fix: remove pthread_mutex that causes self-deadlock on VLE queries

[crdv7]

Remove pthread_mutex from age_global_graph.c — it causes permanent self-deadlock on VLE queries.

ereport(ERROR) (statement timeout, query cancellation, OOM, etc.) triggers siglongjmp which skips pthread_mutex_unlock(). The mutex remains locked, and every subsequent VLE query on the same backend hangs forever in pthread_mutex_lock() with __owner == own PID.

Closes #2432

Analysis

The mutex was introduced in PR #1881 (fix for #1878). It is both unnecessary and harmful:

1. Unnecessary: The protected variable is a process-local static — no concurrent access exists. The test failure in Flaky test age_global_graph fails on slow machines #1878 was a catalog-level race, already fixed by the Assert→runtime check and strndup in the same PR. For cross-backend cache invalidation, PostgreSQL syscache uses sinval callbacks, and PR VLE cache: replace snapshot invalidation with per-graph #2376 already uses lock-free pg_atomic_uint64 version counters in shared memory for this.

2. Harmful: pthread_mutex is incompatible with PostgreSQL's ereport(ERROR) / siglongjmp error handling. Any ERROR during the mutex-held window permanently locks it. This is the only use of pthread in the entire AGE codebase.

Changes

src/backend/utils/adt/age_global_graph.c (1 file, +13 −50):

• Remove #include <pthread.h>
• Remove GRAPH_global_context_container struct (with pthread_mutex_t field)
• Revert to static GRAPH_global_context *global_graph_contexts = NULL
• Remove all pthread_mutex_lock/unlock calls (20 references)

The other fixes from PR #1881 (Assert→runtime check + strndup) remain untouched.

Verification

• All regression tests pass (36/36)
• Deadlock reproduction via statement_timeout: no longer hangs after fix

[Copilot]

Pull request overview

This PR removes the pthread_mutex-based locking around the per-backend global graph-context list in age_global_graph.c to prevent permanent self-deadlock when PostgreSQL errors (ereport(ERROR) via siglongjmp, e.g., statement timeout/cancel/OOM) occur while the mutex is held during VLE-related cache operations.

Changes:

• Removed #include <pthread.h> and the mutex-containing GRAPH_global_context_container.
• Reverted the per-process context head back to a simple static GRAPH_global_context *global_graph_contexts.
• Removed all pthread_mutex_lock() / pthread_mutex_unlock() calls from global-context management and lookup paths.

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[jrgemignani]

@crdv7 Please see the above Copilot message. edit nvm, this is pre-existing and needs to be addressed separately.