api/server: allow empty body for POST /commit again

The error returned by DecodeConfig was changed in
b6d58d749c and caused this to regress.
Allow empty request bodies for this endpoint once again.

Signed-off-by: Cory Snider <csnider@mirantis.com>

api/server/router/container/container_routes.go

@@ -44,7 +44,7 @@ func (s *containerRouter) postCommit(ctx context.Context, w http.ResponseWriter,
 	}
 
 	config, _, _, err := s.decoder.DecodeConfig(r.Body)
-	if err != nil && err != io.EOF { // Do not fail if body is empty.
+	if err != nil && !errors.Is(err, io.EOF) { // Do not fail if body is empty.
 		return err
 	}
 
@@ -486,6 +486,9 @@ func (s *containerRouter) postContainersCreate(ctx context.Context, w http.Respo
 
 	config, hostConfig, networkingConfig, err := s.decoder.DecodeConfig(r.Body)
 	if err != nil {
+		if errors.Is(err, io.EOF) {
+			return errdefs.InvalidParameter(errors.New("invalid JSON: got EOF while reading request body"))
+		}
 		return err
 	}
 	version := httputils.VersionFromContext(ctx)

runconfig/config.go

@@ -77,10 +77,7 @@ func decodeContainerConfig(src io.Reader, si *sysinfo.SysInfo) (*container.Confi
 func loadJSON(src io.Reader, out interface{}) error {
 	dec := json.NewDecoder(src)
 	if err := dec.Decode(&out); err != nil {
-		if err == io.EOF {
-			return validationError("invalid JSON: got EOF while reading request body")
-		}
-		return validationError("invalid JSON: " + err.Error())
+		return invalidJSONError{Err: err}
 	}
 	if dec.More() {
 		return validationError("unexpected content after JSON")

runconfig/errors.go

@@ -40,3 +40,17 @@ func (e validationError) Error() string {
 }
 
 func (e validationError) InvalidParameter() {}
+
+type invalidJSONError struct {
+	Err error
+}
+
+func (e invalidJSONError) Error() string {
+	return "invalid JSON: " + e.Err.Error()
+}
+
+func (e invalidJSONError) Unwrap() error {
+	return e.Err
+}
+
+func (e invalidJSONError) InvalidParameter() {}