Issue #3455915: Try local file first, otherwise remote.

drupalorg_migrate.module

@@ -20,6 +20,9 @@ function drupalorg_migrate_migration_plugins_alter(array &$migrations) {
       if (!empty($paths['source'])) {
         $migrations[$id]['source']['constants']['source_base_path'] = $paths['source'];
       }
+      if (!empty($paths['remote'])) {
+        $migrations[$id]['remote']['constants']['remote_base_path'] = $paths['remote'];
+      }
     }
   }
 }

migrations/drupalorg_migrate_project_files.yml

@@ -15,7 +15,8 @@ source:
   constants:
     # Path changed via hook_migration_plugins_alter
     # and set via $config['drupalorg_migrate']['project_files']['source']
-    source_base_path: 'https://www.drupal.org'
+    source_base_path: '/var/www/html/web/files/d7-files'
+    remote_base_path: 'https://www.drupal.org'
 
 destination:
   plugin: entity:file
@@ -25,15 +26,38 @@ process:
   # the fid field to allow incremental migrations.
   # fid: fid
   filename: filename
-  source_full_path:
+  local_temp_path:
+    plugin: concat
+    delimiter: /
+    source:
+      - constants/source_base_path
+      - filepath
+  local_full_path:
+    -
+      plugin: callback
+      callable: file_exists
+      source: '@local_temp_path'
+    -
+      plugin: skip_on_empty
+      method: process
+    -
+      plugin: get
+      source: '@local_temp_path'
+  remote_full_path:
     -
       plugin: concat
       delimiter: /
       source:
-        - constants/source_base_path
+        - constants/remote_base_path
         - filepath
     -
       plugin: urlencode
+  source_full_path:
+    -
+      plugin: null_coalesce
+      source:
+        - '@local_full_path'
+        - '@remote_full_path'
   uri:
     plugin: file_copy
     source: