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3420851a2c
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