THRIFT-6267: Collect TNonblockingServer reads in a bytearray

[Jens-G]

JIRA: THRIFT-6267
Client: py

Connection.read() in lib/py/src/server/TNonblockingServer.py did self._rbuf += read on a bytes object. A bytes object cannot grow, so each append copied everything received so far, and a frame that arrives in n pieces costs O(n²) bytes of copying. Measured with the new test: a 1 MiB frame in 4 KiB pieces copied 135,790,596 bytes.

Change

• _rbuf is a bytearray, so appending grows it in place. The consumed frame is removed with del self._rbuf[:end].
• Each message gets bytes(self._rbuf[:end]), a copy of exactly its own frame. The processing thread still builds TMemoryBuffer(msg.buffer, msg.offset) as before.

One difference in behaviour: a message used to receive the whole buffer as it stood. When further bytes arrived in the same read, the transport the processor read from therefore continued past the end of its frame into the next one. Now it ends at the frame. What follows a frame stays in the buffer for the next message, as before.

Tests

New lib/py/test/test_nonblocking_server_read_buffer.py, registered in both lists in lib/py/Makefile.am. It uses a socket stand-in that hands out one piece per read(), the way one select() wakeup would.

• Counting test: a 1 MiB frame in 257 pieces has to be collected copying at most twice its size. The pieces are a bytes subclass whose __radd__ counts what buffer + piece copies. Growing a bytearray is not counted.
• Frame boundary: two frames in one read; each message yields its own payload and then nothing.
• Split length: a length word split across three reads.
• Remainder: a frame followed by the start of the next one; the rest arrives after the first message was handled.

Against the unmodified server:

• the counting test fails with 135,790,596 bytes copied against a limit of 2,097,160;
• the boundary test fails, because the first message's transport goes on with \x00\x00\x00\x0ethe second one;
• the remainder test fails for the same reason.

Mutations, each on its own:

• Collecting through bytes(...) + read: the counting test fails.
• Handing the message the buffer itself instead of a copy: all four tests fail, because the later del empties it.
• Copying the whole buffer instead of the frame: both boundary tests fail.

Also run, with the fix:

• test_nonblocking_server_read_buffer.py and test_nonblocking_server_frame_size.py pass on Python 3.10, 3.12 and 3.14;
• thrift_TNonblockingServer.py, a server with a real client, passes on 3.10;
• flake8 is clean.

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