json: add seen-type detection into constructCodec

Also remove named/naked return values from that function.

Author: extemporalgenome

json/codec.go

@@ -126,7 +126,17 @@ func constructCachedCodec(t reflect.Type, cache codecCache) codec {
 	return c
 }
 
-func constructCodec(t reflect.Type, seen codecCache, canAddr bool) (c codec) {
+func constructCodec(t reflect.Type, seen codecCache, canAddr bool) codec {
+	// Check if the codec has already been constructed: if so, return it.
+	// This mitigates infinite recursion due to type definition cycles.
+	c, ok := seen[typeid(t)]
+	if ok {
+		return c
+	}
+
+	// Arrange for the constructed codec to be recorded in seen.
+	defer func() { seen[typeid(t)] = c }()
+
 	switch t {
 	case nullType, nil:
 		c = codec{encode: encoder.encodeNull, decode: decoder.decodeNull}
@@ -163,7 +173,7 @@ func constructCodec(t reflect.Type, seen codecCache, canAddr bool) (c codec) {
 	}
 
 	if c.encode != nil {
-		return
+		return c
 	}
 
 	switch t.Kind() {
@@ -259,7 +269,7 @@ func constructCodec(t reflect.Type, seen codecCache, canAddr bool) (c codec) {
 		c.decode = constructTextUnmarshalerDecodeFunc(t, true)
 	}
 
-	return
+	return c
 }
 
 func constructStringCodec(t reflect.Type, seen codecCache, canAddr bool) codec {