Be more rigorous about local variables in PostgresMain().

tglsfdc · tglsfdc · commit 671bf1cf27cf · 2023-07-10T12:14:34.000-04:00
Since PostgresMain calls sigsetjmp, any local variables that are not marked "volatile" have a risk of unspecified behavior. In practice this means that when control returns via longjmp, such variables might get reset to their values as of the time of sigsetjmp, depending on whether the compiler chose to put them in registers or on the stack. We were careful about this for "send_ready_for_query", but not the other local variables. In the case of the timeout_enabled flags, resetting them to their initial "false" states is actually good, since we do "disable_all_timeouts()" in the longjmp cleanup code path. If that does not happen, we risk uselessly calling "disable_timeout()" later, which is harmless but a little bit expensive. Let's explicitly reset these flags so that the behavior is correct and platform-independent. (This change means that we really don't need the new "volatile" markings after all, but let's install them anyway since any change in this logic could re-introduce a problem.) There is no issue for "firstchar" and "input_message" because those are explicitly reinitialized each time through the query processing loop. To make that clearer, move them to be declared inside the loop. That leaves us with all the function-lifespan locals except the sigjmp_buf itself marked as volatile, which seems like a good policy to have going forward. Because of the possibility of extra disable_timeout() calls, this seems worth back-patching. Sergey Shinderuk and Tom Lane Discussion: https://postgr.es/m/2eda015b-7dff-47fd-d5e2-f1a9899b90a6@postgrespro.ru
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
@@ -3689,11 +3689,11 @@ PostgresMain(int argc, char *argv[],
 			 const char *dbname,
 			 const char *username)
 {
-	int			firstchar;
-	StringInfoData input_message;
 	sigjmp_buf	local_sigjmp_buf;
+
+	/* these must be volatile to ensure state is preserved across longjmp: */
 	volatile bool send_ready_for_query = true;
-	bool		disable_idle_in_transaction_timeout = false;
+	volatile bool disable_idle_in_transaction_timeout = false;
 
 	/* Initialize startup process environment if necessary. */
 	if (!IsUnderPostmaster)
@@ -3978,9 +3978,10 @@ PostgresMain(int argc, char *argv[],
 		 * query cancels from being misreported as timeouts in case we're
 		 * forgetting a timeout cancel.
 		 */
-		disable_all_timeouts(false);
-		QueryCancelPending = false; /* second to avoid race condition */
+		disable_all_timeouts(false);	/* do first to avoid race condition */
+		QueryCancelPending = false;
 		stmt_timeout_active = false;
+		disable_idle_in_transaction_timeout = false;
 
 		/* Not reading from the client anymore. */
 		DoingCommandRead = false;
@@ -4069,6 +4070,9 @@ PostgresMain(int argc, char *argv[],
 
 	for (;;)
 	{
+		int			firstchar;
+		StringInfoData input_message;
+
 		/*
 		 * At top of loop, reset extended-query-message flag, so that any
 		 * errors encountered in "idle" state don't provoke skip.
