feat: check that the CidBuilder hasher is usable

Ref: ipfs/go-merkledag#90


This commit was moved from ipfs/go-merkledag@bb220e8

ipld/merkledag/merkledag_test.go

@@ -25,6 +25,7 @@ import (
 	u "github.com/ipfs/go-ipfs-util"
 	ipld "github.com/ipfs/go-ipld-format"
 	prime "github.com/ipld/go-ipld-prime"
+	mh "github.com/multiformats/go-multihash"
 )
 
 // makeDepthTestingGraph makes a small DAG with two levels. The level-two
@@ -73,6 +74,40 @@ func traverseAndCheck(t *testing.T, root ipld.Node, ds ipld.DAGService, hasF fun
 	}
 }
 
+func TestBadBuilderEncode(t *testing.T) {
+	n := NodeWithData([]byte("boop"))
+	_, err := n.EncodeProtobuf(false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = n.SetCidBuilder(
+		&cid.Prefix{
+			MhType:   mh.SHA2_256,
+			MhLength: -1,
+			Version:  1,
+			Codec:    cid.DagProtobuf,
+		},
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = n.SetCidBuilder(
+		&cid.Prefix{
+			MhType:   mh.SHA2_256_TRUNC254_PADDED,
+			MhLength: 256,
+			Version:  1,
+			Codec:    cid.DagProtobuf,
+		},
+	)
+	if err == nil {
+		t.Fatal("expected SetCidBuilder to error on unusable hasher")
+	}
+	_, err = n.EncodeProtobuf(false)
+	if err != nil {
+		t.Fatalf("expected EncodeProtobuf to use safe CidBuilder: %v", err)
+	}
+}
+
 func TestNode(t *testing.T) {
 
 	n1 := NodeWithData([]byte("beep"))

ipld/merkledag/node.go

@@ -13,6 +13,7 @@ import (
 	dagpb "github.com/ipld/go-codec-dagpb"
 	ipld "github.com/ipld/go-ipld-prime"
 	mh "github.com/multiformats/go-multihash"
+	mhcore "github.com/multiformats/go-multihash/core"
 )
 
 // Common errors
@@ -96,14 +97,26 @@ func (n *ProtoNode) CidBuilder() cid.Builder {
 }
 
 // SetCidBuilder sets the CID builder if it is non nil, if nil then it
-// is reset to the default value
-func (n *ProtoNode) SetCidBuilder(builder cid.Builder) {
+// is reset to the default value. An error will be returned if the builder
+// is not usable.
+func (n *ProtoNode) SetCidBuilder(builder cid.Builder) error {
 	if builder == nil {
 		n.builder = v0CidPrefix
-	} else {
-		n.builder = builder.WithCodec(cid.DagProtobuf)
-		n.cached = cid.Undef
+		return nil
+	}
+	if p, ok := builder.(*cid.Prefix); ok {
+		mhLen := p.MhLength
+		if mhLen <= 0 {
+			mhLen = -1
+		}
+		_, err := mhcore.GetVariableHasher(p.MhType, mhLen)
+		if err != nil {
+			return err
+		}
 	}
+	n.builder = builder.WithCodec(cid.DagProtobuf)
+	n.cached = cid.Undef
+	return nil
 }
 
 // LinkSlice is a slice of format.Links