utils: do not use errno after success

[eriksjolund]

I noticed this when reading the source code.

[gemini-code-assist]

Summary of Changes

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This pull request refines the error handling logic within a utility function by ensuring that the errno global variable is only consulted and reported when a preceding operation has explicitly indicated a failure. This change prevents the reporting of stale or irrelevant error codes, leading to more accurate and reliable error diagnostics.

Highlights

• Error Handling: Corrected the usage of errno in the create_file_if_missing_at function to ensure it is only read and passed to the error reporting mechanism when an actual error has occurred, preventing potentially misleading error codes.

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Changelog

• src/libcrun/utils.c
  • Modified the crun_make_error call to conditionally pass errno based on the return value of the operation, ensuring errno is only used after a failure.

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[gemini-code-assist]

Code Review

This pull request aims to fix incorrect errno usage after a successful function call. The change is correct in principle, but it leads to a loss of information in error messages. I've suggested an alternative approach that preserves the original error information while still being correct regarding errno usage.

[gemini-code-assist]

While this change correctly avoids using errno after a successful function call (get_file_type_at), it replaces the original error from openat with 0. This loses valuable information. For instance, if openat failed because a directory with the same name exists (EISDIR), this information would be lost.

A better approach, consistent with other parts of this file (e.g., line 227), is to save errno after the failing openat call and use it later.

I recommend an implementation like this:

if (fd_write < 0)
  {
    int saved_errno = errno;
    mode_t tmp_mode;
    int ret;

    /* On errors, check if the file already exists.  */
    ret = get_file_type_at (dirfd, &tmp_mode, false, file);
    if (ret == 0 && S_ISREG (tmp_mode))
      return 0;

    return crun_make_error (err, ret ? errno : saved_errno, "create file `%s`", file);
  }

Since I can only suggest a change to this specific line, my suggestion below assumes you've declared and assigned saved_errno earlier in the block.

      return crun_make_error (err, ret ? errno : saved_errno, "create file `%s`", file);

[eriksjolund]

Seems like a good advise. I'll check later.