proposal: os: support copy_file_range for SectionReader

[hanwen-flow]

Proposal Details

Writing to a file from an io.SectionReader backed by a file could
use copy_file_range(2).

The machinery for this mostly exists: SectionReader.Outer() gives
access to the underlying *os.File and offset. Then, the offset
should be passed into unix.CopyFileRange. After the call completes,
the number of bytes (or the updated offset) can be passed to
SectionReader.Seek.

The only missing piece is that CopyFileRange in internal/poll
does not accept offset arguments. Is this something that can be
changed, and if yes, does it require a formal proposal?

For reference, this is similar to
#61727.

[gabyhelp]

Related Issues

• io: add SectionReader.Outer method #61870 (closed)

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