unix: return UV_EAGAIN if uv_try_write cannot write any data

include/uv.h

@@ -707,8 +707,9 @@ UV_EXTERN int uv_write2(uv_write_t* req,
  * Same as `uv_write()`, but won't queue write request if it can't be completed
  * immediately.
  * Will return either:
- * - >= 0: number of bytes written (can be less than the supplied buffer size)
- * - < 0: negative error code
+ * - > 0: number of bytes written (can be less than the supplied buffer size)
+ * - < 0: negative error code (UV_EAGAIN is returned if no data can be sent
+ *   immediately)
  */
 UV_EXTERN int uv_try_write(uv_stream_t* handle,
                            const uv_buf_t bufs[],

src/unix/stream.c

@@ -1374,7 +1374,7 @@ int uv_try_write(uv_stream_t* stream,
 
   /* Connecting or already writing some data */
   if (stream->connect_req != NULL || stream->write_queue_size != 0)
-    return 0;
+    return -EAGAIN;
 
   has_pollout = uv__io_active(&stream->io_watcher, UV__POLLOUT);
 
@@ -1404,7 +1404,10 @@ int uv_try_write(uv_stream_t* stream,
     uv__stream_osx_interrupt_select(stream);
   }
 
-  return (int) written;
+  if (written == 0)
+    return -EAGAIN;
+  else
+    return written;
 }
 
 

test/test-tcp-try-write.c

@@ -63,12 +63,14 @@ static void connect_cb(uv_connect_t* req, int status) {
   do {
     buf = uv_buf_init(zeroes, sizeof(zeroes));
     r = uv_try_write((uv_stream_t*) &client, &buf, 1);
-    ASSERT(r >= 0);
-    bytes_written += r;
+    ASSERT(r > 0 || r == UV_EAGAIN);
+    if (r > 0) {
+      bytes_written += r;
 
-    /* Partial write */
-    if (r != (int) sizeof(zeroes))
-      break;
+      /* Partial write */
+      if (r != (int) sizeof(zeroes))
+        break;
+    }
   } while (1);
   uv_close((uv_handle_t*) &client, close_cb);
 }