Do not re-lock in SimpleGraph.Adj - #237
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Adj already holds RLock and then called Degree, which takes RLock again. A concurrent AddEdge writer waiting for the lock made that re-acquire hang forever. See Workiva#232
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Adjalready holdsRLockand then calledDegree, which takesRLockagain. Go'sRWMutexis not reentrant: a concurrentAddEdgewriter waiting for the lock made that re-acquire hang forever.Compute the adjacency list under the existing lock.
Fixes #232
Test
go test ./graph -run TestAdjDoesNotDeadlockWithAddEdge -count=1