Merge pull request #387 An error occurred from haberman/json-null

JSON: Only perform multiple oneof check for non-null fields.

bazel/workspace_deps.bzl

@@ -36,8 +36,8 @@ def upb_deps():
     maybe(
         http_archive,
         name = "bazel_skylib",
-        strip_prefix = "bazel-skylib-master",
-        urls = ["https://github.com/bazelbuild/bazel-skylib/archive/master.tar.gz"],
+        strip_prefix = "bazel-skylib-main",
+        urls = ["https://github.com/bazelbuild/bazel-skylib/archive/main.tar.gz"],
     )
 
     maybe(

upb/json_decode.c

@@ -939,17 +939,17 @@ static void jsondec_field(jsondec *d, upb_msg *msg, const upb_msgdef *m) {
     return;
   }
 
-  if (upb_fielddef_realcontainingoneof(f) &&
-      upb_msg_whichoneof(msg, upb_fielddef_containingoneof(f))) {
-    jsondec_err(d, "More than one field for this oneof.");
-  }
-
   if (jsondec_peek(d) == JD_NULL && !jsondec_isvalue(f)) {
     /* JSON "null" indicates a default value, so no need to set anything. */
     jsondec_null(d);
     return;
   }
 
+  if (upb_fielddef_realcontainingoneof(f) &&
+      upb_msg_whichoneof(msg, upb_fielddef_containingoneof(f))) {
+    jsondec_err(d, "More than one field for this oneof.");
+  }
+
   preserved = d->debug_field;
   d->debug_field = f;