Backups: minor adaption of upload file size warning

Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/backups/templates/backups_upload.html

@@ -32,9 +32,11 @@
       You can choose the apps you wish to import after uploading a backup file.
     {% endblocktrans %}
   </p>
-  <div class="alert alert-info" role="alert">
-    {% trans "Free disk space" %}: {{ free_space }}<br />
-    {% trans "Make sure the uploaded file is smaller than that." %}
+  <div class="alert alert-warning" role="alert">
+    {% blocktrans trimmed %}
+      WARNING: You have {{ max_filesize }} available to restore a backup.<br />
+      Exceeding this limit can leave your {{ box_name }} unusable.
+    {% endblocktrans %}
   </div>
 
   <form class="form" enctype="multipart/form-data" method="post">

plinth/modules/backups/views.py

@@ -151,8 +151,15 @@ class UploadArchiveView(SuccessMessageMixin, FormView):
         except (PlinthError, PermissionError):
             logger.error('Error getting information about root partition.')
         else:
-            context['free_space'] = storage.format_bytes(
-                    disk_info["free_bytes"])
+            # The maximum file size that can be uploaded and restored is at
+            # most half of the available disk space:
+            # - Django stores uploaded files that do not fit into memory to
+            #   disk (/tmp/). These are only copied by form_valid() after
+            #   the upload is finished.
+            # - For restoring it's highly advisable to have at least as much
+            #   free disk space as the file size.
+            context['max_filesize'] = storage.format_bytes(
+                    disk_info["free_bytes"] / 2)
         return context
 
     def form_valid(self, form):