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 #9086 will not alter performance

_{Comparing andreacorbellini:comm-safer-is_dir (6bc9f47) with main (8f7afa7)}

Summary

✅ 125 untouched

[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)