s3/transfermanager: fix panic in concurrentReader when len(p) is smaller than cap(p)

[islishude]

Summary

This fixes a panic in concurrentReader.read when the caller passes a slice where len(p) < cap(p).

The reader was using cap(p) to:

• detect empty input
• calculate the max part index to process
• decide whether p[index:] was safe to slice

That is not safe for an io.Reader, because reads must be bounded by len(p), not cap(p).

Read reads up to len(p) bytes into p. It returns the number of bytes read (0 <= n <= len(p)) and any error encountered.

In this case, the old logic could treat an offset as valid and then panic with a slice-bounds error when evaluating p[index:].

Changes

• replace cap(p) with len(p) in concurrentReader.read
• keep part selection and direct chunk writes aligned with the actual readable window
• add regression coverage for both paths that previously panicked:
  • reading from a buffered chunk
  • reading from a newly received chunk

Testing

• added TestConcurrentReaderReadUsesSliceLenForBounds
• ran the concurrent reader test file successfully

[Copilot]

Pull request overview

Fixes a slice-bounds panic in feature/s3/transfermanager’s concurrentReader.read by ensuring all read-window calculations and slicing are bounded by len(p) (per io.Reader contract), not cap(p).

Changes:

• Replace cap(p) with len(p) for empty-buffer detection, max part selection, and safe slicing in concurrentReader.read.
• Add a regression test covering both previously-panicking paths: reading from an already-buffered chunk and from a newly received chunk.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
feature/s3/transfermanager/concurrent_reader.go	Bounds all indexing/slicing in read to len(p) to prevent panics with short slices.
feature/s3/transfermanager/concurrent_reader_test.go	Adds regression coverage to ensure short len(p) with larger cap(p) does not panic and does not advance buffered chunk state.

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