kv: fix GenerateForcedRetryableError to return a bumped epoch

This is needed for PR #74563, where we change how txn is reset.
Without this change GenerateForcedRetryableError returns an
error with an inner txn that has an epoch 0. With this change
the epoch is copied from the original txn.

Release note: None

pkg/kv/db_test.go

@@ -699,3 +699,19 @@ func TestDBDecommissionedOperations(t *testing.T) {
 		})
 	}
 }
+
+// TestGenerateForcedRetryableError verifies that GenerateForcedRetryableError
+// returns an error with a transaction that had the epoch bumped (and not epoch 0).
+func TestGenerateForcedRetryableError(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+	ctx := context.Background()
+	s, db := setup(t)
+	defer s.Stopper().Stop(context.Background())
+	txn := db.NewTxn(ctx, "test: TestGenerateForcedRetryableError")
+	require.Equal(t, 0, int(txn.Epoch()))
+	err := txn.GenerateForcedRetryableError(ctx, "testing TestGenerateForcedRetryableError")
+	var retryErr *roachpb.TransactionRetryWithProtoRefreshError
+	require.True(t, errors.As(err, &retryErr))
+	require.Equal(t, 1, int(retryErr.Transaction.Epoch))
+}

pkg/kv/txn.go

@@ -1468,17 +1468,7 @@ func (txn *Txn) GenerateForcedRetryableError(ctx context.Context, msg string) er
 	now := txn.db.clock.NowAsClockTimestamp()
 	txn.mu.sender.ManualRestart(ctx, txn.mu.userPriority, now.ToTimestamp())
 	txn.resetDeadlineLocked()
-	return roachpb.NewTransactionRetryWithProtoRefreshError(
-		msg,
-		txn.mu.ID,
-		roachpb.MakeTransaction(
-			txn.debugNameLocked(),
-			nil, // baseKey
-			txn.mu.userPriority,
-			now.ToTimestamp(),
-			txn.db.clock.MaxOffset().Nanoseconds(),
-			int32(txn.db.ctx.NodeID.SQLInstanceID())),
-	)
+	return txn.mu.sender.PrepareRetryableError(ctx, msg)
 }
 
 // PrepareRetryableError returns a