Improve "merkledag: not found" error

Based on #33, taking advantage of the rather new Is() methods from the errors
package to check for equality.

batch_test.go

@@ -2,6 +2,8 @@ package format
 
 import (
 	"context"
+	"errors"
+	"fmt"
 	"sync"
 	"testing"
 
@@ -24,7 +26,7 @@ func (d *testDag) Get(ctx context.Context, cid cid.Cid) (Node, error) {
 	if n, ok := d.nodes[cid.KeyString()]; ok {
 		return n, nil
 	}
-	return nil, ErrNotFound
+	return nil, ErrNotFoundCid{cid}
 }
 
 func (d *testDag) GetMany(ctx context.Context, cids []cid.Cid) <-chan *NodeOption {
@@ -35,7 +37,7 @@ func (d *testDag) GetMany(ctx context.Context, cids []cid.Cid) <-chan *NodeOptio
 		if n, ok := d.nodes[c.KeyString()]; ok {
 			out <- &NodeOption{Node: n}
 		} else {
-			out <- &NodeOption{Err: ErrNotFound}
+			out <- &NodeOption{Err: ErrNotFoundCid{c}}
 		}
 	}
 	close(out)
@@ -144,3 +146,22 @@ func TestBatchOptions(t *testing.T) {
 		t.Fatalf("maxNodes incorrect, want: %d, got: %d", wantMaxNodes, b.opts.maxNodes)
 	}
 }
+
+func TestErrorTypes(t *testing.T) {
+	d := newTestDag()
+	notFoundNode := &EmptyNode{}
+	_, err := d.Get(context.Background(), notFoundNode.Cid())
+	if err == nil {
+		t.Fatal("should throw NotFound error")
+	}
+
+	err2 := fmt.Errorf("could not read: %w", err)
+
+	if !errors.Is(err, ErrNotFound) {
+		t.Fatal("should be an ErrNotFound")
+	}
+
+	if !errors.Is(err2, ErrNotFound) {
+		t.Fatal("should be an ErrNotFound")
+	}
+}

go.mod

 module github.com/ipfs/go-ipld-format
 
+go 1.14
+
 require (
 	github.com/ipfs/go-block-format v0.0.2
 	github.com/ipfs/go-cid v0.0.2

merkledag.go

@@ -7,7 +7,37 @@ import (
 	cid "github.com/ipfs/go-cid"
 )
 
-var ErrNotFound = fmt.Errorf("merkledag: not found")
+// ErrNotFound is used to signal when a Node could not be found. The specific
+// meaning will depend on the DAGService implementation, which may be trying
+// to read nodes locally but also, trying to find them remotely.
+var ErrNotFound = ErrNotFoundCid{}
+
+// ErrNotFoundCid can be use to provide specific CID information in a NotFound
+// error.
+type ErrNotFoundCid struct {
+	c cid.Cid
+}
+
+// Error implements the error interface and returns a human-readable
+// message for this error.
+func (e ErrNotFoundCid) Error() string {
+	if e.c == cid.Undef {
+		return "ipld: node not found"
+	}
+
+	return fmt.Sprintf("ipld: %s not found", e.c)
+}
+
+// Is allows to check whether any error is of this ErrNotFoundCid type.
+// Do not use this directly, but rather errors.Is(yourError, ErrNotFound).
+func (e ErrNotFoundCid) Is(err error) bool {
+	switch err.(type) {
+	case ErrNotFoundCid:
+		return true
+	default:
+		return false
+	}
+}
 
 // Either a node or an error.
 type NodeOption struct {