jrgemignani commented on code in PR #2376:
URL: https://github.com/apache/age/pull/2376#discussion_r3097642246
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src/backend/utils/adt/age_global_graph.c:
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@@ -1531,3 +1676,291 @@ Datum age_graph_stats(PG_FUNCTION_ARGS)
PG_RETURN_POINTER(agtype_value_to_agtype(result.res));
}
+
+/*
+ * ============================================================================
+ * Graph Version Counter Implementation
+ *
+ * Provides per-graph monotonic version counters in shared memory for
+ * cross-backend VLE cache invalidation. Three modes are supported:
+ *
+ * DSM (PG 17+): Uses GetNamedDSMSegment — works without shared_preload_libs
+ * SHMEM (PG <17): Uses shmem_request/startup hooks — needs shared_preload_libs
+ * SNAPSHOT: Falls back to original snapshot-based invalidation
+ * ============================================================================
+ */
+
+#if PG_VERSION_NUM >= 170000
+/*
+ * DSM path: GetNamedDSMSegment init callback.
+ * Called once when the DSM segment is first created.
+ */
+static void age_dsm_init_callback(void *ptr)
+{
+ GraphVersionState *state = (GraphVersionState *) ptr;
+
+ LWLockInitialize(&state->lock,
+ LWLockNewTrancheId());
+ LWLockRegisterTranche(state->lock.tranche, "age_graph_version");
+ state->num_entries = 0;
+ memset(state->entries, 0, sizeof(state->entries));
+}
+
+/*
+ * Get the shared GraphVersionState via DSM registry.
+ * The segment is created on first access and persists until server shutdown.
+ */
+static GraphVersionState *get_version_state_dsm(void)
+{
+ bool found;
+
+ return (GraphVersionState *)
+ GetNamedDSMSegment("age_graph_versions",
+ sizeof(GraphVersionState),
+ age_dsm_init_callback,
+ &found);
+}
+#endif /* PG_VERSION_NUM >= 170000 */
+
+/*
+ * SHMEM path: request and startup hooks for PG < 17.
+ * These are registered in _PG_init when shared_preload_libraries is used.
+ * On PG 17+, DSM is used instead and these functions are not called.
+ */
+#if PG_VERSION_NUM < 170000
+void age_graph_version_shmem_request(void)
+{
+ RequestAddinShmemSpace(MAXALIGN(sizeof(GraphVersionState)));
+}
+
+void age_graph_version_shmem_startup(void)
+{
+ bool found;
+
+ LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
+
+ shmem_version_state =
+ (GraphVersionState *) ShmemInitStruct("AGE Graph Version State",
+ sizeof(GraphVersionState),
+ &found);
+ if (!found)
+ {
+ LWLockInitialize(&shmem_version_state->lock,
+ LWLockNewTrancheId());
+ LWLockRegisterTranche(shmem_version_state->lock.tranche,
+ "age_graph_version");
+ shmem_version_state->num_entries = 0;
+ memset(shmem_version_state->entries, 0,
+ sizeof(shmem_version_state->entries));
+ }
+
+ LWLockRelease(AddinShmemInitLock);
+}
+#endif /* PG_VERSION_NUM < 170000 */
+
+/*
+ * Detect which version mode to use. Called once per backend on first access.
+ * Emits a DEBUG1 log message indicating the chosen mode.
+ */
+static void detect_version_mode(void)
+{
+#if PG_VERSION_NUM >= 170000
+ version_mode = VERSION_MODE_DSM;
+ elog(DEBUG1, "AGE: VLE cache using DSM version counter");
+#else
+ if (shmem_version_state != NULL)
+ {
+ version_mode = VERSION_MODE_SHMEM;
+ elog(DEBUG1, "AGE: VLE cache using SHMEM version counter");
+ }
+ else
+ {
+ version_mode = VERSION_MODE_SNAPSHOT;
+ elog(DEBUG1, "AGE: VLE cache using snapshot-based invalidation "
+ "(add AGE to shared_preload_libraries for better caching)");
+ }
+#endif
+}
+
+/*
+ * Get a pointer to the GraphVersionState, regardless of mode.
+ * Returns NULL only in SNAPSHOT mode (no shared memory available).
+ */
+static GraphVersionState *get_version_state(void)
+{
+ if (version_mode == VERSION_MODE_UNKNOWN)
+ detect_version_mode();
+
+#if PG_VERSION_NUM >= 170000
+ if (version_mode == VERSION_MODE_DSM)
Review Comment:
needed for this
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