Transfer current command counter ID to parallel workers.

Commit 924bcf4f correctly forbade
parallel workers to modify the command counter while in parallel mode,
but it inexplicably neglected to actually transfer the current command
counter from leader to workers.  This can result in the workers seeing
a different set of tuples from the leader, which is bad.  Repair.

src/backend/access/transam/xact.c

@@ -4786,8 +4786,8 @@ Size
 EstimateTransactionStateSpace(void)
 {
 	TransactionState s;
-	Size		nxids = 5;		/* iso level, deferrable, top & current XID,
-								 * XID count */
+	Size		nxids = 6;		/* iso level, deferrable, top & current XID,
+								 * command counter, XID count */
 
 	for (s = CurrentTransactionState; s != NULL; s = s->parent)
 	{
@@ -4807,12 +4807,13 @@ EstimateTransactionStateSpace(void)
  *
  * We need to save and restore XactDeferrable, XactIsoLevel, and the XIDs
  * associated with this transaction.  The first eight bytes of the result
- * contain XactDeferrable and XactIsoLevel; the next eight bytes contain the
- * XID of the top-level transaction and the XID of the current transaction
- * (or, in each case, InvalidTransactionId if none).  After that, the next 4
- * bytes contain a count of how many additional XIDs follow; this is followed
- * by all of those XIDs one after another.  We emit the XIDs in sorted order
- * for the convenience of the receiving process.
+ * contain XactDeferrable and XactIsoLevel; the next twelve bytes contain the
+ * XID of the top-level transaction, the XID of the current transaction
+ * (or, in each case, InvalidTransactionId if none), and the current command
+ * counter.  After that, the next 4 bytes contain a count of how many
+ * additional XIDs follow; this is followed by all of those XIDs one after
+ * another.  We emit the XIDs in sorted order for the convenience of the
+ * receiving process.
  */
 void
 SerializeTransactionState(Size maxsize, char *start_address)
@@ -4820,14 +4821,16 @@ SerializeTransactionState(Size maxsize, char *start_address)
 	TransactionState s;
 	Size		nxids = 0;
 	Size		i = 0;
+	Size		c = 0;
 	TransactionId *workspace;
 	TransactionId *result = (TransactionId *) start_address;
 
-	Assert(maxsize >= 5 * sizeof(TransactionId));
-	result[0] = (TransactionId) XactIsoLevel;
-	result[1] = (TransactionId) XactDeferrable;
-	result[2] = XactTopTransactionId;
-	result[3] = CurrentTransactionState->transactionId;
+	result[c++] = (TransactionId) XactIsoLevel;
+	result[c++] = (TransactionId) XactDeferrable;
+	result[c++] = XactTopTransactionId;
+	result[c++] = CurrentTransactionState->transactionId;
+	result[c++] = (TransactionId) currentCommandId;
+	Assert(maxsize >= c * sizeof(TransactionId));
 
 	/*
 	 * If we're running in a parallel worker and launching a parallel worker
@@ -4836,9 +4839,9 @@ SerializeTransactionState(Size maxsize, char *start_address)
 	 */
 	if (nParallelCurrentXids > 0)
 	{
-		Assert(maxsize > (nParallelCurrentXids + 4) * sizeof(TransactionId));
-		result[4] = nParallelCurrentXids;
-		memcpy(&result[5], ParallelCurrentXids,
+		result[c++] = nParallelCurrentXids;
+		Assert(maxsize >= (nParallelCurrentXids + c) * sizeof(TransactionId));
+		memcpy(&result[c], ParallelCurrentXids,
 			   nParallelCurrentXids * sizeof(TransactionId));
 		return;
 	}
@@ -4853,7 +4856,7 @@ SerializeTransactionState(Size maxsize, char *start_address)
 			nxids = add_size(nxids, 1);
 		nxids = add_size(nxids, s->nChildXids);
 	}
-	Assert(nxids * sizeof(TransactionId) < maxsize);
+	Assert((c + 1 + nxids) * sizeof(TransactionId) <= maxsize);
 
 	/* Copy them to our scratch space. */
 	workspace = palloc(nxids * sizeof(TransactionId));
@@ -4871,8 +4874,8 @@ SerializeTransactionState(Size maxsize, char *start_address)
 	qsort(workspace, nxids, sizeof(TransactionId), xidComparator);
 
 	/* Copy data into output area. */
-	result[4] = (TransactionId) nxids;
-	memcpy(&result[5], workspace, nxids * sizeof(TransactionId));
+	result[c++] = (TransactionId) nxids;
+	memcpy(&result[c], workspace, nxids * sizeof(TransactionId));
 }
 
 /*
@@ -4892,8 +4895,9 @@ StartParallelWorkerTransaction(char *tstatespace)
 	XactDeferrable = (bool) tstate[1];
 	XactTopTransactionId = tstate[2];
 	CurrentTransactionState->transactionId = tstate[3];
-	nParallelCurrentXids = (int) tstate[4];
-	ParallelCurrentXids = &tstate[5];
+	currentCommandId = tstate[4];
+	nParallelCurrentXids = (int) tstate[5];
+	ParallelCurrentXids = &tstate[6];
 
 	CurrentTransactionState->blockState = TBLOCK_PARALLEL_INPROGRESS;
 }