Add asyncio message assembler.

Author: aaugustin

src/websockets/asyncio/messages.py

@@ -0,0 +1,254 @@
+from __future__ import annotations
+
+import asyncio
+import codecs
+import collections
+from typing import (
+    Any,
+    AsyncIterator,
+    Callable,
+    Generic,
+    TypeVar,
+)
+
+from ..frames import OP_BINARY, OP_CONT, OP_TEXT, Frame
+from ..typing import Data
+
+
+__all__ = ["Assembler"]
+
+UTF8Decoder = codecs.getincrementaldecoder("utf-8")
+
+T = TypeVar("T")
+
+
+class SimpleQueue(Generic[T]):
+    """
+    Simplified version of asyncio.Queue.
+
+    Doesn't support maxsize nor concurrent calls to get().
+
+    """
+
+    def __init__(self) -> None:
+        self.loop = asyncio.get_running_loop()
+        self.get_waiter: asyncio.Future[None] | None = None
+        self.queue: collections.deque[T] = collections.deque()
+
+    def __len__(self) -> int:
+        return len(self.queue)
+
+    def put(self, item: T) -> None:
+        """Put an item into the queue without waiting."""
+        self.queue.append(item)
+        if self.get_waiter is not None and not self.get_waiter.done():
+            self.get_waiter.set_result(None)
+
+    async def get(self) -> T:
+        """Remove and return an item from the queue, waiting if necessary."""
+        if not self.queue:
+            if self.get_waiter is not None:
+                raise RuntimeError("get is already running")
+            self.get_waiter = self.loop.create_future()
+            try:
+                await self.get_waiter
+            finally:
+                self.get_waiter.cancel()
+                self.get_waiter = None
+        return self.queue.popleft()
+
+    def abort(self) -> None:
+        if self.get_waiter is not None and not self.get_waiter.done():
+            self.get_waiter.set_exception(EOFError("stream of frames ended"))
+        self.queue.clear()  # TODO add test for this
+
+
+class Assembler:
+    """
+    Assemble messages from frames.
+
+    :class:`Assembler` expects only data frames. The stream of frames must
+    respect the protocol; if it doesn't, the behavior is undefined.
+
+    Args:
+        pause: Called when the buffer of frames goes above the high water mark;
+            should pause reading from the network.
+        resume: Called when the buffer of frames goes below the low water mark;
+            should resume reading from the network.
+
+    """
+
+    # coverage reports incorrectly: "line NN didn't jump to the function exit"
+    def __init__(  # pragma: no cover
+        self,
+        pause: Callable[[], Any] = lambda: None,
+        resume: Callable[[], Any] = lambda: None,
+    ) -> None:
+        # Queue of incoming messages. Each item is a queue of frames.
+        self.frames: SimpleQueue[Frame] = SimpleQueue()
+
+        # We cannot put a hard limit on the size of the queues because a single
+        # call to Protocol.data_received() could produce thousands of frames,
+        # which must be buffered. Instead, we pause reading when the buffer goes
+        # above the high limit and we resume when it goes under the low limit.
+        self.high = 16
+        self.low = 4
+        self.paused = False
+        self.pause = pause
+        self.resume = resume
+
+        # This flag prevents concurrent calls to get() by user code.
+        self.get_in_progress = False
+
+        # This flag marks the end of the connection.
+        self.closed = False
+
+    async def get(self, decode: bool | None = None) -> Data:
+        """
+        Read the next message.
+
+        :meth:`get` returns a single :class:`str` or :class:`bytes`.
+
+        If the message is fragmented, :meth:`get` waits until the last frame is
+        received, then it reassembles the message and returns it. To receive
+        messages frame by frame, use :meth:`get_iter` instead.
+
+        Raises:
+            EOFError: If the stream of frames has ended.
+            RuntimeError: If two coroutines run :meth:`get` or :meth:`get_iter`
+                concurrently.
+
+        """
+        if self.closed:
+            raise EOFError("stream of frames ended")
+
+        if self.get_in_progress:
+            raise RuntimeError("get or get_iter is already running")
+
+        # Locking with get_in_progress ensures only one coroutine can get here.
+        self.get_in_progress = True
