asyncio.StreamReader raises BufferError with active memoryview

Bug report

Bug description:

Summary

When StreamReader has an active memoryview export of its internal buffer, methods that mutate the buffer in place can raise:

BufferError: Existing exports of data: object cannot be re-sized

This appears in real workloads with async database drivers, but the root cause is in StreamReader buffer mutation behavior.

Steps to reproduce

import asyncio

async def main():
    r = asyncio.StreamReader()
    r.feed_data(b"AABBCCDD")

    # Simulate a consumer that keeps a memoryview alive
    mv = memoryview(r._buffer)

    # Triggers BufferError in current implementation
    await r.readexactly(4)

    mv.release()

asyncio.run(main())

Expected behavior

readexactly(4) should return b"AABB" without raising BufferError.

Actual behavior

BufferError: Existing exports of data: object cannot be re-sized

Why this happens

Some StreamReader paths mutate bytearray in place, for example slice deletion and clear operations. In-place resize is not allowed while a memoryview export exists.

Affected methods

• read
• readline
• readuntil
• readexactly

Environment

• Python: 3.13.x, 3.14.x (also reproducible in local tests with newer builds)
• OS: macOS (also observed in Linux server workloads)
• Third-party context where this is frequently hit: async DB drivers using memoryview over packet buffers

Possible fix direction

Replace in-place buffer mutations with non-mutating replacement assignments, for example:

• replace slice delete with reassignment to remaining bytes
• replace clear with assigning a new empty bytearray

If useful, I can open a PR with tests covering the regression paths.

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Linked PRs

• gh-146380

Comienza con asyncio.StreamReader y las rutas de read, readline, readuntil y readexactly descritas en el informe; ejecuta la reproducción proporcionada para observar el BufferError con un memoryview activo. Se considera completado cuando estos métodos devuelvan los datos esperados sin generar una excepción, con cobertura de regresión para las rutas afectadas de mutación del búfer.