client, roachpb: eliminate log spam when loadgens are killed

When the TPCC loadgen is CTRL-C'ed, the logs are spammed with:
I180906 22:02:41.239771 27177127 internal/client/txn.go:625  [n1] async rollback failed: TransactionStatusError: already committed (REASON_UNKNOWN): "sql txn" id=57c518aa key=/Table/61/1/1224/0 rw=true pri=0.03781170 iso=SERIALIZABLE stat=COMMITTED epo=0 ts=1536271361.068457287,0 orig=1536271361.068457287,0 max=1536271361.073978228,0 wto=false rop=false seq=11 int=5

The "async rollback" part refers to the rollback being done with a
canceled ctx (presumably a dropped connection's ctx). I believe the
error happens because there's a commit in flight when the ctx is
canceled. This patch lowers the message's level for this case.

Release note: None

pkg/internal/client/txn.go

@@ -626,7 +626,16 @@ func (txn *Txn) rollback(ctx context.Context) *roachpb.Error {
 		var ba roachpb.BatchRequest
 		ba.Add(endTxnReq(false /* commit */, nil /* deadline */, false /* systemConfigTrigger */))
 		if _, pErr := txn.Send(ctx, ba); pErr != nil {
-			log.Infof(ctx, "async rollback failed: %s", pErr)
+			if statusErr, ok := pErr.GetDetail().(*roachpb.TransactionStatusError); ok &&
+				statusErr.Reason == roachpb.TransactionStatusError_REASON_TXN_COMMITTED {
+				// A common cause of these async rollbacks failing is when they're
+				// triggered by a ctx canceled while a commit is in-flight (and it's too
+				// late for it to be canceled), and so the rollback finds the txn to be
+				// already committed. We don't spam the logs with those.
+				log.VEventf(ctx, 2, "async rollback failed: %s", pErr)
+			} else {
+				log.Infof(ctx, "async rollback failed: %s", pErr)
+			}
 		}
 	}); err != nil {
 		cancel()

pkg/roachpb/errors.go

@@ -410,6 +410,15 @@ func NewTransactionNotFoundStatusError() *TransactionStatusError {
 	}
 }
 
+// NewTransactionCommittedStatusError initializes a new TransactionStatusError
+// with a REASON_TXN_COMMITTED.
+func NewTransactionCommittedStatusError() *TransactionStatusError {
+	return &TransactionStatusError{
+		Msg:    "already committed",
+		Reason: TransactionStatusError_REASON_TXN_COMMITTED,
+	}
+}
+
 func (e *TransactionStatusError) Error() string {
 	return fmt.Sprintf("TransactionStatusError: %s (%s)", e.Msg, e.Reason)
 }