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_with_compare::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_with_compare::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 addresses a server crash (MDEV-30297) when dropping a period with an empty name by adding null-pointer and length checks to Lex_cstring_with_compare::streq. A test case is also included to verify the fix. The review feedback suggests optimizing the comparison logic in streq by checking for pointer equality first to provide a fast-path return.

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[Copilot]

Pull request overview

Fixes a crash/assertion failure when dropping a PERIOD with an empty name by making identifier string equality checks safe for default-constructed (NULL) lex strings, and adds a regression test for the reported case (MDEV-30297).

Changes:

• Add NULL-pointer handling in Lex_cstring_with_compare::streq() to avoid calling Compare_identifiers (and similar comparators) with NULL str pointers.
• Add a mysql-test regression case for `ALTER TABLE ... DROP PERIOD IF EXISTS FOR ```.
• Update the expected test output to include the resulting warning.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
sql/vers_string.h	Makes streq() robust against NULL str pointers to prevent comparator crashes/asserts.
mysql-test/suite/period/t/alter.test	Adds a regression test for dropping a PERIOD with an empty name.
mysql-test/suite/period/r/alter.result	Captures the expected warning output for the new test case.

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[sanja-byelkin]

Please fix commit comment as we agreed and look on copilot complains. OK to push after fixing