GH-12940 ext/pdo_pgsql: using PQclosePrepared to free statement resources.

PQclosePrepared allows the statement's name to be reused thus allowing
cache solutions to work properly ; whereas, for now, the `DEALLOCATE
<statement>` query is used frees entirely the statement's resources.

Author: devnexen

ext/pdo_pgsql/config.m4

@@ -68,6 +68,8 @@ if test "$PHP_PDO_PGSQL" != "no"; then
 
   AC_CHECK_LIB(pq, PQlibVersion,, AC_MSG_ERROR([Unable to build the PDO PostgreSQL driver: at least libpq 9.1 is required]))
 
+  PHP_CHECK_FUNC(PQclosePrepared, pq)
+
   LIBS=$old_LIBS
   LDFLAGS=$old_LDFLAGS
 

ext/pdo_pgsql/pgsql_statement.c

@@ -74,12 +74,17 @@ static int pgsql_stmt_dtor(pdo_stmt_t *stmt)
 	if (S->stmt_name) {
 		if (S->is_prepared && server_obj_usable) {
 			pdo_pgsql_db_handle *H = S->H;
-			char *q = NULL;
 			PGresult *res;
-
+#ifndef HAVE_PQCLOSEPREPARED
+			// TODO (??) libpq does not support close statement protocol < postgres 17
+			// check if we can circumvent this.
+			char *q = NULL;
 			spprintf(&q, 0, "DEALLOCATE %s", S->stmt_name);
 			res = PQexec(H->server, q);
 			efree(q);
+#else
+			res = PQclosePrepared(H->server, S->stmt_name);
+#endif
 			if (res) {
 				PQclear(res);
 			}