Rework MDL enum constants values

[pranavktiwari]

fixes MDEV-39184

Problem:

MDL constants values of 0 assign some default values (MDL_INTENTION_EXCLUSIVE, BACKUP) before MDL_REQUEST_INIT(). That does not make sense if MDL_REQUEST_INIT() was not called. Please rework the values of 0 into (MDL_NOT_INITIALIZED, NOT_INITIALIZED). Described by these comments in the code:

Fix

1- Increment all values in enum_mdl_type by 1.
2- Add a 0 at the start of the incompatibility arrays so the enum indices continue to match their corresponding bitmasks.

This ensures that uninitialized MDL constants are correctly handled while preserving existing bitmask mappings.

[CLAassistant]

All committers have signed the CLA.

[midenok]

Hi @pranavktiwari,

Thank you for the patch! Now I see that change for MDL_BIT has negative effect on performance which is unwanted. I'd better tell in the description that it is investigation task and should be evaluated for push. Nonetheless, if we rework enum_mdl_type into bitmask values we would not need MDL_BIT() at all. What other problems occur if we try that?

It seems that I cannot recognize the changes for the second part of the ticket (about enum_mdl_namespace). Is it feasible to do these changes? Can you try to do it on the minimal set of code, without plugins and minor storage engines?

[Copilot]

Pull request overview

This PR reworks MDL enum constant values so that 0 represents an uninitialized MDL state (instead of mapping to real lock types), and updates related mappings/output so metadata lock reporting continues to work.

Changes:

• Shifted enum_mdl_type / backup-stage lock constants so MDL_NOT_INITIALIZED becomes 0.
• Introduced MDL_key::NOT_INITIALIZED=0 namespace and updated handlers/mappings accordingly.
• Updated incompatibility matrices, metadata-lock name tables, and adjusted mysql-test expected results.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
sql/sp.h	Adds handling for MDL_key::NOT_INITIALIZED in namespace→handler switch.
sql/mdl.h	Updates MDL type and namespace enums and shifts backup lock constants.
sql/mdl.cc	Adds NOT_INITIALIZED PSI stage entry, pads name tables, and pads incompatibility arrays.
plugin/metadata_lock_info/metadata_lock_info.cc	Adds “Not initialised” entry to namespace name table.
mysql-test/main/mdl_sync.result	Updates expected output ordering for MDL metadata_lock_info.
mysql-test/main/mdl.result	Updates expected output ordering for MDL metadata_lock_info.

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[Copilot]

The new stage name string has a spelling error: "Not initilised". This ends up visible in processlist/performance schema stage output; please fix the typo (and ideally keep spelling consistent with the information_schema.metadata_lock_info strings).

Suggested change

	{0, "Not initilised", 0},
	{0, "Not initialised", 0},

[Copilot]

The 0th entry in lock_types is now an empty LEX_STRING (str==NULL). If any code path ends up calling get_mdl_lock_name()/get_type_name() with MDL_NOT_INITIALIZED (e.g., debug logging on an uninitialized MDL_request), this can lead to undefined behavior when printing "%s". Consider providing a real name like "MDL_NOT_INITIALIZED" instead of an empty initializer to keep it safe and debuggable.

Suggested change

	{},
	{ C_STRING_WITH_LEN("MDL_NOT_INITIALIZED") },

[Copilot]

Same issue as lock_types[]: backup_lock_types[0] is now an empty LEX_STRING (str==NULL). If a backup-namespace MDL_request/ticket is ever inspected or logged with type==0, printing the name can dereference/format a NULL string. Using an explicit "MDL_NOT_INITIALIZED" (or similar) entry at index 0 would avoid surprises.

Suggested change

	{},
	{ C_STRING_WITH_LEN("MDL_BACKUP_NOT_INITIALIZED") },