gob: make robust when decoding a struct with non-struct data.

The decoder was crashing when handling an rpc that expected
a struct but was delivered something else.  This diagnoses the
problem.  The other direction (expecting non-struct but getting
one) was already handled.

R=rsc
CC=golang-dev
https://golang.org/cl/2246041

src/pkg/gob/decode.go

@@ -843,12 +843,17 @@ func (dec *Decoder) compileDec(remoteId typeId, rt reflect.Type) (engine *decEng
 		return dec.compileSingle(remoteId, rt)
 	}
 	var wireStruct *structType
-	// Builtin types can come from global pool; the rest must be defined by the decoder
+	// Builtin types can come from global pool; the rest must be defined by the decoder.
+	// Also we know we're decoding a struct now, so the client must have sent one.
 	if t, ok := builtinIdToType[remoteId]; ok {
-		wireStruct = t.(*structType)
+		wireStruct, _ = t.(*structType)
 	} else {
 		wireStruct = dec.wireType[remoteId].structT
 	}
+	if wireStruct == nil {
+		return nil, os.ErrorString("gob: type mismatch in decoder: want struct type " +
+			rt.String() + "; got non-struct")
+	}
 	engine = new(decEngine)
 	engine.instr = make([]decInstr, len(wireStruct.field))
 	// Loop over the fields of the wire type.

src/pkg/gob/encoder_test.go

@@ -327,3 +327,31 @@ func TestSingletons(t *testing.T) {
 		}
 	}
 }
+
+func TestStructNonStruct(t *testing.T) {
+	type Struct struct {
+		a string
+	}
+	type NonStruct string
+	s := Struct{"hello"}
+	var sp Struct
+	if err := encAndDec(s, &sp); err != nil {
+		t.Error(err)
+	}
+	var ns NonStruct
+	if err := encAndDec(s, &ns); err == nil {
+		t.Error("should get error for struct/non-struct")
+	} else if strings.Index(err.String(), "type") < 0 {
+		t.Error("for struct/non-struct expected type error; got", err)
+	}
+	// Now try the other way
+	var nsp NonStruct
+	if err := encAndDec(ns, &nsp); err != nil {
+		t.Error(err)
+	}
+	if err := encAndDec(ns, &s); err == nil {
+		t.Error("should get error for non-struct/struct")
+	} else if strings.Index(err.String(), "type") < 0 {
+		t.Error("for non-struct/struct expected type error; got", err)
+	}
+}

src/pkg/gob/type.go

@@ -52,10 +52,20 @@ func (t typeId) gobType() gobType {
 }
 
 // string returns the string representation of the type associated with the typeId.
-func (t typeId) string() string { return t.gobType().string() }
+func (t typeId) string() string {
+	if t.gobType() == nil {
+		return "<nil>"
+	}
+	return t.gobType().string()
+}
 
 // Name returns the name of the type associated with the typeId.
-func (t typeId) Name() string { return t.gobType().Name() }
+func (t typeId) Name() string {
+	if t.gobType() == nil {
+		return "<nil>"
+	}
+	return t.gobType().Name()
+}
 
 // Common elements of all types.
 type commonType struct {