1 files changed, 101 insertions, 0 deletions
diff --git a/devtools/client/aboutdebugging/src/modules/runtime-default-preferences.js b/devtools/client/aboutdebugging/src/modules/runtime-default-preferences.js
new file mode 100644
index 0000000000..02c06334f7
--- /dev/null
+++ b/devtools/client/aboutdebugging/src/modules/runtime-default-preferences.js
@@ -0,0 +1,101 @@
+/* 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";
+
+/**
+ * This module provides a workaround for remote debugging when a preference is
+ * defined in the firefox preference file (browser/app/profile/firefox.js) but
+ * still read from the server, without any default value.
+ *
+ * This causes the server to crash and can't easily be recovered.
+ *
+ * While we work on better linting to prevent such issues (Bug 1660182), this
+ * module will be able to set default values for all missing preferences.
+ */
+
+const PREFERENCE_TYPES = {
+  BOOL: "BOOL",
+  CHAR: "CHAR",
+  INT: "INT",
+};
+exports.PREFERENCE_TYPES = PREFERENCE_TYPES;
+
+/**
+ * Expected properties for the preference descriptors:
+ * - prefName {String}: the name of the preference.
+ * - defaultValue {String|Bool|Number}: the value to set if the preference is
+ *   missing.
+ * - trait {String}: the name of the trait corresponding to this pref on the
+ *   PreferenceFront.
+ * - type {String}: the preference type (either BOOL, CHAR or INT).
+ */
+const DEFAULT_PREFERENCES = [];
+exports.DEFAULT_PREFERENCES = DEFAULT_PREFERENCES;
+
+const METHODS = {
+  [PREFERENCE_TYPES.BOOL]: {
+    setPref: "setBoolPref",
+    getPref: "getBoolPref",
+  },
+  [PREFERENCE_TYPES.CHAR]: {
+    setPref: "setCharPref",
+    getPref: "getCharPref",
+  },
+  [PREFERENCE_TYPES.INT]: {
+    setPref: "setIntPref",
+    getPref: "getIntPref",
+  },
+};
+
+/**
+ * Set default values for all the provided preferences on the runtime
+ * corresponding to the provided clientWrapper, if needed.
+ *
+ * Note: prefDescriptors will most likely be DEFAULT_PREFERENCES when
+ * used in production code, but can be parameterized for tests.
+ *
+ * @param {ClientWrapper} clientWrapper
+ * @param {Array} prefDescriptors
+ *        Array of preference descriptors, see DEFAULT_PREFERENCES.
+ */
+async function setDefaultPreferencesIfNeeded(clientWrapper, prefDescriptors) {
+  if (!prefDescriptors || prefDescriptors.length === 0) {
+    return;
+  }
+
+  const preferenceFront = await clientWrapper.getFront("preference");
+  const preferenceTraits = await preferenceFront.getTraits();
+
+  // Note: using Promise.all here fails because the request/responses get mixed.
+  for (const prefDescriptor of prefDescriptors) {
+    // If the fix for this preference is already on this server, skip it.
+    if (preferenceTraits[prefDescriptor.trait]) {
+      continue;
+    }
+
+    await setDefaultPreference(preferenceFront, prefDescriptor);
+  }
+}
+exports.setDefaultPreferencesIfNeeded = setDefaultPreferencesIfNeeded;
+
+async function setDefaultPreference(preferenceFront, prefDescriptor) {
+  const { prefName, type, defaultValue } = prefDescriptor;
+
+  if (!Object.values(PREFERENCE_TYPES).includes(type)) {
+    throw new Error(`Unsupported type for setDefaultPreference "${type}"`);
+  }
+
+  const prefMethods = METHODS[type];
+  try {
+    // Try to read the preference only to check if the call is successful.
+    // If not, this means the preference is missing and should be initialized.
+    await preferenceFront[prefMethods.getPref](prefName);
+  } catch (e) {
+    console.warn(
+      `Preference "${prefName}"" is not set on the remote runtime. Setting default value.`
+    );
+    await preferenceFront[prefMethods.setPref](prefName, defaultValue);
+  }
+}