Add "tumble" distributed transaction processing service for appengine.

appengine/tumble/tumble.go

Index: appengine/tumble/tumble.go
diff --git a/appengine/tumble/tumble.go b/appengine/tumble/tumble.go
new file mode 100644
index 0000000000000000000000000000000000000000..5f42372bfe60a23bab2b46522942cbafa497cea9
--- /dev/null
+++ b/appengine/tumble/tumble.go
@@ -0,0 +1,59 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package tumble
+
+import (
+	"fmt"
+
+	"github.com/luci/gae/filter/txnBuf"
+	"github.com/luci/gae/service/datastore"
+	"golang.org/x/net/context"
+)
+
+// EnterTransaction is the method to use when doing transactional operations
+// in the tumble ecosystem. It allows work to be done in the entity group
+// specified by `fromRoot`, and any returned Mutation objects will be
+// transactionally queued for the tumble backend.
+//
+// Usually this is called from your application's handlers to begin a tumble
+// state machine as a result of some API interaction.
+func EnterTransaction(c context.Context, fromRoot *datastore.Key, f func(context.Context) ([]Mutation, error)) error {
+	shardSet, _, err := enterTransactionInternal(txnBuf.FilterRDS(c), fromRoot, f)
+	if err != nil {
+		return err
+	}
+	fireTasks(c, shardSet)
+	return nil
+}
+
+func enterTransactionInternal(c context.Context, fromRoot *datastore.Key, f func(context.Context) ([]Mutation, error)) (map[uint64]struct{}, uint, error) {
+	if fromRoot == nil {
+		return nil, 0, fmt.Errorf("tumble: Passing nil as fromRoot is illegal")
+	}
+
+	shardSet := map[uint64]struct{}(nil)
+	numNewMuts := uint(0)
+
+	err := datastore.Get(c).RunInTransaction(func(c context.Context) error {
+		// do a Get on the fromRoot to ensure that this transaction is associated
+		// with that entity group.
+		_, _ = datastore.Get(c).Exists(fromRoot)
+
+		muts, err := f(c)
+		if err != nil {
+			return err
+		}
+
+		numNewMuts = uint(len(muts))
+		shardSet, err = putMutations(c, fromRoot, muts)
+
+		return err
+	}, nil)
+	if err != nil {
+		return nil, 0, err
+	}
+
+	return shardSet, numNewMuts, nil
+}