test(conformance): add ConformanceOptions to RunIOTests and fix EOF handling

[funkyshu]

Summary

Prepares the canonical backend/testsuite conformance suite so it can be reused by backends (and a future Testcontainers module, see #294) instead of being duplicated. Two changes:

Added

• RunIOTests now accepts optional ConformanceOptions (the same options type already used by RunConformanceTests), so a backend can opt out of IO sequences it genuinely cannot support. SkipFTPSpecificTests skips the Write, Seek, Write (partial write) cases — FTP streams writes directly to the server and cannot rewind an in-progress write.
• The FTP TestIOConformance (vfsintegration) now passes SkipFTPSpecificTests: true; previously it had no way to, so those cases would fail against a real FTP server.

Fixed

• ExecuteSequence EOF handling. A fixed-size read (R(n)) that returns io.EOF alongside the final bytes is a valid end-of-file condition on some backends. The sequence runner treated any error — including io.EOF — as fatal and aborted. It now recognizes io.EOF as a successful read and continues the sequence.

Notes

• Reuses the existing ConformanceOptions struct rather than introducing a parallel IOOptions type, keeping one coherent options surface across the suite.
• The RunIOTests signature change is backward compatible (variadic), so existing callers (mem, os, s3, gs, azure, sftp, dropbox contrib) are unaffected.
• Skipped cases now use t.Skip (visible as skipped) rather than a silent early return.

Testing

• go build ./...
• go vet -tags=vfsintegration ./backend/ftp/... ./backend/testsuite/...
• go test -tags=vfsintegration -run TestConformance -run TestIOConformance ./backend/mem/... ./backend/os/... — all IO sequences pass, including read-to-EOF cases (mem/os support partial writes, so Write, Seek, Write runs and passes there).
• golangci-lint run --build-tags=vfsintegration ./backend/testsuite/... ./backend/ftp/... — 0 issues.

Part of a series extracted from #294 (this is "PR B"; the Azure/FTP backend fixes landed in #346).

Made with Cursor

[Copilot]

Pull request overview

This PR updates the backend/testsuite IO conformance suite to be more reusable across backends by (1) allowing IO tests to accept ConformanceOptions (consistent with RunConformanceTests) so backends can explicitly skip unsupported IO sequences, and (2) improving the sequence runner’s handling of io.EOF on fixed-size reads to avoid incorrectly failing valid EOF scenarios.

Changes:

• Extended RunIOTests to accept optional ConformanceOptions, enabling selective skipping of IO sequences (e.g., FTP partial-write patterns).
• Added IO conformance unit tests (mem-backed) to cover both the new skip path and the updated EOF behavior.
• Updated the FTP integration conformance test to pass SkipFTPSpecificTests: true for unsupported seek-within-write sequences.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
CHANGELOG.md	Documents the new IO conformance options support and the EOF-handling fix under Unreleased.
backend/testsuite/io_conformance.go	Adds opt-in skipping for seek-within-write sequences and adjusts EOF handling during fixed-size reads.
backend/testsuite/io_conformance_test.go	Adds unit tests for the new RunIOTests options behavior and EOF-on-full-read handling.
backend/ftp/conformance_test.go	Passes SkipFTPSpecificTests to IO conformance to avoid unsupported FTP sequences.

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