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

Bắt đầu với asyncio.StreamReader và các nhánh read, readline, readuntil và readexactly được mô tả trong báo cáo; chạy bản tái hiện được cung cấp để quan sát BufferError khi có một memoryview đang hoạt động. Được xem là hoàn thành khi các phương thức này trả về dữ liệu mong đợi mà không phát sinh ngoại lệ, đồng thời có kiểm thử hồi quy bao phủ các nhánh biến đổi buffer bị ảnh hưởng.