MDEV-39788: Fix master.info not upgrading master_use_gtid

[ParadoxV5]

Note

This PR is based on #5147, which provides the regression test.

• Closes MDEV-39788
  • Supercedes MDEV-39788 CHANGE MASTER savefiles read and write one line to many #5148

The line-count line in master.info and relay-log.info has been inconsistent (off by one) since its introduction.
MDEV-37530 corrected this by changing master.info to use relay-log.info’s line-count definition by chance, but at the cost of an upgrade incompatibility.
It now reads one more line beyond the pre-upgraded file’s line-based section, which for MariaDB 10.0–12.2 means an upgrade doesn’t carry over the first option stored in key=value, master_use_gtid.

Since older versions can read the new format in a downgrade, rather than encouraging the old inconsistency, this commit focuses on the upgrade problem by adding a shim entry to master.info’s list to emulate prior versions’ reading behaviour.
Although this implementation can only restore compatibility with versions 10.0+, versions before MariaDB 10 have long been EOL.

While here, this commit also fixes code and comments that contradict the actual effect.

[gemini-code-assist]

Code Review

This pull request addresses compatibility issues in parsing *.info files (specifically master.info and relay-log.info) from older MariaDB/MySQL versions, ensuring that line counts and position values greater than 2^31 are handled correctly. It introduces a LINE_COUNT_FIX pseudo-value in Master_info_file to restore compatibility with files generated by MariaDB 10.0+ and updates the padding and saving logic in Info_file. Additionally, new tests are added to verify these changes. The review feedback suggests naming the unnamed struct LINE_COUNT_FIX to Line_count_fix_t to improve readability and prevent potential compiler-specific issues, along with a minor typo correction in the documentation.

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[gemini-code-assist]

Naming the unnamed struct Line_count_fix_t improves code readability and avoids potential compiler-specific issues with unnamed classes overriding virtual functions or being used in template-based pointer-to-member wrappers. Additionally, there is a minor typo in the comment: 'use common code' should be 'use of common code'.

protected:
  /**
    @ref Master_info_file has been inconsistent that its line count included
    the line-count "value", while @ref Relay_log_info_file's excludes it.
    Although the use of common code now makes @ref Master_info_file match,
    files from older versions are now off-by-one to Info_file::load_from_file().
    The presence of this non-reading pseudo-value restores
    compatibility with files generated by MariaDB 10.0+,
    as the driver loop will increment the line counter for it,
    thus emulating counting the line-count "value".
    @deprecated This is not needed after all pre-12.3 versions go EoL.
  */
  struct Line_count_fix_t : Persistent
  {
    bool load_from(IO_CACHE *file) override { return false; } ///< No-op
    void save_to(IO_CACHE *file) override {} ///< No-op
  } LINE_COUNT_FIX;