Create unbuffered Tumble entry point for LogDog.

tumble/tumble.go

Index: tumble/tumble.go
diff --git a/tumble/tumble.go b/tumble/tumble.go
index f5e8f6924b55df6b440ecf477814cde22bfbeafd..0818ce2d305bfc5cb73ce656036fbd6e0e773583 100644
--- a/tumble/tumble.go
+++ b/tumble/tumble.go
@@ -24,7 +24,7 @@ import (
 // state machine as a result of some API interaction.
 func RunMutation(c context.Context, m Mutation) error {
 	cfg := getConfig(c)
-	shardSet, _, _, err := enterTransactionInternal(txnBuf.FilterRDS(c), cfg, m, 0)
+	shardSet, _, _, err := enterTransactionMutation(txnBuf.FilterRDS(c), cfg, m, 0)
 	if err != nil {
 		return err
 	}
@@ -32,11 +32,36 @@ func RunMutation(c context.Context, m Mutation) error {
 	return nil
 }
 
-func enterTransactionInternal(c context.Context, cfg *Config, m Mutation, round uint64) (map[taskShard]struct{}, []Mutation, []*ds.Key, error) {
-	fromRoot := m.Root(c)
+// RegisterInTransaction opens a lightweight unbuffered transaction on "root"
+// runs "fn" inside of it. Any mutations returned by "fn" will be registered
+// at the end of the transaction if "fn" doesn't return an error.
+//
+// This is useful as an initial starting point without incurring all of the
+// overhead of spinning up a new buffered transaction.
+//
+// During "fn"'s execution, standard Tumble operations such as PutNamedMutation
+// and CancelNamedMutation may be performed.
+func RegisterInTransaction(c context.Context, root *ds.Key, fn func(context.Context) ([]Mutation, error)) error {
+	cfg := getConfig(c)
+	shardSet, _, _, err := enterTransactionInternal(c, cfg, root, fn, 0)
+	if err != nil {
+		return err
+	}
+	fireTasks(c, cfg, shardSet)
+	return nil
+}
+
+func enterTransactionMutation(c context.Context, cfg *Config, m Mutation, round uint64) (
+	map[taskShard]struct{}, []Mutation, []*ds.Key, error) {
+
+	return enterTransactionInternal(c, cfg, m.Root(c), m.RollForward, round)
+}
+
+func enterTransactionInternal(c context.Context, cfg *Config, root *ds.Key, fn func(context.Context) ([]Mutation, error), round uint64) (
+	map[taskShard]struct{}, []Mutation, []*ds.Key, error) {
 
-	if fromRoot == nil {
-		return nil, nil, nil, fmt.Errorf("tumble: Passing nil as fromRoot is illegal")
+	if root == nil {
+		return nil, nil, nil, fmt.Errorf("tumble: Passing nil as root is illegal")
 	}
 
 	shardSet := map[taskShard]struct{}(nil)
@@ -44,17 +69,17 @@ func enterTransactionInternal(c context.Context, cfg *Config, m Mutation, round
 	retMutKeys := []*ds.Key(nil)
 
 	err := ds.RunInTransaction(c, func(c context.Context) error {
-		// do a Get on the fromRoot to ensure that this transaction is associated
+		// do a Get on the root to ensure that this transaction is associated
 		// with that entity group.
-		_, _ = ds.Exists(c, fromRoot)
+		_, _ = ds.Exists(c, root)
 
-		muts, err := m.RollForward(c)
+		muts, err := fn(c)
 		if err != nil {
 			return err
 		}
 
 		retMuts = muts
-		shardSet, retMutKeys, err = putMutations(c, cfg, fromRoot, muts, round)
+		shardSet, retMutKeys, err = putMutations(c, cfg, root, muts, round)
 
 		return err
 	}, nil)