perf: snapshot *io.SectionReader bodies by shallow copy

[knight42]

Summary

Follow-up to #11. A *io.SectionReader body previously fell through snapshotReader to sectionReaderFor, which Seek-probes the shared body (3 calls per attempt) and wraps it in another SectionReader, making every read go through two indirection layers (view.ReadAt → orig.ReadAt → underlying.ReadAt).

io.SectionReader's only mutable field is the read cursor off — r, base, and limit are constant after creation, and Read goes through s.r.ReadAt(p, s.off). So a shallow struct copy gives each attempt an independent cursor over the same stateless ReaderAt, exactly the bytes.Reader/strings.Reader idiom already in the switch. The copy reads [off, limit), the identical window sectionReaderFor would have constructed — zero probes, zero extra wrapping.

The safety assumption (the underlying ReadAt tolerates concurrent calls) is the io.ReaderAt contract, the same one sectionReaderFor already relies on.

Behavior

• Framing unchanged: http.NewRequest cannot size a SectionReader, so the body still goes out chunked.
• sectionReaderFor still handles types that expose ReaderAt+Seeker without being a SectionReader (e.g. *os.File).

Testing

Added a "section reader body" case to the TestRetryBodyNotCorruptedByInFlightUpload table (64 MiB in-flight-upload corruption repro, asserts chunked framing). Full suite passes with -race.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The changes are covered by regression tests with no blocking issues identified.

Pull request overview

Optimizes retry snapshots for *io.SectionReader request bodies while preserving framing and replay behavior.

Changes:

• Shallow-copies SectionReader state per retry attempt.
• Adds regression coverage for in-flight uploads and chunked framing.

File summaries

File	Description
retry_test.go	Tests retry integrity for section-reader bodies.
middleware.go	Adds efficient SectionReader snapshot handling.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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