MDEV-30297 Server crash / assertion failure in Compare_identifiers::operator upon dropping period with empty name

[pranavktiwari]

fixes MDEV-30297

Problem:

`ALTER TABLE ... DROP PERIOD IF EXISTS FOR ``` can trigger an assertion failure in debug builds and crash in non-debug builds.

Cause:

Lex_cstring::streq() may invoke Compare_identifiers on default-constructed Lex_cstring objects (str == NULL, length == 0). Compare_identifiers assumes non-null string pointers and asserts or crashes when called with a null pointer.

Fix:

Add null-pointer handling to Lex_cstring::streq() and avoid calling Compare_identifiers when either string pointer is null. This prevents invalid comparisons while preserving existing behavior for initialized strings.

[gemini-code-assist]

Code Review

This pull request updates the streq method in sql/vers_string.h to safely handle null pointers during string comparisons. The reviewer suggested optimizing this logic by checking for length mismatches first to fail fast, skipping the comparison call when the length is zero, and simplifying the null pointer checks.

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[gemini-code-assist]

We can optimize this comparison by failing fast when the lengths do not match, which is the most common case for non-equal strings. Additionally, we can avoid calling Compare() entirely when the length is 0 (since two empty strings are always equal under any collation). This also simplifies the NULL pointer handling.

    if (length != b.length)
      return false;
    if (str == NULL || b.str == NULL)
      return str == b.str;
    return length == 0 || 0 == Compare()(*this, b);

[sanja-byelkin]

why it is for 11.4 not for 10.11?

[sanja-byelkin]

add version marker