better channel behavior and documentation

fetcher.go

@@ -26,6 +26,8 @@ type FetcherConfig struct {
 type Fetcher interface {
 	// NodeMatching traverses a node graph starting with the provided node using the given selector and possibly crossing
 	// block boundaries. Each matched node is sent to the FetchResult channel.
+	// The results and error channels will be closed on query completion or error. The error channel is buffered,
+	// will emit at most one error and must be checked after processing the results.
 	NodeMatching(ctx context.Context, node ipld.Node, match selector.Selector) (<-chan FetchResult, <-chan error)
 
 	// BlockOfType fetches a node graph of the provided type corresponding to single block by link.
@@ -34,6 +36,8 @@ type Fetcher interface {
 	// BlockMatchingOfType traverses a node graph starting with the given link using the given selector and possibly
 	// crossing block boundaries. The nodes will be typed using the provided prototype. Each matched node is sent to
 	// the FetchResult channel.
+	// The results and error channels will be closed on query completion or error. The error channel is buffered,
+	// will emit at most one error and must be checked after processing the results.
 	BlockMatchingOfType(ctx context.Context, root ipld.Link, match selector.Selector, ptype ipld.NodePrototype) (<-chan FetchResult, <-chan error)
 }
 
@@ -75,10 +79,11 @@ func (f *fetcherSession) BlockOfType(ctx context.Context, link ipld.Link, ptype
 
 func (f *fetcherSession) NodeMatching(ctx context.Context, node ipld.Node, match selector.Selector) (<-chan FetchResult, <-chan error) {
 	results := make(chan FetchResult)
-	errors := make(chan error)
+	errors := make(chan error, 1)
 
 	go func() {
 		defer close(results)
+		defer close(errors)
 
 		err := f.fetch(ctx, node, match, results)
 		if err != nil {
@@ -92,10 +97,11 @@ func (f *fetcherSession) NodeMatching(ctx context.Context, node ipld.Node, match
 
 func (f *fetcherSession) BlockMatchingOfType(ctx context.Context, root ipld.Link, match selector.Selector, ptype ipld.NodePrototype) (<-chan FetchResult, <-chan error) {
 	results := make(chan FetchResult)
-	errors := make(chan error)
+	errors := make(chan error, 1)
 
 	go func() {
 		defer close(results)
+		defer close(errors)
 
 		// retrieve first node
 		node, err := f.BlockOfType(ctx, root, ptype)

fetcher_test.go

@@ -208,24 +208,16 @@ func TestHelpers(t *testing.T) {
 
 func assertNodesInOrder(t *testing.T, nodeCh <-chan fetcher.FetchResult, errCh <-chan error, nodeCount int, nodes map[int]ipld.Node) {
 	order := 0
-Loop:
-	for {
-		select {
-		case res, ok := <-nodeCh:
-			if !ok {
-				break Loop
-			}
-
-			expectedNode, ok := nodes[order]
-			if ok {
-				assert.Equal(t, expectedNode, res.Node)
-			}
-
-			order++
-		case err := <-errCh:
-			require.FailNow(t, err.Error())
+	for res := range nodeCh {
+		expectedNode, ok := nodes[order]
+		if ok {
+			assert.Equal(t, expectedNode, res.Node)
 		}
+		order++
 	}
+
+	err := <-errCh
+	require.NoError(t, err)
 	assert.Equal(t, nodeCount, order)
 }