Re-add login saving exception when it is somehow removed

- Add onLoginSavingChanged event to Logins API experiment to react to
  changes in login saving exceptions

- Watch for the removal of the login saving exception for this add-on's
  management pages. Re-add when removed.

Fixes #57

src/background/environment.js

@@ -4,11 +4,22 @@
 
 export async function initializeEnvironment() {
   // Issue #57: Disable login saving for extension pages.
-  const origin = browser.extension.getURL("").slice(0, -1);
+  const ourOrigin = browser.extension.getURL("").slice(0, -1);
+
+  // If login saving is enabled for us on startup, add an exception.
   const savingEnabled = await browser.experiments.logins
-    .getLoginSavingEnabled(origin);
+    .getLoginSavingEnabled(ourOrigin);
   if (savingEnabled) {
     await browser.experiments.logins
-      .setLoginSavingEnabled(origin, false);
+      .setLoginSavingEnabled(ourOrigin, false);
   }
+
+  // HACK: If our exception to login saving is removed, immediately re-add it.
+  browser.experiments.logins.onLoginSavingChanged
+    .addListener((origin, isEnabled) => {
+      if (origin === ourOrigin && !isEnabled) {
+        browser.experiments.logins
+          .setLoginSavingEnabled(ourOrigin, false);
+      }
+    });
 }

src/experiments/logins/api.js

@@ -230,6 +230,34 @@ this.logins = class extends ExtensionAPI {
               Services.obs.removeObserver(observer, "passwordmgr-storage-changed");
             };
           }).api(),
+          onLoginSavingChanged: new EventManager(context, "logins.onLoginSavingChanged", fire => {
+            const observer = {
+              observe: (permission, topic, type) => {
+                const { principal, type: permissionType } = permission;
+                const { origin } = principal;
+
+                if (permissionType != "login-saving") {
+                  return;
+                }
+
+                let isEnabled;
+                if (type === "added") {
+                  isEnabled = true;
+                } else if (type === "deleted") {
+                  isEnabled = false;
+                } else {
+                  return;
+                }
+
+                fire.async(origin, isEnabled);
+              },
+            };
+            const observedTopic = "perm-changed";
+            Services.obs.addObserver(observer, observedTopic);
+            return () => {
+              Services.obs.removeObserver(observer, observedTopic);
+            };
+          }).api(),
         },
       },
     };

src/experiments/logins/schema.json

@@ -196,6 +196,23 @@
         "name": "onAllRemoved",
         "type": "function",
         "description": "Fired when all Logins are removed."
+      },
+      {
+        "name": "onLoginSavingChanged",
+        "type": "function",
+        "description": "Fired when exceptions for login saving have changed",
+        "parameters": [
+          {
+            "name": "aHost",
+            "type": "string",
+            "description": "The origin for login saving exception"
+          },
+          {
+            "name": "isEnabled",
+            "type": "boolean",
+            "description": "Whether the exception was enabled (true) or removed (false)"
+          }
+        ]
       }
     ],
     "functions": [