Add a plugin API for coverage data

Change-Id: Id09bf2da548b8def10420ea85addc9575c734ab4

polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-js-api.js

@@ -26,6 +26,8 @@
     // notifying their listeners in the notify function.
     this._annotationLayers = [];
 
+    this._coverageProvider = null;
+
     // Default impl is a no-op.
     this._addLayerFunc = annotationActionsContext => {};
   }
@@ -55,6 +57,37 @@
     return this;
   };
 
+  /**
+   * The specified function will be called when a gr-diff component is built,
+   * and feeds the returned coverage data into the diff. Optional.
+   *
+   * Be sure to call this only once and only from one plugin. Multiple coverage
+   * providers are not supported. A second call will just overwrite the
+   * provider of the first call.
+   *
+   * TODO(brohlfs): Replace Array<Object> type by Array<Gerrit.CoverageRange>.
+   *
+   * @param {function(changeNum, path, basePatchNum, patchNum):
+   * !Promise<!Array<Object>>} coverageProvider
+   * @return {GrAnnotationActionsInterface}
+   */
+  GrAnnotationActionsInterface.prototype.setCoverageProvider = function(
+      coverageProvider) {
+    if (this._coverageProvider) {
+      console.warn('Overwriting an existing coverage provider.');
+    }
+    this._coverageProvider = coverageProvider;
+    return this;
+  };
+
+  /**
+   * Used by Gerrit to look up the coverage provider. Not intended to be called
+   * by plugins.
+   */
+  GrAnnotationActionsInterface.prototype.getCoverageProvider = function() {
+    return this._coverageProvider;
+  };
+
   /**
    * Returns a checkbox HTMLElement that can be used to toggle annotations
    * on/off. The checkbox will be initially disabled. Plugins should enable it

polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-js-api-interface.js

@@ -228,6 +228,36 @@
       return layers;
     },
 
+    /**
+     * Retrieves coverage data possibly provided by a plugin.
+     *
+     * Will wait for plugins to be loaded. If multiple plugins offer a coverage
+     * provider, the first one is used. If no plugin offers a coverage provider,
+     * will resolve to [].
+     *
+     * TODO(brohlfs): Replace Array<Object> type by Array<Gerrit.CoverageRange>.
+     *
+     * @param {string|number} changeNum
+     * @param {string} path
+     * @param {string|number} basePatchNum
+     * @param {string|number} patchNum
+     * @return {!Promise<!Array<Object>>}
+     */
+    getCoverageRanges(changeNum, path, basePatchNum, patchNum) {
+      return Gerrit.awaitPluginsLoaded().then(() => {
+        for (const annotationApi of
+            this._getEventCallbacks(EventType.ANNOTATE_DIFF)) {
+          const provider = annotationApi.getCoverageProvider();
+          // Only one coverage provider makes sense. If there are more, then we
+          // simply ignore them.
+          if (provider) {
+            return provider(changeNum, path, basePatchNum, patchNum);
+          }
+        }
+        return [];
+      });
+    },
+
     getAdminMenuLinks() {
       const links = [];
       for (const adminApi of