From 095eed36f8799419ff300688e5d5af89e124f2f8 Mon Sep 17 00:00:00 2001
From: Steven Allen <steven@stebalien.com>
Date: Wed, 27 Jun 2018 17:42:09 -0700
Subject: [PATCH 1/2] add a Poll function to node promises

(and improve the documentation)
---
 promise.go | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/promise.go b/promise.go
index 02743b0..cb5c182 100644
--- a/promise.go
+++ b/promise.go
@@ -4,12 +4,8 @@ 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{}),
@@ -17,6 +13,12 @@ func NewNodePromise(ctx context.Context) *NodePromise {
 	}
 }
 
+// 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.

From c01f809fab73b130d890a32c924aa3bd0e98db66 Mon Sep 17 00:00:00 2001
From: Steven Allen <steven@stebalien.com>
Date: Thu, 12 Jul 2018 10:45:52 +0200
Subject: [PATCH 2/2] don't block in poll

---
 promise.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/promise.go b/promise.go
index cb5c182..c8c3e8d 100644
--- a/promise.go
+++ b/promise.go
@@ -54,12 +54,16 @@ func (np *NodePromise) Send(nd Node) {
 }
 
 // Poll returns the result of the promise if ready but doesn't block.
+//
+// Returns nil, nil if not ready.
 func (np *NodePromise) Poll() (Node, error) {
 	select {
 	case <-np.done:
 		return np.value, np.err
 	case <-np.ctx.Done():
 		return nil, np.ctx.Err()
+	default:
+		return nil, nil
 	}
 }