Fix busted logic for parallel lock grouping in TopoSort().

A "break" statement erroneously left behind by commit a1c1af2a1
caused TopoSort to do the wrong thing if a lock's wait list
contained multiple members of the same locking group.

Because parallel workers don't normally need any locks not already
taken by their leader, this is very hard --- maybe impossible ---
to hit in production.  Still, if it did happen, the queries involved
in an otherwise-resolvable deadlock would block until canceled.

In addition to removing the bogus "break", add an Assert showing
that the conflicting uses of the beforeConstraints[] array (for both
counts and flags) don't overlap, and add some commentary explaining
why not; because it's not obvious without explanation, IMHO.

Original report and patch from Rui Hai Jiang; additional assert
and commentary by me.  Back-patch to 9.6 where the bug came in.

Discussion: https://postgr.es/m/CAEri+mLd3bpHLyW+a9pSe1y=aEkeuJpwBSwvo-+m4n7-ceRmXw@mail.gmail.com

diff --git a/src/backend/storage/lmgr/deadlock.c b/src/backend/storage/lmgr/deadlock.c
index 990d48377d8368ed0b064fc2b859edf55b2ca704..c687496b0415dd78caf55965a6a1931eab4ae70f 100644 (file)
--- a/src/backend/storage/lmgr/deadlock.c
+++ b/src/backend/storage/lmgr/deadlock.c
@@ -922,6 +922,12 @@ TopoSort(LOCK *lock,
                 * in the same lock group on the queue, set their number of
                 * beforeConstraints to -1 to indicate that they should be emitted
                 * with their groupmates rather than considered separately.
+                *
+                * In this loop and the similar one just below, it's critical that we
+                * consistently select the same representative member of any one lock
+                * group, so that all the constraints are associated with the same
+                * proc, and the -1's are only associated with not-representative
+                * members.  We select the last one in the topoProcs array.
                 */
                proc = constraints[i].waiter;
                Assert(proc != NULL);
@@ -940,7 +946,6 @@ TopoSort(LOCK *lock,
                                        Assert(beforeConstraints[j] <= 0);
                                        beforeConstraints[j] = -1;
                                }
-                               break;
                        }
                }
 
@@ -977,6 +982,7 @@ TopoSort(LOCK *lock,
                if (kk < 0)
                        continue;
 
+               Assert(beforeConstraints[jj] >= 0);
                beforeConstraints[jj]++;        /* waiter must come before */
                /* add this constraint to list of after-constraints for blocker */
                constraints[i].pred = jj;