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

Start with asyncio.StreamReader and the read, readline, readuntil, and readexactly paths described in the report; run the supplied reproduction to observe the BufferError with an active memoryview. Done means these methods return the expected data without raising, with regression coverage for the affected buffer-mutation paths.