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

A "break" statement erroneously left behind by commita1c1af2caused TopoSort to do the wrong thing if a lock's wait listcontained multiple members of the same locking group.Because parallel workers don't normally need any locks not alreadytaken by their leader, this is very hard --- maybe impossible ---to hit in production. Still, if it did happen, the queries involvedin an otherwise-resolvable deadlock would block until canceled.In addition to removing the bogus "break", add an Assert showingthat the conflicting uses of the beforeConstraints[] array (for bothcounts and flags) don't overlap, and add some commentary explainingwhy not; because it's not obvious without explanation, IMHO.Original report and patch from Rui Hai Jiang; additional assertand 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

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src/backend/storage/lmgr
- deadlock.c

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`‎src/backend/storage/lmgr/deadlock.c`

Lines changed: 7 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -922,6 +922,12 @@ TopoSort(LOCK *lock,`
`922`	`922`	`* in the same lock group on the queue, set their number of`
`923`	`923`	`* beforeConstraints to -1 to indicate that they should be emitted`
`924`	`924`	`* with their groupmates rather than considered separately.`
	`925`	`+ *`
	`926`	`+ * In this loop and the similar one just below, it's critical that we`
	`927`	`+ * consistently select the same representative member of any one lock`
	`928`	`+ * group, so that all the constraints are associated with the same`
	`929`	`+ * proc, and the -1's are only associated with not-representative`
	`930`	`+ * members. We select the last one in the topoProcs array.`
`925`	`931`	`*/`
`926`	`932`	`proc=constraints[i].waiter;`
`927`	`933`	`Assert(proc!=NULL);`
`@@ -940,7 +946,6 @@ TopoSort(LOCK *lock,`
`940`	`946`	`Assert(beforeConstraints[j] <=0);`
`941`	`947`	`beforeConstraints[j]=-1;`
`942`	`948`	`}`
`943`		`-break;`
`944`	`949`	`}`
`945`	`950`	`}`
`946`	`951`
`@@ -977,6 +982,7 @@ TopoSort(LOCK *lock,`
`977`	`982`	`if (kk<0)`
`978`	`983`	`continue;`
`979`	`984`
	`985`	`+Assert(beforeConstraints[jj] >=0);`
`980`	`986`	`beforeConstraints[jj]++;/* waiter must come before */`
`981`	`987`	`/* add this constraint to list of after-constraints for blocker */`
`982`	`988`	`constraints[i].pred=jj;`

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`‎src/backend/storage/lmgr/deadlock.c`

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