Addressed PR comments.

upb/util/required_fields.c

@@ -193,13 +193,9 @@ static void upb_FindContext_Pop(upb_FindContext* ctx) {
   ctx->stack.size--;
 }
 
-static void upb_util_FindUnsetRequiredInternal(upb_FindContext* ctx,
+static void upb_util_FindUnsetInMessage(upb_FindContext* ctx,
                                         const upb_msg* msg,
                                         const upb_msgdef* m) {
-  // OPT: add markers in the schema for where we can avoid iterating:
-  // 1. messages with no required fields.
-  // 2. messages that cannot possibly reach any required fields.
-
   // Iterate over all fields to see if any required fields are missing.
   for (int i = 0, n = upb_msgdef_fieldcount(m); i < n; i++) {
     const upb_fielddef* f = upb_msgdef_field(m, i);
@@ -223,6 +219,16 @@ static void upb_util_FindUnsetRequiredInternal(upb_FindContext* ctx,
       }
     }
   }
+}
+
+static void upb_util_FindUnsetRequiredInternal(upb_FindContext* ctx,
+                                               const upb_msg* msg,
+                                               const upb_msgdef* m) {
+  // OPT: add markers in the schema for where we can avoid iterating:
+  // 1. messages with no required fields.
+  // 2. messages that cannot possibly reach any required fields.
+
+  upb_util_FindUnsetInMessage(ctx, msg, m);
 
   // Iterate over all present fields to find sub-messages that might be missing
   // required fields.  This may revisit some of the fields already inspected

upb/util/required_fields.h

@@ -67,6 +67,8 @@ typedef union {
 //
 // The output buffer `buf` will always be NULL-terminated. If the output data
 // (including NULL terminator) exceeds `size`, the result will be truncated.
+// Returns the string length of the data we attempted to write, excluding the
+// terminating NULL.
 size_t upb_FieldPath_ToText(upb_FieldPathEntry **path, char *buf, size_t size);
 
 // Checks whether `msg` or any of its children has unset required fields,