add a Poll function to node promises

promise.go

@@ -4,19 +4,21 @@ import (
 	"context"
 )
 
-// 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.
-//
-// Thread Safety: This is multiple-consumer/single-producer safe.
+// NewNodePromise constructs a NodePromise with the given context. Canceling the
+// context will immediately cancel the NodePromise.
 func NewNodePromise(ctx context.Context) *NodePromise {
 	return &NodePromise{
 		done: make(chan struct{}),
 		ctx:  ctx,
 	}
 }
 
+// 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.
+//
+// Thread Safety: This is multiple-consumer/single-producer safe.
 type NodePromise struct {
 	value Node
 	err   error
@@ -25,7 +27,7 @@ type NodePromise struct {
 	ctx context.Context
 }
 
-// Call this function to fail a promise.
+// Fail fails this promise.
 //
 // Once a promise has been failed or fulfilled, further attempts to fail it will
 // be silently dropped.
@@ -38,7 +40,7 @@ func (np *NodePromise) Fail(err error) {
 	close(np.done)
 }
 
-// Fulfill this promise.
+// Send fulfills this promise.
 //
 // Once a promise has been fulfilled or failed, calling this function will
 // panic.
@@ -51,6 +53,16 @@ func (np *NodePromise) Send(nd Node) {
 	close(np.done)
 }
 
+// Poll returns the result of the promise if ready but doesn't block.
+func (np *NodePromise) Poll() (Node, error) {
+	select {
+	case <-np.done:
+		return np.value, np.err
+	case <-np.ctx.Done():
+		return nil, np.ctx.Err()
+	}
+}
+
 // Get the value of this promise.
 //
 // This function is safe to call concurrently from any number of goroutines.