comm: report all I/O errors

[andreacorbellini]

If a read error happens after opening the file, that error is never reported and comm just quits without printing any error message. This commit fixes that.

This also removes the is_dir() check, preventing a potential race condition where the path could be swapped with a directory in between the is_dir() check and the open() call. On Unix, this correctly results in the "is a directory" error message to be printed (this is detected at read() time).

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will degrade performance by 3.26%

_{Comparing andreacorbellini:comm-safer-is_dir (65cd809) with main (ca13e33)}

Summary

❌ 1 regressed benchmark
✅ 281 untouched benchmarks
⏩ 38 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	dd_copy_partial	129.1 KB	133.5 KB	-3.26%

Footnotes

1. 38 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[andreacorbellini]

For Windows I decided to stick with the same behavior of Unix: report the error from the operating system. In the case of Windows, if the file is a directory, you get "permission denied". Not very user friendly, but I think this is the best approach, mostly because it's not racy.

If the maintainers don't like that, we can add a metadata check just for Windows (not on Unix), maybe something like: try to open the file, if it fails with a "permission denied", check if it's a directory, and based on that report the most appropriate error message. Let me know what you prefer and I will update the PR accordingly.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/pr/pr-tests was skipped on 'main' but is now failing.