FEATURE: Show diff of local changes before updating remote theme (#7443)

app/assets/javascripts/admin/models/theme.js.es6

@@ -1,6 +1,9 @@
 import RestModel from "discourse/models/rest";
 import { default as computed } from "ember-addons/ember-computed-decorators";
 import { popupAjaxError } from "discourse/lib/ajax-error";
+import { ajax } from "discourse/lib/ajax";
+import { escapeExpression } from "discourse/lib/utilities";
+import highlightSyntax from "discourse/lib/highlight-syntax";
 
 const THEME_UPLOAD_VAR = 2;
 
@@ -277,9 +280,36 @@ const Theme = RestModel.extend({
   },
 
   updateToLatest() {
-    return this.save({ remote_update: true }).then(() =>
-      this.set("changed", false)
-    );
+    return ajax(`/admin/themes/${this.id}/diff_local_changes`).then(json => {
+      if (json && json.error) {
+        bootbox.alert(
+          I18n.t("generic_error_with_reason", {
+            error: json.error
+          })
+        );
+      } else if (json && json.diff) {
+        bootbox.confirm(
+          I18n.t("admin.customize.theme.update_confirm") +
+            `<pre><code class="diff">${escapeExpression(
+              json.diff
+            )}</code></pre>`,
+          I18n.t("cancel"),
+          I18n.t("admin.customize.theme.update_confirm_yes"),
+          result => {
+            if (result) {
+              return this.save({ remote_update: true }).then(() =>
+                this.set("changed", false)
+              );
+            }
+          }
+        );
+        highlightSyntax();
+      } else {
+        return this.save({ remote_update: true }).then(() =>
+          this.set("changed", false)
+        );
+      }
+    });
   },
 
   changed: false,

app/assets/stylesheets/common/base/modal.scss

@@ -265,6 +265,15 @@
         }
       }
     }
+
+    pre {
+      background-color: blend-primary-secondary(5%);
+      max-height: 300px;
+      padding: 0.5em;
+      code {
+        max-height: none;
+      }
+    }
   }
   .password-confirmation {
     display: none;

app/controllers/admin/themes_controller.rb

@@ -250,6 +250,15 @@ class Admin::ThemesController < Admin::AdminController
     exporter.cleanup!
   end
 
+  def diff_local_changes
+    theme = Theme.find_by(id: params[:id])
+    raise Discourse::InvalidParameters.new(:id) unless theme
+    changes = theme.remote_theme&.diff_local_changes
+    respond_to do |format|
+      format.json { render json: changes || {} }
+    end
+  end
+
   private
 
   def update_default_theme

app/models/remote_theme.rb

@@ -170,6 +170,21 @@ class RemoteTheme < ActiveRecord::Base
     end
   end
 
+  def diff_local_changes
+    return unless is_git?
+    importer = ThemeStore::GitImporter.new(remote_url, private_key: private_key, branch: branch)
+    begin
+      importer.import!
+    rescue RemoteTheme::ImportError => err
+      { error: err.message }
+    else
+      changes = importer.diff_local_changes(self.id)
+      return nil if changes.blank?
+
+      { diff: changes }
+    end
+  end
+
   def normalize_override(hex)
     return unless hex