Redesign procedure 2PC skip: static analysis via PLpgSQL plugin (Backport PR#8566 to 14.2)

[Muhammad Usama (codeforall)]

DESCRIPTION: Avoid coordinated transactions for single-statement single-shard procedure calls

Replace the runtime-counter approach from PR#8524 with pre-execution static analysis of procedure bodies using the PLpgSQL plugin func_beg hook. The prior design detected multi-statement procedures only after the first statement had run, which could produce a partial commit followed by an ERROR. Analyzing the body before any statement executes lets multi-statement procedures fall back cleanly to a coordinated (2PC) transaction with no error and no partial commit.

How it works
A new PLpgSQL plugin walks the statement tree at func_beg time, before any statement runs and sets the ProcedureBodyIsSingleStatement flag. The executor gate (CanSkipProcedureCoordination) then treats a single-statement body as necessary but not sufficient: the existing single-task / single-placement / coordinated-transaction checks still apply before a skip is allowed.

The walker:

Counts SQL-producing statements: EXECSQL, PERFORM, DYNEXECUTE, and CALL (the motivating use case).
Disqualifies on COMMIT, ROLLBACK, and every loop type (LOOP, WHILE, FOR/FORI/FORS/FORC, DYNFORS, FOREACH), a statement inside a loop can run many times, which is exactly the partial-commit risk we avoid.
Recurses into container statements: a BLOCK contributes the sum of its body and its exception-handler actions (both may run), while an IF or CASE contributes the maximum SQL count across its branches (then/ELSIF/else, or WHEN/else) because only one branch executes at runtime. A disqualifier in any branch still disqualifies the whole body.

Key changes

New [procedure_body_analysis.c] PLpgSQL plugin performing the pre-execution walk.
Max-based IF/CASE branch counting, so a procedure with a single SQL statement per branch stays eligible for the skip.
Multi-statement or disqualified procedures fall back to normal coordinated (2PC) transactions instead of raising an ERROR.
Removed the ProcedureNonCoordinatedExecutionCount global counter and its reset sites in utility_hook.c; the single-statement flag is now also defensively reset on the procedure error path so it can never survive a failed procedure.
The optimization stays behind the off-by-default citus.enable_procedure_transaction_skip GUC (kept off here to keep the change backport-friendly for 14.2 & 13.4; enabling-by-default / GUC removal is deferred to a follow-up on main). Note the analysis only applies to PLpgSQL procedures.
Regression coverage for the IF, CASE, nested-block, and exception walker paths (plus the single-CALL and loop cases), asserting the 2PC skip via citus.log_remote_commands.

[codecov]

Codecov Report

❌ Patch coverage is 68.21705% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.58%. Comparing base (f70aeb5) to head (73c74ff).

Additional details and impacted files

@@               Coverage Diff                @@
##           release-14.0    #8694      +/-   ##
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- Coverage         88.62%   88.58%   -0.04%     
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  Files               288      289       +1     
  Lines             64375    64503     +128     
  Branches           8087     8107      +20     
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+ Hits              57050    57140      +90     
- Misses             4991     5024      +33     
- Partials           2334     2339       +5     

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