Acquire ProcArrayLock exclusively in ProcArrayClearTransaction.

This corrects an oversight by me in 2072932407, which made
ProcArrayClearTransaction() increment xactCompletionCount. That requires an
exclusive lock, obviously.

There's other approaches that avoid the exclusive acquisition, but given that a
2PC commit is fairly heavyweight, it doesn't seem worth doing so. I've not been
able to measure a performance difference, unsurprisingly.  I did add a
comment documenting that we could do so, should it ever become a bottleneck.

Reported-By: Tom Lane <tgl@sss.pgh.pa.us>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/1355915.1597794204@sss.pgh.pa.us

src/backend/storage/ipc/procarray.c

@@ -840,13 +840,20 @@ ProcArrayClearTransaction(PGPROC *proc)
 	size_t		pgxactoff;
 
 	/*
-	 * We can skip locking ProcArrayLock exclusively here, because this action
-	 * does not actually change anyone's view of the set of running XIDs: our
-	 * entry is duplicate with the gxact that has already been inserted into
-	 * the ProcArray. But need it in shared mode for pgproc->pgxactoff to stay
-	 * the same.
+	 * Currently we need to lock ProcArrayLock exclusively here, as we
+	 * increment xactCompletionCount below. We also need it at least in shared
+	 * mode for pgproc->pgxactoff to stay the same below.
+	 *
+	 * We could however, as this action does not actually change anyone's view
+	 * of the set of running XIDs (our entry is duplicate with the gxact that
+	 * has already been inserted into the ProcArray), lower the lock level to
+	 * shared if we were to make xactCompletionCount an atomic variable. But
+	 * that doesn't seem worth it currently, as a 2PC commit is heavyweight
+	 * enough for this not to be the bottleneck.  If it ever becomes a
+	 * bottleneck it may also be worth considering to combine this with the
+	 * subsequent ProcArrayRemove()
 	 */
-	LWLockAcquire(ProcArrayLock, LW_SHARED);
+	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
 	pgxactoff = proc->pgxactoff;