Fix miserable coding in pg_stat_get_activity().

Commit dd1a3bccc replaced a test on whether a subroutine returned a
null pointer with a test on whether &pointer->backendStatus was null.
This accidentally failed to fail, at least on common compilers, because
backendStatus is the first field in the struct; but it was surely trouble
waiting to happen.  Commit f91feba87 then messed things up further,
changing the logic to

	local_beentry = pgstat_fetch_stat_local_beentry(curr_backend);
	if (!local_beentry)
		continue;
	beentry = &local_beentry->backendStatus;
	if (!beentry)
	{

where the second "if" is now dead code, so that the intended behavior of
printing a row with "<backend information not available>" cannot occur.

I suspect this is all moot because pgstat_fetch_stat_local_beentry
will never actually return null in this function's usage, but it's still
very poor coding.  Repair back to 9.4 where the original problem was
introduced.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 44ccd37e998..688eea12a93 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -633,15 +633,13 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
 		{
 			/* Get specific pid slot */
 			local_beentry = pgstat_fetch_stat_local_beentry(*(int *) (funcctx->user_fctx));
-			beentry = &local_beentry->backendStatus;
 		}
 		else
 		{
 			/* Get the next one in the list */
 			local_beentry = pgstat_fetch_stat_local_beentry(funcctx->call_cntr + 1);	/* 1-based index */
-			beentry = &local_beentry->backendStatus;
 		}
-		if (!beentry)
+		if (!local_beentry)
 		{
 			int			i;
 
@@ -655,6 +653,8 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
 			SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
 		}
 
+		beentry = &local_beentry->backendStatus;
+
 		/* Values available to all callers */
 		values[0] = ObjectIdGetDatum(beentry->st_databaseid);
 		values[1] = Int32GetDatum(beentry->st_procpid);