Hi, hackers!
While analyzing -Wclobbered warnings from gcc we found a true one in
PostgresMain():
postgres.c: In function ‘PostgresMain’:
postgres.c:4118:25: warning: variable
‘idle_in_transaction_timeout_enabled’ might be clobbered by ‘longjmp’ or
‘vfork’ [-Wclobbered]
4118 | bool idle_in_transaction_timeout_enabled =
false;
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
postgres.c:4119:25: warning: variable ‘idle_session_timeout_enabled’
might be clobbered by ‘longjmp’ or ‘vfork’ [-Wclobbered]
4119 | bool idle_session_timeout_enabled = false;
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
These variables must be declared volatile, because they are read after
longjmp(). send_ready_for_query declared there is volatile.
Without volatile, these variables are kept in registers and restored by
longjump(). I think, this is harmless because the error handling code
calls disable_all_timeouts() anyway. But strictly speaking the code is
invoking undefined behavior by reading those variables after longjmp(),
so it's worth fixing. And for consistency with send_ready_for_query too.
I believe, making them volatile doesn't affect performance in any way.
I also moved firstchar's declaration inside the loop where it's used, to
make it clear that this variable needn't be volatile and is not
preserved after longjmp().
Best regards,
--
Sergey Shinderuk https://postgrespro.com/
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 01b6cc1f7d3..56d8b0814b5 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -4111,12 +4111,11 @@ PostgresSingleUserMain(int argc, char *argv[],
void
PostgresMain(const char *dbname, const char *username)
{
- int firstchar;
StringInfoData input_message;
sigjmp_buf local_sigjmp_buf;
volatile bool send_ready_for_query = true;
- bool idle_in_transaction_timeout_enabled = false;
- bool idle_session_timeout_enabled = false;
+ volatile bool idle_in_transaction_timeout_enabled = false;
+ volatile bool idle_session_timeout_enabled = false;
Assert(dbname != NULL);
Assert(username != NULL);
@@ -4418,6 +4417,8 @@ PostgresMain(const char *dbname, const char *username)
for (;;)
{
+ int firstchar;
+
/*
* At top of loop, reset extended-query-message flag, so that any
* errors encountered in "idle" state don't provoke skip.
