PANIC when the shared memory is corrupted

shmNumGxacts and shmGxactArray are accessed under the protection of
shmControlLock, this commit add some defensive code and PANIC at the earliest
when the shared memory is corrupted.

src/backend/cdb/cdbtm.c

@@ -1676,6 +1676,7 @@ tmShmemInit(void)
 			elog(PANIC, "cannot generate global transaction id");
 		}
 
+		MemSet(shared, 0, tmShmemSize());
 		*shmDistribTimeStamp = (DistributedTransactionTimeStamp)t;
   		elog(DEBUG1, "DTM start timestamp %u", *shmDistribTimeStamp);
 
@@ -1827,9 +1828,10 @@ isMppTxOptions_ExplicitBegin(int txnOptions)
 void
 getTmLock(void)
 {
-
 	if (ControlLockCount++ == 0)
 		LWLockAcquire(shmControlLock, LW_EXCLUSIVE);
+
+	Assert(LWLockHeldByMe(shmControlLock));
 }
 
 /* release tm lw lock */
@@ -1838,7 +1840,6 @@ releaseTmLock(void)
 {
 	if (--ControlLockCount == 0)
 		LWLockRelease(shmControlLock);
-
 }
 
 /*
@@ -2650,6 +2651,7 @@ releaseGxact_UnderLocks(void)
 {
 	int			i;
 	int			curr;
+	int			numGxacts;
 
 	if (currentGxact == NULL)
 	{
@@ -2661,8 +2663,8 @@ releaseGxact_UnderLocks(void)
 		 currentGxact->gid, currentGxact->debugIndex);
 
 	/* find slot of current transaction */
-	curr = *shmNumGxacts;	/* A bad value we can safely test. */
-	for (i = 0; i < *shmNumGxacts; i++)
+	numGxacts = curr = *shmNumGxacts;	/* A bad value we can safely test. */
+	for (i = 0; i < numGxacts; i++)
 	{
 		if (shmGxactArray[i] == currentGxact)
 		{
@@ -2671,6 +2673,12 @@ releaseGxact_UnderLocks(void)
 		}
 	}
 
+	if (curr == numGxacts)
+		ereport(PANIC, (errcode(ERRCODE_INTERNAL_ERROR),
+						errmsg("Cannot find my global transaction in the array"),
+						errdetail("ControlLockCount: %u, numGxacts: %u, gid: %s, currentGxact: %p",
+								   ControlLockCount, numGxacts, currentGxact->gid, currentGxact)));
+
 	/* move this to the next available slot */
 	(*shmNumGxacts)--;
 	if (curr != *shmNumGxacts)
@@ -2679,6 +2687,10 @@ releaseGxact_UnderLocks(void)
 		shmGxactArray[*shmNumGxacts] = currentGxact;
 	}
 
+	if (*shmNumGxacts != numGxacts - 1)
+		ereport(PANIC, (errcode(ERRCODE_INTERNAL_ERROR),
+						errmsg("Shared memory corrupted")));
+
 	currentGxact = NULL;
 }
 

src/include/cdb/cdbtm.h

@@ -264,7 +264,7 @@ typedef struct TmControlBlock
 
 
 #define TMCONTROLBLOCK_BYTES(num_gxacts) \
-	(offsetof(TmControlBlock, gxact_array) + sizeof(TMGXACT) * (num_gxacts))
+	(offsetof(TmControlBlock, gxact_array) + sizeof(TMGXACT *) * (num_gxacts))
 
 extern DtxContext DistributedTransactionContext;