Fix for pg_duckdb cache invalidation (#192)

* List `duckdb_only_functions` will be cleaned only if cache is
considered installed (i.e. list should be populated at this point).
* Should fix #187

src/pgduckdb_metadata_cache.cpp

@@ -62,7 +62,10 @@ InvalidateCaches(Datum arg, int cache_id, uint32 hash_value) {
 		return;
 	}
 	cache.valid = false;
-	list_free(cache.duckdb_only_functions);
+	if (cache.installed) {
+		list_free(cache.duckdb_only_functions);
+		cache.duckdb_only_functions = NIL;
+	}
 }
 
 /*

test/regression/expected/basic.out

@@ -43,3 +43,7 @@ SET duckdb.max_threads_per_query TO default;
 SET client_min_messages TO default;
 DROP TABLE t;
 DROP TABLE empty;
+-- Check that DROP / CREATE extension works
+DROP EXTENSION pg_duckdb;
+CREATE EXTENSION pg_duckdb;
+WARNING:  To actually execute queries using DuckDB you need to run "SET duckdb.execution TO true;"

test/regression/sql/basic.sql

@@ -20,3 +20,8 @@ SET client_min_messages TO default;
 
 DROP TABLE t;
 DROP TABLE empty;
+
+-- Check that DROP / CREATE extension works
+
+DROP EXTENSION pg_duckdb;
+CREATE EXTENSION pg_duckdb;