Add checksums (used as a timestamp) to VFS.

Deprecate mj_makeEmptyFileVFS in favor of mj_addBufferVFS.

PiperOrigin-RevId: 608915388
Change-Id: Id9e9b042799bc1bf547f0795ae6f386447d53cdd

src/engine/engine_resource.c

@@ -105,7 +105,9 @@ mjResource* mju_openResource(const char* name, char* error, size_t error_sz) {
     memcpy(&fb->mtime, &file_stat.st_mtime, sizeof(time_t));
   } else {
     memset(&fb->mtime, 0, sizeof(time_t));
+    resource->timestamp[0] = '\0';
   }
+  strftime(resource->timestamp, 512, "%Y-%m-%d-%H:%M:%S", localtime(&(fb->mtime)));
   return resource;
 }
 
@@ -185,24 +187,20 @@ static int mju_isModifiedFile(const char* name, const file_buffer* fb) {
     if (stat(name, &file_stat) == 0) {
       return difftime(fb->mtime, file_stat.st_mtime) < 0;
     }
-    return -1;
   }
-  return -2;
+  return 1;  // modified (default)
 }
 
 
 
-// Returns > 0 if resource has been modified since last read, 0 if not, and < 0
-// if inconclusive
-int mju_isModifiedResource(const mjResource* resource) {
-  if (resource == NULL) {
-    return -2;
-  }
-
+// return 0 if the resource's timestamp matches the provided timestamp
+// return > 0 if the the resource is younger than the given timestamp
+// return < 0 if the resource is older than the given timestamp
+int mju_isModifiedResource(const mjResource* resource, const char* timestamp) {
   // provider is not OS filesystem
   if (resource->provider) {
     if (resource->provider->modified) {
-      return resource->provider->modified(resource);
+      return resource->provider->modified(resource, timestamp);
     }
     return 1;  // default (modified)
   }