From bd6bf29983ecb4796ac36221fe4de21c4ccc4801 Mon Sep 17 00:00:00 2001
From: Marco Slot <marco.slot@gmail.com>
Date: Mon, 21 Aug 2017 10:27:33 +0200
Subject: [PATCH] Don't add procs multiple times in BuildWaitGraphForSourceNode

---
 .../distributed/transaction/lock_graph.c      | 48 +++++++++++--------
 1 file changed, 27 insertions(+), 21 deletions(-)

diff --git a/src/backend/distributed/transaction/lock_graph.c b/src/backend/distributed/transaction/lock_graph.c
index 2e5961976..4eb5a5802 100644
--- a/src/backend/distributed/transaction/lock_graph.c
+++ b/src/backend/distributed/transaction/lock_graph.c
@@ -32,12 +32,14 @@
 
 /*
  * PROCStack is a stack of PGPROC pointers used to perform a depth-first search
- * through the lock graph.
+ * through the lock graph. It also keeps track of which processes have been
+ * added to the stack to avoid visiting the same process multiple times.
  */
 typedef struct PROCStack
 {
 	int procCount;
 	PGPROC **procs;
+	bool *procAdded;
 } PROCStack;
 
 
@@ -56,6 +58,7 @@ static void AddEdgesForWaitQueue(WaitGraph *waitGraph, PGPROC *waitingProc,
 static void AddWaitEdge(WaitGraph *waitGraph, PGPROC *waitingProc, PGPROC *blockingProc,
 						PROCStack *remaining);
 static WaitEdge * AllocWaitEdge(WaitGraph *waitGraph);
+static void AddProcToVisit(PROCStack *remaining, PGPROC *proc);
 static bool IsSameLockGroup(PGPROC *leftProc, PGPROC *rightProc);
 static bool IsConflictingLockMask(int holdMask, int conflictMask);
 
@@ -392,11 +395,8 @@ BuildWaitGraphForSourceNode(int sourceNodeId)
 {
 	WaitGraph *waitGraph = NULL;
 	int curBackend = 0;
-	bool visitedProcs[MaxBackends];
 	PROCStack remaining;
 
-	memset(visitedProcs, 0, MaxBackends);
-
 	/*
 	 * Try hard to avoid allocations while holding lock. Thus we pre-allocate
 	 * space for locks in large batches - for common scenarios this should be
@@ -410,6 +410,7 @@ BuildWaitGraphForSourceNode(int sourceNodeId)
 	waitGraph->edges = (WaitEdge *) palloc(waitGraph->allocatedSize * sizeof(WaitEdge));
 
 	remaining.procs = (PGPROC **) palloc(sizeof(PGPROC *) * MaxBackends);
+	remaining.procAdded = (bool *) palloc0(sizeof(bool *) * MaxBackends);
 	remaining.procCount = 0;
 
 	LockLockData();
@@ -418,8 +419,7 @@ BuildWaitGraphForSourceNode(int sourceNodeId)
 	 * Build lock-graph.  We do so by first finding all procs which we are
 	 * interested in (originating on our source system, and blocked).  Once
 	 * those are collected, do depth first search over all procs blocking
-	 * those.  To avoid redundantly visiting procs, keep track of which procs
-	 * already have been visited in a pgproc-indexed visitedProcs[] array.
+	 * those.
 	 */
 
 	/* build list of starting procs */
@@ -453,26 +453,13 @@ BuildWaitGraphForSourceNode(int sourceNodeId)
 			continue;
 		}
 
-		remaining.procs[remaining.procCount++] = currentProc;
+		AddProcToVisit(&remaining, currentProc);
 	}
 
 	while (remaining.procCount > 0)
 	{
 		PGPROC *waitingProc = remaining.procs[--remaining.procCount];
 
-		/*
-		 * We might find a process again if multiple distributed transactions are
-		 * waiting for it, but we add all edges on the first visit so we don't need
-		 * to visit it again. This also avoids getting into an infinite loop in
-		 * case of a local deadlock.
-		 */
-		if (visitedProcs[waitingProc->pgprocno])
-		{
-			continue;
-		}
-
-		visitedProcs[waitingProc->pgprocno] = true;
-
 		/* only blocked processes result in wait edges */
 		if (!IsProcessWaitingForLock(waitingProc))
 		{
@@ -643,7 +630,7 @@ AddWaitEdge(WaitGraph *waitGraph, PGPROC *waitingProc, PGPROC *blockingProc,
 	curEdge->isBlockingXactWaiting = IsProcessWaitingForLock(blockingProc);
 	if (curEdge->isBlockingXactWaiting)
 	{
-		remaining->procs[remaining->procCount++] = blockingProc;
+		AddProcToVisit(remaining, blockingProc);
 	}
 
 	curEdge->waitingPid = waitingProc->pid;
@@ -705,6 +692,25 @@ AllocWaitEdge(WaitGraph *waitGraph)
 }
 
 
+/*
+ * AddProcToVisit adds a process to the stack of processes to visit
+ * in the depth-first search, unless it was already added.
+ */
+static void
+AddProcToVisit(PROCStack *remaining, PGPROC *proc)
+{
+	if (remaining->procAdded[proc->pgprocno])
+	{
+		return;
+	}
+
+	Assert(remaining->procCount < MaxBackends);
+
+	remaining->procs[remaining->procCount++] = proc;
+	remaining->procAdded[proc->pgprocno] = true;
+}
+
+
 /*
  * IsProcessWaitingForLock returns whether a given process is waiting for a lock.
  */