iox v0.3.4 — Fix ErrUnexpectedEOF contract for policy copy functions

[hayabusa-cloud]

Introduction

iox provides non-blocking I/O semantics for Go, extending the standard io package with first-class signals for would-block and partial-completion states.

In traditional blocking I/O, operations either succeed or fail. Non-blocking I/O introduces a third state: "would block" — the operation cannot proceed right now but isn't a failure. Additionally, streaming operations may return "more" — progress was made, but the operation isn't finished. The iox package treats these states as control flow signals, not errors.

The package defines two semantic errors: ErrWouldBlock (no progress, retry later) and ErrMore (progress made, more data available).

The Copy family of functions (Copy, CopyBuffer, CopyN, CopyBufferPolicy) preserve non-blocking semantics that io.Copy would swallow. They support WriterTo/ReaderFrom fast paths and implement seeker rollback for partial writes, enabling safe retry without data loss.

Usage

n, err := iox.CopyNPolicy(dst, src, 1024, policy)
if errors.Is(err, io.ErrUnexpectedEOF) {
    // Source had fewer than 1024 bytes — short copy
}

Highlights

CopyNPolicy and CopyNBufferPolicy now correctly return io.ErrUnexpectedEOF when the source provides fewer bytes than requested, matching the contract established by CopyN and CopyNBuffer. The slow-path write loop in copyBufferPolicy also gains rollback on wrapped semantic write errors, preventing potential data loss on seekable readers.

What's Changed

Bug Fixes

• CopyNPolicy and CopyNBufferPolicy now return io.ErrUnexpectedEOF on short copies instead of (written, nil) — aligns with CopyN/CopyNBuffer contract (fix: return ErrUnexpectedEOF on short copies #23)
• copyBufferPolicy slow-path write loop now performs seeker rollback on wrapped semantic write errors, consistent with copyBuffer (fix: return ErrUnexpectedEOF on short copies #23)

Tests

• Add coverage tests for CopyNPolicy, CopyNBufferPolicy error passthrough, and copyBufferPolicy wrapped semantic error rollback (fix: return ErrUnexpectedEOF on short copies #23)

Compatibility

• Go: 1.26+
• Platforms: linux/amd64, linux/arm64, linux/riscv64, linux/loong64

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This discussion was created from the release iox v0.3.4 — Fix ErrUnexpectedEOF contract for policy copy functions.