feat(cmds): add dag diff

core/commands/dag/dag.go

@@ -41,6 +41,7 @@ the existing 'ipfs object' command moving forward.
 		"import":  DagImportCmd,
 		"export":  DagExportCmd,
 		"stat":    DagStatCmd,
+		"diff":    DagDiffCmd,
 	},
 }
 
@@ -322,3 +323,37 @@ Note: This command skips duplicate blocks in reporting both size and the number
 		}),
 	},
 }
+
+type DagDiff struct {
+	s string
+}
+
+// DagDiffCmd is a command for comparing DAG nodes.
+// FIXME: Right now only supports DAG-PB encoding (for the roadmap of this
+//  command see https://github.com/ipfs/go-ipfs/issues/4801).
+var DagDiffCmd = &cmds.Command{
+	Helptext: cmds.HelpText{
+		Tagline: "Diffs two DAG nodes.",
+		ShortDescription: `
+'ipfs dag diff' fetches two DAG nodes and returns their differences.
+`,
+	},
+	Arguments: []cmds.Argument{
+		cmds.StringArg("node_before", true, false, "First node, \"before\", in the diff."),
+		cmds.StringArg("node_after", true, false, "First node, \"after\", in the diff."),
+	},
+	Options: []cmds.Option{ // FIXME: Remove if unused.
+	},
+	Run:  dagDiff,
+	Type: DagDiff{},
+	Encoders: cmds.EncoderMap{
+		cmds.Text: cmds.MakeTypedEncoder(func(req *cmds.Request, w io.Writer, d *DagDiff) error {
+			_, err := fmt.Fprintf(
+				w,
+				"%s\n",
+				d.s,
+			)
+			return err
+		}),
+	},
+}

core/commands/dag/diff.go

@@ -0,0 +1,56 @@
+package dagcmd
+
+import (
+	"encoding/json"
+	cmds "github.com/ipfs/go-ipfs-cmds"
+	"github.com/ipfs/go-ipfs/core/commands/cmdenv"
+	"io/ioutil"
+
+	"github.com/wI2L/jsondiff"
+)
+
+func dagDiff(req *cmds.Request, res cmds.ResponseEmitter, env cmds.Environment) error {
+	api, err := cmdenv.GetApi(env, req)
+	if err != nil {
+		return err
+	}
+
+	getArgNodeAsJson := func(argNumber int) ([]byte, error) {
+		r, err := getNodeWithCodec(req.Context, req.Arguments[argNumber], "dag-json", api)
+		if err != nil {
+			return nil, err
+		}
+
+		jsonOutput, err := ioutil.ReadAll(r)
+		if err != nil {
+			return nil, err
+		}
+
+		return jsonOutput, nil
+	}
+
+	nodeBefore, err := getArgNodeAsJson(0)
+	if err != nil {
+		return err
+	}
+	nodeAfter, err := getArgNodeAsJson(1)
+	if err != nil {
+		return err
+	}
+
+	patch, err := jsondiff.CompareJSONOpts(nodeBefore, nodeAfter, jsondiff.Invertible())
+	if err != nil {
+		return err
+	}
+
+	indented, err := json.MarshalIndent(patch, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	if err := res.Emit(DagDiff{string(indented)}); err != nil {
+		return err
+	}
+
+	return nil
+}