1 files changed, 119 insertions, 0 deletions
diff --git a/tools/lint/eslint/eslint-plugin-mozilla/lib/rules/balanced-observers.js b/tools/lint/eslint/eslint-plugin-mozilla/lib/rules/balanced-observers.js
new file mode 100644
index 0000000000..f714b31cda
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+++ b/tools/lint/eslint/eslint-plugin-mozilla/lib/rules/balanced-observers.js
@@ -0,0 +1,119 @@
+/**
+ * @fileoverview Check that there's a Services.(prefs|obs).removeObserver for
+ * each addObserver.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+"use strict";
+
+// -----------------------------------------------------------------------------
+// Rule Definition
+// -----------------------------------------------------------------------------
+
+module.exports = function(context) {
+  // ---------------------------------------------------------------------------
+  // Helpers
+  // ---------------------------------------------------------------------------
+
+  var addedObservers = [];
+  var removedObservers = [];
+
+  function getObserverAPI(node) {
+    const object = node.callee.object;
+    if (
+      object.type == "MemberExpression" &&
+      object.property.type == "Identifier"
+    ) {
+      return object.property.name;
+    }
+    return null;
+  }
+
+  function isServicesObserver(api) {
+    return api == "obs" || api == "prefs";
+  }
+
+  function getObservableName(node, api) {
+    if (api === "obs") {
+      return node.arguments[1].value;
+    }
+    return node.arguments[0].value;
+  }
+
+  function addAddedObserver(node) {
+    const api = getObserverAPI(node);
+    if (!isServicesObserver(api)) {
+      return;
+    }
+
+    addedObservers.push({
+      functionName: node.callee.property.name,
+      observable: getObservableName(node, api),
+      node: node.callee.property,
+    });
+  }
+
+  function addRemovedObserver(node) {
+    const api = getObserverAPI(node);
+    if (!isServicesObserver(api)) {
+      return;
+    }
+
+    removedObservers.push({
+      functionName: node.callee.property.name,
+      observable: getObservableName(node, api),
+    });
+  }
+
+  function getUnbalancedObservers() {
+    const unbalanced = addedObservers.filter(
+      observer => !hasRemovedObserver(observer)
+    );
+    addedObservers = removedObservers = [];
+
+    return unbalanced;
+  }
+
+  function hasRemovedObserver(addedObserver) {
+    return removedObservers.some(
+      observer => addedObserver.observable === observer.observable
+    );
+  }
+
+  // ---------------------------------------------------------------------------
+  // Public
+  // ---------------------------------------------------------------------------
+
+  return {
+    CallExpression(node) {
+      if (node.arguments.length === 0) {
+        return;
+      }
+
+      if (node.callee.type === "MemberExpression") {
+        var methodName = node.callee.property.name;
+
+        if (methodName === "addObserver") {
+          addAddedObserver(node);
+        } else if (methodName === "removeObserver") {
+          addRemovedObserver(node);
+        }
+      }
+    },
+
+    "Program:exit": function() {
+      getUnbalancedObservers().forEach(function(observer) {
+        context.report(
+          observer.node,
+          "No corresponding 'removeObserver(\"{{observable}}\")' was found.",
+          {
+            observable: observer.observable,
+          }
+        );
+      });
+    },
+  };
+};