+        try:
+            # First frame
+            frame = await self.frames.get()
+            self.maybe_resume()
+            assert frame.opcode is OP_TEXT or frame.opcode is OP_BINARY
+            if decode is None:
+                decode = frame.opcode is OP_TEXT
+            frames = [frame]
+            # Following frames, for fragmented messages
+            while not frame.fin:
+                frame = await self.frames.get()
+                self.maybe_resume()
+                assert frame.opcode is OP_CONT
+                frames.append(frame)
+        finally:
+            self.get_in_progress = False
+
+        data = b"".join(frame.data for frame in frames)
+        if decode:
+            return data.decode()
+        else:
+            return data
+
+    async def get_iter(self, decode: bool | None = None) -> AsyncIterator[Data]:
+        """
+        Stream the next message.
+
+        Iterating the return value of :meth:`get_iter` asynchronously yields a
+        :class:`str` or :class:`bytes` for each frame in the message.
+
+        The iterator must be fully consumed before calling :meth:`get_iter` or
+        :meth:`get` again. Else, :exc:`RuntimeError` is raised.
+
+        This method only makes sense for fragmented messages. If messages aren't
+        fragmented, use :meth:`get` instead.
+
+        Raises:
+            EOFError: If the stream of frames has ended.
+            RuntimeError: If two coroutines run :meth:`get` or :meth:`get_iter`
+                concurrently.
+
+        """
+        if self.closed:
+            raise EOFError("stream of frames ended")
+
+        if self.get_in_progress:
+            raise RuntimeError("get or get_iter is already running")
+
+        # Locking with get_in_progress ensures only one coroutine can get here.
+        self.get_in_progress = True
+        try:
+            # First frame
+            frame = await self.frames.get()
+            self.maybe_resume()
+            assert frame.opcode is OP_TEXT or frame.opcode is OP_BINARY
+            if decode is None:
+                decode = frame.opcode is OP_TEXT
+            if decode:
+                decoder = UTF8Decoder()
+                yield decoder.decode(frame.data, frame.fin)
+            else:
+                yield frame.data
+            # Following frames, for fragmented messages
+            while not frame.fin:
+                frame = await self.frames.get()
+                self.maybe_resume()
+                assert frame.opcode is OP_CONT
+                if decode:
+                    yield decoder.decode(frame.data, frame.fin)
+                else:
+                    yield frame.data
+        finally:
+            self.get_in_progress = False
+
+    def put(self, frame: Frame) -> None:
+        """
+        Add ``frame`` to the next message.
+
+        Raises:
+            EOFError: If the stream of frames has ended.
+
+        """
+        if self.closed:
+            raise EOFError("stream of frames ended")
+
+        self.frames.put(frame)
+        self.maybe_pause()
+
+    def get_limits(self) -> tuple[int, int]:
+        """Return low and high water marks for flow control."""
+        return self.low, self.high
+
+    def set_limits(self, low: int = 4, high: int = 16) -> None:
+        """Configure low and high water marks for flow control."""
+        self.low, self.high = low, high
+
+    def maybe_pause(self) -> None:
+        """Pause the writer if queue is above the high water mark."""
+        # Check for "> high" to support high = 0
+        if len(self.frames) > self.high and not self.paused:
+            self.paused = True
+            self.pause()
+
+    def maybe_resume(self) -> None:
+        """Resume the writer if queue is below the low water mark."""
+        # Check for "<= low" to support low = 0
+        if len(self.frames) <= self.low and self.paused:
+            self.paused = False
+            self.resume()
+
+    def close(self) -> None:
+        """
+        End the stream of frames.
+
+        Callling :meth:`close` concurrently with :meth:`get`, :meth:`get_iter`,
+        or :meth:`put` is safe. They will raise :exc:`EOFError`.
+
+        """
+        if self.closed:
+            return
+
+        self.closed = True
+
+        # Unblock get or get_iter.
+        self.frames.abort()