From cf7f5da42659ea1dab7e3aec6b3950cf2b91773c Mon Sep 17 00:00:00 2001
From: Jeromy <jeromyj@gmail.com>
Date: Wed, 10 Feb 2016 21:42:17 -0800
Subject: [PATCH] don't fail promises that already succeeded

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>
---
 merkledag/merkledag.go      | 57 ++++++++++++++++++++++++++++---------
 merkledag/merkledag_test.go | 44 ++++++++++++++++++++++++++++
 2 files changed, 87 insertions(+), 14 deletions(-)

diff --git a/merkledag/merkledag.go b/merkledag/merkledag.go
index 3761096cc..6a6ad0ecd 100644
--- a/merkledag/merkledag.go
+++ b/merkledag/merkledag.go
@@ -176,9 +176,8 @@ func GetNodes(ctx context.Context, ds DAGService, keys []key.Key) []NodeGetter {
 	}
 
 	promises := make([]NodeGetter, len(keys))
-	sendChans := make([]chan<- *Node, len(keys))
 	for i := range keys {
-		promises[i], sendChans[i] = newNodePromise(ctx)
+		promises[i] = newNodePromise(ctx)
 	}
 
 	dedupedKeys := dedupeKeys(keys)
@@ -199,7 +198,9 @@ func GetNodes(ctx context.Context, ds DAGService, keys []key.Key) []NodeGetter {
 				}
 
 				if opt.Err != nil {
-					log.Error("error fetching: ", opt.Err)
+					for _, p := range promises {
+						p.Fail(opt.Err)
+					}
 					return
 				}
 
@@ -214,7 +215,7 @@ func GetNodes(ctx context.Context, ds DAGService, keys []key.Key) []NodeGetter {
 				is := FindLinks(keys, k, 0)
 				for _, i := range is {
 					count++
-					sendChans[i] <- nd
+					promises[i].Send(nd)
 				}
 			case <-ctx.Done():
 				return
@@ -237,18 +238,18 @@ func dedupeKeys(ks []key.Key) []key.Key {
 	return out
 }
 
-func newNodePromise(ctx context.Context) (NodeGetter, chan<- *Node) {
-	ch := make(chan *Node, 1)
+func newNodePromise(ctx context.Context) NodeGetter {
 	return &nodePromise{
-		recv: ch,
+		recv: make(chan *Node, 1),
 		ctx:  ctx,
 		err:  make(chan error, 1),
-	}, ch
+	}
 }
 
 type nodePromise struct {
 	cache *Node
-	recv  <-chan *Node
+	clk   sync.Mutex
+	recv  chan *Node
 	ctx   context.Context
 	err   chan error
 }
@@ -260,20 +261,49 @@ type nodePromise struct {
 type NodeGetter interface {
 	Get(context.Context) (*Node, error)
 	Fail(err error)
+	Send(*Node)
 }
 
 func (np *nodePromise) Fail(err error) {
+	np.clk.Lock()
+	v := np.cache
+	np.clk.Unlock()
+
+	// if promise has a value, don't fail it
+	if v != nil {
+		return
+	}
+
 	np.err <- err
 }
 
-func (np *nodePromise) Get(ctx context.Context) (*Node, error) {
+func (np *nodePromise) Send(nd *Node) {
+	var already bool
+	np.clk.Lock()
 	if np.cache != nil {
-		return np.cache, nil
+		already = true
+	}
+	np.cache = nd
+	np.clk.Unlock()
+
+	if already {
+		panic("sending twice to the same promise is an error!")
+	}
+
+	np.recv <- nd
+}
+
+func (np *nodePromise) Get(ctx context.Context) (*Node, error) {
+	np.clk.Lock()
+	c := np.cache
+	np.clk.Unlock()
+	if c != nil {
+		return c, nil
 	}
 
 	select {
-	case blk := <-np.recv:
-		np.cache = blk
+	case nd := <-np.recv:
+		return nd, nil
 	case <-np.ctx.Done():
 		return nil, np.ctx.Err()
 	case <-ctx.Done():
@@ -281,7 +311,6 @@ func (np *nodePromise) Get(ctx context.Context) (*Node, error) {
 	case err := <-np.err:
 		return nil, err
 	}
-	return np.cache, nil
 }
 
 type Batch struct {
diff --git a/merkledag/merkledag_test.go b/merkledag/merkledag_test.go
index 8137496d8..e475fa680 100644
--- a/merkledag/merkledag_test.go
+++ b/merkledag/merkledag_test.go
@@ -20,6 +20,7 @@ import (
 	imp "github.com/ipfs/go-ipfs/importer"
 	chunk "github.com/ipfs/go-ipfs/importer/chunk"
 	. "github.com/ipfs/go-ipfs/merkledag"
+	dstest "github.com/ipfs/go-ipfs/merkledag/test"
 	"github.com/ipfs/go-ipfs/pin"
 	uio "github.com/ipfs/go-ipfs/unixfs/io"
 	u "gx/ipfs/QmZNVWh8LLjAavuQ2JXuFmuYH3C11xo988vSgp7UQrTRj1/go-ipfs-util"
@@ -323,3 +324,46 @@ func TestEnumerateChildren(t *testing.T) {
 
 	traverse(root)
 }
+
+func TestFetchFailure(t *testing.T) {
+	ds := dstest.Mock()
+	ds_bad := dstest.Mock()
+
+	top := new(Node)
+	for i := 0; i < 10; i++ {
+		nd := &Node{Data: []byte{byte('a' + i)}}
+		_, err := ds.Add(nd)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		err = top.AddNodeLinkClean(fmt.Sprintf("AA%d", i), nd)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	for i := 0; i < 10; i++ {
+		nd := &Node{Data: []byte{'f', 'a' + byte(i)}}
+		_, err := ds_bad.Add(nd)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		err = top.AddNodeLinkClean(fmt.Sprintf("BB%d", i), nd)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	getters := GetDAG(context.Background(), ds, top)
+	for i, getter := range getters {
+		_, err := getter.Get(context.Background())
+		if err != nil && i < 10 {
+			t.Fatal(err)
+		}
+		if err == nil && i >= 10 {
+			t.Fatal("should have failed request")
+		}
+	}
+}