Bug report

Bug description:

Summary

WhenStreamReader 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 inStreamReader buffer mutation behavior.

Steps to reproduce

importasyncioasyncdefmain():r=asyncio.StreamReader()r.feed_data(b"AABBCCDD")# Simulate a consumer that keeps a memoryview alivemv=memoryview(r._buffer)# Triggers BufferError in current implementationawaitr.readexactly(4)mv.release()asyncio.run(main())

Expected behavior

readexactly(4) should returnb"AABB" without raisingBufferError.

Actual behavior

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

Why this happens

SomeStreamReader paths mutatebytearray 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-146379: Avoid StreamReader BufferError with active memoryview exports #146380