fix(usockets) fix last_write_failed flag (#25496)

https://github.com/oven-sh/bun/pull/25361 needs to be merged before this
PR

## Summary
- Move `last_write_failed` flag from loop-level to per-socket flag for
correctness

## Changes

- Move `last_write_failed` from `loop->data` to `socket->flags`
- More semantically correct since write status is per-socket, not
per-loop

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

packages/bun-usockets/src/crypto/openssl.c

@@ -417,8 +417,7 @@ void us_internal_update_handshake(struct us_internal_ssl_socket_t *s) {
     }
     s->handshake_state = HANDSHAKE_PENDING;
     s->ssl_write_wants_read = 1;
-    s->s.context->loop->data.last_write_failed = 1;
-
+    s->s.flags.last_write_failed = 1;
     return;
   }
   // success

packages/bun-usockets/src/eventing/libuv.c

@@ -71,7 +71,7 @@ void us_poll_init(struct us_poll_t *p, LIBUS_SOCKET_DESCRIPTOR fd,
 }
 
 void us_poll_free(struct us_poll_t *p, struct us_loop_t *loop) {
-  // poll was adopted and dont own uv_poll_t anymore
+  // poll was resized and dont own uv_poll_t anymore
   if(!p->uv_p) {
     free(p);
     return;

packages/bun-usockets/src/internal/internal.h

@@ -176,6 +176,8 @@ struct us_socket_flags {
     bool adopted: 1;
     /* If true, the socket is a TLS socket */
     bool is_tls: 1;
+    /* If true, the last write to this socket failed (would block) */
+    bool last_write_failed: 1;
 
 } __attribute__((packed));
 

packages/bun-usockets/src/internal/loop_data.h

@@ -37,7 +37,6 @@ struct us_internal_loop_data_t {
     struct us_timer_t *sweep_timer;
     int sweep_timer_count;
     struct us_internal_async *wakeup_async;
-    int last_write_failed;
     struct us_socket_context_t *head;
     struct us_socket_context_t *iterator;
     struct us_socket_context_t *closed_context_head;

packages/bun-usockets/src/loop.c

@@ -391,14 +391,12 @@ void us_internal_dispatch_ready_poll(struct us_poll_t *p, int error, int eof, in
             /* The context can change after calling a callback but the loop is always the same */
             struct us_loop_t* loop = s->context->loop;
             if (events & LIBUS_SOCKET_WRITABLE && !error) {
-                /* Note: if we failed a write as a socket of one loop then adopted
-                 * to another loop, this will be wrong. Absurd case though */
-                loop->data.last_write_failed = 0;
+                s->flags.last_write_failed = 0;
                 #ifdef LIBUS_USE_KQUEUE
                 /* Kqueue is one-shot so is not writable anymore */
                 p->state.poll_type = us_internal_poll_type(p) | ((events & LIBUS_SOCKET_READABLE) ? POLL_TYPE_POLLING_IN : 0);
                 #endif
-                
+
                 s = s->context->on_writable(s);
                 /* After socket adoption, track the new socket; the old one becomes invalid */
                 if(s && s->flags.adopted && s->prev) {
@@ -410,7 +408,7 @@ void us_internal_dispatch_ready_poll(struct us_poll_t *p, int error, int eof, in
                 }
 
                 /* If we have no failed write or if we shut down, then stop polling for more writable */
-                if (!loop->data.last_write_failed || us_socket_is_shut_down(0, s)) {
+                if (!s->flags.last_write_failed || us_socket_is_shut_down(0, s)) {
                     us_poll_change(&s->p, loop, us_poll_events(&s->p) & LIBUS_SOCKET_READABLE);
                 } else {
                     #ifdef LIBUS_USE_KQUEUE

packages/bun-usockets/src/socket.c

@@ -370,7 +370,7 @@ int us_socket_write(int ssl, struct us_socket_t *s, const char *data, int length
 
     int written = bsd_send(us_poll_fd(&s->p), data, length);
     if (written != length) {
-        s->context->loop->data.last_write_failed = 1;
+        s->flags.last_write_failed = 1;
         us_poll_change(&s->p, s->context->loop, LIBUS_SOCKET_READABLE | LIBUS_SOCKET_WRITABLE);
     }
 
@@ -407,7 +407,7 @@ int us_socket_ipc_write_fd(struct us_socket_t *s, const char* data, int length,
     int sent = bsd_sendmsg(us_poll_fd(&s->p), &msg, 0);
 
     if (sent != length) {
-        s->context->loop->data.last_write_failed = 1;
+        s->flags.last_write_failed = 1;
         us_poll_change(&s->p, s->context->loop, LIBUS_SOCKET_READABLE | LIBUS_SOCKET_WRITABLE);
     }
 

src/deps/libuwsockets.cpp

@@ -1701,7 +1701,7 @@ size_t uws_req_get_header(uws_req_t *res, const char *lower_case_header,
   {
     us_socket_r s = (us_socket_t *)res;
     if(us_socket_is_closed(s->flags.is_tls, s)) return;
-    s->context->loop->data.last_write_failed = 1;
+    s->flags.last_write_failed = 1;
     us_poll_change(&s->p, s->context->loop,
                    LIBUS_SOCKET_READABLE | LIBUS_SOCKET_WRITABLE);
   }
@@ -1866,7 +1866,7 @@ __attribute__((callback (corker, ctx)))
 
   void us_socket_sendfile_needs_more(us_socket_r s) {
     if(us_socket_is_closed(s->flags.is_tls, s)) return;
-    s->context->loop->data.last_write_failed = 1;
+    s->flags.last_write_failed = 1;
     us_poll_change(&s->p, s->context->loop, LIBUS_SOCKET_READABLE | LIBUS_SOCKET_WRITABLE);
   }
 

src/deps/uws/InternalLoopData.zig

@@ -4,7 +4,6 @@ pub const InternalLoopData = extern struct {
     sweep_timer: ?*Timer,
     sweep_timer_count: i32,
     wakeup_async: ?*us_internal_async,
-    last_write_failed: i32,
     head: ?*SocketContext,
     iterator: ?*SocketContext,
     closed_context_head: ?*SocketContext,