NodePromise: replace interface with concrete type

Also:

1. Specify the threading guarantees.
2. Vastly simplify it to use a single channel for synchronization.

daghelpers.go

@@ -21,7 +21,7 @@ func FindLinks(links []*cid.Cid, c *cid.Cid, start int) []int {
 // GetDAG will fill out all of the links of the given Node.
 // It returns a channel of nodes, which the caller can receive
 // all the child nodes of 'root' on, in proper order.
-func GetDAG(ctx context.Context, ds DAGService, root Node) []NodePromise {
+func GetDAG(ctx context.Context, ds DAGService, root Node) []*NodePromise {
 	var cids []*cid.Cid
 	for _, lnk := range root.Links() {
 		cids = append(cids, lnk.Cid)
@@ -32,16 +32,16 @@ func GetDAG(ctx context.Context, ds DAGService, root Node) []NodePromise {
 
 // GetNodes returns an array of 'FutureNode' promises, with each corresponding
 // to the key with the same index as the passed in keys
-func GetNodes(ctx context.Context, ds DAGService, keys []*cid.Cid) []NodePromise {
+func GetNodes(ctx context.Context, ds DAGService, keys []*cid.Cid) []*NodePromise {
 
 	// Early out if no work to do
 	if len(keys) == 0 {
 		return nil
 	}
 
-	promises := make([]NodePromise, len(keys))
+	promises := make([]*NodePromise, len(keys))
 	for i := range keys {
-		promises[i] = newNodePromise(ctx)
+		promises[i] = NewNodePromise(ctx)
 	}
 
 	dedupedKeys := dedupeKeys(keys)

promise.go

@@ -2,87 +2,65 @@ package format
 
 import (
 	"context"
-	"sync"
 )
 
-// TODO: I renamed this to NodePromise because:
-// 1. NodeGetter is a naming conflict.
-// 2. It's a promise...
-
-// TODO: Should this even be an interface? It seems like a simple struct would
-// suffice.
-
 // NodePromise provides a promise like interface for a dag Node
 // the first call to Get will block until the Node is received
 // from its internal channels, subsequent calls will return the
 // cached node.
-type NodePromise interface {
-	Get(context.Context) (Node, error)
-	Fail(err error)
-	Send(Node)
-}
-
-func newNodePromise(ctx context.Context) NodePromise {
-	return &nodePromise{
-		recv: make(chan Node, 1),
+//
+// Thread Safety: This is multiple-consumer/single-producer safe.
+func NewNodePromise(ctx context.Context) *NodePromise {
+	return &NodePromise{
+		done: make(chan struct{}),
 		ctx:  ctx,
-		err:  make(chan error, 1),
 	}
 }
 
-type nodePromise struct {
-	cache Node
-	clk   sync.Mutex
-	recv  chan Node
-	ctx   context.Context
-	err   chan error
-}
+type NodePromise struct {
+	value Node
+	err   error
+	done  chan struct{}
 
-func (np *nodePromise) Fail(err error) {
-	np.clk.Lock()
-	v := np.cache
-	np.clk.Unlock()
+	ctx context.Context
+}
 
-	// if promise has a value, don't fail it
-	if v != nil {
+// Call this function to fail a promise.
+//
+// Once a promise has been failed or fulfilled, further attempts to fail it will
+// be silently dropped.
+func (np *NodePromise) Fail(err error) {
+	if np.err != nil || np.value != nil {
+		// Already filled.
 		return
 	}
-
-	np.err <- err
+	np.err = err
+	close(np.done)
 }
 
-func (np *nodePromise) Send(nd Node) {
-	var already bool
-	np.clk.Lock()
-	if np.cache != nil {
-		already = true
-	}
-	np.cache = nd
-	np.clk.Unlock()
-
-	if already {
-		panic("sending twice to the same promise is an error!")
+// Fulfill this promise.
+//
+// Once a promise has been fulfilled or failed, calling this function will
+// panic.
+func (np *NodePromise) Send(nd Node) {
+	// if promise has a value, don't fail it
+	if np.err != nil || np.value != nil {
+		panic("already filled")
 	}
-
-	np.recv <- nd
+	np.value = nd
+	close(np.done)
 }
 
-func (np *nodePromise) Get(ctx context.Context) (Node, error) {
-	np.clk.Lock()
-	c := np.cache
-	np.clk.Unlock()
-	if c != nil {
-		return c, nil
-	}
-
+// Get the value of this promise.
+//
+// This function is safe to call concurrently from any number of goroutines.
+func (np *NodePromise) Get(ctx context.Context) (Node, error) {
 	select {
-	case nd := <-np.recv:
-		return nd, nil
+	case <-np.done:
+		return np.value, np.err
 	case <-np.ctx.Done():
 		return nil, np.ctx.Err()
 	case <-ctx.Done():
 		return nil, ctx.Err()
-	case err := <-np.err:
-		return nil, err
 	}
 }