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+"""
+h2/windows
+~~~~~~~~~~
+
+Defines tools for managing HTTP/2 flow control windows.
+
+The objects defined in this module are used to automatically manage HTTP/2
+flow control windows. Specifically, they keep track of what the size of the
+window is, how much data has been consumed from that window, and how much data
+the user has already used. It then implements a basic algorithm that attempts
+to manage the flow control window without user input, trying to ensure that it
+does not emit too many WINDOW_UPDATE frames.
+"""
+from __future__ import annotations
+
+from .exceptions import FlowControlError
+
+# The largest acceptable value for a HTTP/2 flow control window.
+LARGEST_FLOW_CONTROL_WINDOW = 2**31 - 1
+
+
+class WindowManager:
+ """
+ A basic HTTP/2 window manager.
+
+ :param max_window_size: The maximum size of the flow control window.
+ :type max_window_size: ``int``
+ """
+
+ def __init__(self, max_window_size: int) -> None:
+ assert max_window_size <= LARGEST_FLOW_CONTROL_WINDOW
+ self.max_window_size = max_window_size
+ self.current_window_size = max_window_size
+ self._bytes_processed = 0
+
+ def window_consumed(self, size: int) -> None:
+ """
+ We have received a certain number of bytes from the remote peer. This
+ necessarily shrinks the flow control window!
+
+ :param size: The number of flow controlled bytes we received from the
+ remote peer.
+ :type size: ``int``
+ :returns: Nothing.
+ :rtype: ``None``
+ """
+ self.current_window_size -= size
+ if self.current_window_size < 0:
+ msg = "Flow control window shrunk below 0"
+ raise FlowControlError(msg)
+
+ def window_opened(self, size: int) -> None:
+ """
+ The flow control window has been incremented, either because of manual
+ flow control management or because of the user changing the flow
+ control settings. This can have the effect of increasing what we
+ consider to be the "maximum" flow control window size.
+
+ This does not increase our view of how many bytes have been processed,
+ only of how much space is in the window.
+
+ :param size: The increment to the flow control window we received.
+ :type size: ``int``
+ :returns: Nothing
+ :rtype: ``None``
+ """
+ self.current_window_size += size
+
+ if self.current_window_size > LARGEST_FLOW_CONTROL_WINDOW:
+ msg = f"Flow control window mustn't exceed {LARGEST_FLOW_CONTROL_WINDOW}"
+ raise FlowControlError(msg)
+
+ self.max_window_size = max(self.current_window_size, self.max_window_size)
+
+ def process_bytes(self, size: int) -> int | None:
+ """
+ The application has informed us that it has processed a certain number
+ of bytes. This may cause us to want to emit a window update frame. If
+ we do want to emit a window update frame, this method will return the
+ number of bytes that we should increment the window by.
+
+ :param size: The number of flow controlled bytes that the application
+ has processed.
+ :type size: ``int``
+ :returns: The number of bytes to increment the flow control window by,
+ or ``None``.
+ :rtype: ``int`` or ``None``
+ """
+ self._bytes_processed += size
+ return self._maybe_update_window()
+
+ def _maybe_update_window(self) -> int | None:
+ """
+ Run the algorithm.
+
+ Our current algorithm can be described like this.
+
+ 1. If no bytes have been processed, we immediately return 0. There is
+ no meaningful way for us to hand space in the window back to the
+ remote peer, so let's not even try.
+ 2. If there is no space in the flow control window, and we have
+ processed at least 1024 bytes (or 1/4 of the window, if the window
+ is smaller), we will emit a window update frame. This is to avoid
+ the risk of blocking a stream altogether.
+ 3. If there is space in the flow control window, and we have processed
+ at least 1/2 of the window worth of bytes, we will emit a window
+ update frame. This is to minimise the number of window update frames
+ we have to emit.
+
+ In a healthy system with large flow control windows, this will
+ irregularly emit WINDOW_UPDATE frames. This prevents us starving the
+ connection by emitting eleventy bajillion WINDOW_UPDATE frames,
+ especially in situations where the remote peer is sending a lot of very
+ small DATA frames.
+ """
+ # TODO: Can the window be smaller than 1024 bytes? If not, we can
+ # streamline this algorithm.
+ if not self._bytes_processed:
+ return None
+
+ max_increment = (self.max_window_size - self.current_window_size)
+ increment = 0
+
+ # Note that, even though we may increment less than _bytes_processed,
+ # we still want to set it to zero whenever we emit an increment. This
+ # is because we'll always increment up to the maximum we can.
+ if ((self.current_window_size == 0) and (
+ self._bytes_processed > min(1024, self.max_window_size // 4))) or self._bytes_processed >= (self.max_window_size // 2):
+ increment = min(self._bytes_processed, max_increment)
+ self._bytes_processed = 0
+
+ self.current_window_size += increment
+ return increment