Try decoding as JSON first then YAML, for speed (#693)

Fixes #680

.github/workflows/go.yml

@@ -97,6 +97,11 @@ jobs:
       run: |
         ! grep -IErn '\s$' --exclude-dir={.git,target,pgdata}
 
+    - if: runner.os == 'Linux'
+      name: Ensure use of unmarshal
+      run: |
+        [[ "$(git grep -F yaml. -- openapi3/ | grep -v _test.go | wc -l)" = 1 ]]
+
     - if: runner.os == 'Linux'
       name: Missing specification object link to definition
       run: |

openapi3/loader.go

@@ -115,7 +115,7 @@ func (loader *Loader) loadSingleElementFromURI(ref string, rootPath *url.URL, el
 	if err != nil {
 		return nil, err
 	}
-	if err := yaml.Unmarshal(data, element); err != nil {
+	if err := unmarshal(data, element); err != nil {
 		return nil, err
 	}
 
@@ -133,7 +133,7 @@ func (loader *Loader) readURL(location *url.URL) ([]byte, error) {
 func (loader *Loader) LoadFromData(data []byte) (*T, error) {
 	loader.resetVisitedPathItemRefs()
 	doc := &T{}
-	if err := yaml.Unmarshal(data, doc); err != nil {
+	if err := unmarshal(data, doc); err != nil {
 		return nil, err
 	}
 	if err := loader.ResolveRefsIn(doc, nil); err != nil {
@@ -162,7 +162,7 @@ func (loader *Loader) loadFromDataWithPathInternal(data []byte, location *url.UR
 	doc := &T{}
 	loader.visitedDocuments[uri] = doc
 
-	if err := yaml.Unmarshal(data, doc); err != nil {
+	if err := unmarshal(data, doc); err != nil {
 		return nil, err
 	}
 	if err := loader.ResolveRefsIn(doc, location); err != nil {
@@ -172,6 +172,14 @@ func (loader *Loader) loadFromDataWithPathInternal(data []byte, location *url.UR
 	return doc, nil
 }
 
+func unmarshal(data []byte, v interface{}) error {
+	// See https://github.com/getkin/kin-openapi/issues/680
+	if err := json.Unmarshal(data, v); err != nil {
+		return yaml.Unmarshal(data, v)
+	}
+	return nil
+}
+
 // ResolveRefsIn expands references if for instance spec was just unmarshalled
 func (loader *Loader) ResolveRefsIn(doc *T, location *url.URL) (err error) {
 	if loader.Context == nil {
@@ -319,7 +327,7 @@ func (loader *Loader) resolveComponent(
 		if err2 != nil {
 			return nil, err
 		}
-		if err2 = yaml.Unmarshal(data, &cursor); err2 != nil {
+		if err2 = unmarshal(data, &cursor); err2 != nil {
 			return nil, err
 		}
 		if cursor, err2 = drill(cursor); err2 != nil || cursor == nil {