[PR #8933/8f3b1f44 backport][3.10] Small cleanups to the websocket fr…

…ame sender (#8978)

Co-authored-by: J. Nick Koston <nick@koston.org>

aiohttp/http_websocket.py

@@ -115,6 +115,7 @@ class WSMsgType(IntEnum):
 PACK_RANDBITS = Struct("!L").pack
 MSG_SIZE: Final[int] = 2**14
 DEFAULT_LIMIT: Final[int] = 2**16
+MASK_LEN: Final[int] = 4
 
 
 class WSMessage(NamedTuple):
@@ -625,12 +626,18 @@ async def _send_frame(
         if self._closing and not (opcode & WSMsgType.CLOSE):
             raise ConnectionResetError("Cannot write to closing transport")
 
+        # RSV are the reserved bits in the frame header. They are used to
+        # indicate that the frame is using an extension.
+        # https://datatracker.ietf.org/doc/html/rfc6455#section-5.2
         rsv = 0
-
         # Only compress larger packets (disabled)
         # Does small packet needs to be compressed?
         # if self.compress and opcode < 8 and len(message) > 124:
         if (compress or self.compress) and opcode < 8:
+            # RSV1 (rsv = 0x40) is set for compressed frames
+            # https://datatracker.ietf.org/doc/html/rfc7692#section-7.2.3.1
+            rsv = 0x40
+
             if compress:
                 # Do not set self._compress if compressing is for this frame
                 compressobj = self._make_compress_obj(compress)
@@ -649,40 +656,56 @@ async def _send_frame(
             )
             if message.endswith(_WS_DEFLATE_TRAILING):
                 message = message[:-4]
-            rsv = rsv | 0x40
 
         msg_length = len(message)
 
         use_mask = self.use_mask
-        if use_mask:
-            mask_bit = 0x80
-        else:
-            mask_bit = 0
+        mask_bit = 0x80 if use_mask else 0
 
+        # Depending on the message length, the header is assembled differently.
+        # The first byte is reserved for the opcode and the RSV bits.
+        first_byte = 0x80 | rsv | opcode
         if msg_length < 126:
-            header = PACK_LEN1(0x80 | rsv | opcode, msg_length | mask_bit)
+            header = PACK_LEN1(first_byte, msg_length | mask_bit)
+            header_len = 2
         elif msg_length < (1 << 16):
-            header = PACK_LEN2(0x80 | rsv | opcode, 126 | mask_bit, msg_length)
+            header = PACK_LEN2(first_byte, 126 | mask_bit, msg_length)
+            header_len = 4
         else:
-            header = PACK_LEN3(0x80 | rsv | opcode, 127 | mask_bit, msg_length)
+            header = PACK_LEN3(first_byte, 127 | mask_bit, msg_length)
+            header_len = 10
+
+        # https://datatracker.ietf.org/doc/html/rfc6455#section-5.3
+        # If we are using a mask, we need to generate it randomly
+        # and apply it to the message before sending it. A mask is
+        # a 32-bit value that is applied to the message using a
+        # bitwise XOR operation. It is used to prevent certain types
+        # of attacks on the websocket protocol. The mask is only used
+        # when aiohttp is acting as a client. Servers do not use a mask.
         if use_mask:
             mask = PACK_RANDBITS(self.get_random_bits())
             message = bytearray(message)
             _websocket_mask(mask, message)
             self._write(header + mask + message)
-            self._output_size += len(header) + len(mask) + msg_length
+            self._output_size += header_len + MASK_LEN + msg_length
+
         else:
             if msg_length > MSG_SIZE:
                 self._write(header)
                 self._write(message)
             else:
                 self._write(header + message)
 
-            self._output_size += len(header) + msg_length
+            self._output_size += header_len + msg_length
 
         # It is safe to return control to the event loop when using compression
         # after this point as we have already sent or buffered all the data.
 
+        # Once we have written output_size up to the limit, we call the
+        # drain helper which waits for the transport to be ready to accept
+        # more data. This is a flow control mechanism to prevent the buffer
+        # from growing too large. The drain helper will return right away
+        # if the writer is not paused.
         if self._output_size > self._limit:
             self._output_size = 0
             await self.protocol._drain_helper()