                            .std_out, .std_err => {
+                                this.flags.is_stdio_sync = true;
+                            },
+                            else => {},
+                        }
+                    }
                     const socket = This.Socket.fromFd(this.socket_context.?, f, This, this, null, false) orelse return error.ConnectionFailed;
                     this.onOpen(socket);
                 },
@@ -2509,6 +2520,11 @@ fn NewSocket(comptime ssl: bool) type {
                 const remaining = bytes[uwrote..];
                 if (remaining.len > 0) {
                     this.buffered_data_for_node_net.append(bun.default_allocator, remaining) catch bun.outOfMemory();
+
+                    // Flush immediately for stdout/stderr sockets to ensure synchronous behavior
+                    if (this.flags.is_stdio_sync) {
+                        this.internalFlush();
+                    }
                 }
             }
 
@@ -2537,16 +2553,29 @@ fn NewSocket(comptime ssl: bool) type {
 
         fn internalFlush(this: *This) void {
             if (this.buffered_data_for_node_net.len > 0) {
-                const written: usize = @intCast(@max(this.socket.write(this.buffered_data_for_node_net.slice(), false), 0));
-                this.bytes_written += written;
-                if (written > 0) {
-                    if (this.buffered_data_for_node_net.len > written) {
-                        const remaining = this.buffered_data_for_node_net.slice()[written..];
-                        _ = bun.c.memmove(this.buffered_data_for_node_net.ptr, remaining.ptr, remaining.len);
-                        this.buffered_data_for_node_net.len = @truncate(remaining.len);
-                    } else {
-                        this.buffered_data_for_node_net.deinitWithAllocator(bun.default_allocator);
-                        this.buffered_data_for_node_net = .{};
+                // For stdio sockets, keep writing until all data is flushed
+                while (this.buffered_data_for_node_net.len > 0) {
+                    const written: usize = @intCast(@max(this.socket.write(this.buffered_data_for_node_net.slice(), false), 0));
+                    this.bytes_written += written;
+                    if (written > 0) {
+                        if (this.buffered_data_for_node_net.len > written) {
+                            const remaining = this.buffered_data_for_node_net.slice()[written..];
+                            _ = bun.c.memmove(this.buffered_data_for_node_net.ptr, remaining.ptr, remaining.len);
+                            this.buffered_data_for_node_net.len = @truncate(remaining.len);
+                        } else {
+                            this.buffered_data_for_node_net.deinitWithAllocator(bun.default_allocator);
+                            this.buffered_data_for_node_net = .{};
+                        }
+                    }
+
+                    // For non-stdio sockets, only do one write attempt
+                    if (!this.flags.is_stdio_sync) {
+                        break;
+                    }
+
+                    // If no data was written, avoid infinite loop
+                    if (written == 0) {
+                        break;
                     }
                 }
             }

test/js/node/test/sequential/test-stream2-stderr-sync.js

@@ -0,0 +1,95 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+'use strict';
+// Make sure that sync writes to stderr get processed before exiting.
+
+require('../common');
+
+function parent() {
+  const spawn = require('child_process').spawn;
+  const assert = require('assert');
+  let i = 0;
+  children.forEach(function(_, c) {
+    const child = spawn(process.execPath, [__filename, String(c)]);
+    let err = '';
+
+    child.stderr.on('data', function(c) {
+      err += c;
+    });
+
+    child.on('close', function() {
+      assert.strictEqual(err, `child ${c}\nfoo\nbar\nbaz\n`);
+      console.log(`ok ${++i} child #${c}`);
+      if (i === children.length)
+        console.log(`1..${i}`);
+    });
+  });
+}
+
+// using console.error
+function child0() {
+  console.error('child 0');
+  console.error('foo');
+  console.error('bar');
+  console.error('baz');
+}
+
+// Using process.stderr
+function child1() {
+  process.stderr.write('child 1\n');
+  process.stderr.write('foo\n');
+  process.stderr.write('bar\n');
+  process.stderr.write('baz\n');
+}
+
+// using a net socket
+function child2() {
+  const net = require('net');
+  const socket = new net.Socket({
+    fd: 2,
+    readable: false,
+    writable: true,
+  });
+  socket.write('child 2\n');
+  socket.write('foo\n');
+  socket.write('bar\n');
+  socket.write('baz\n');
+}
+
+
+function child3() {
+  console.error('child 3\nfoo\nbar\nbaz');
+}
+
+function child4() {
+  process.stderr.write('child 4\nfoo\nbar\nbaz\n');
+}
+
+const children = [ child0, child1, child2, child3, child4 ];
+
+if (!process.argv[2]) {
+  parent();
+} else {
+  children[process.argv[2]]();
+  // Immediate process.exit to kill any waiting stuff.
+  process.exit();
+}