1 files changed, 92 insertions, 0 deletions
diff --git a/browser/components/sessionstore/SessionMigration.sys.mjs b/browser/components/sessionstore/SessionMigration.sys.mjs
new file mode 100644
index 0000000000..26e1e27d50
--- /dev/null
+++ b/browser/components/sessionstore/SessionMigration.sys.mjs
@@ -0,0 +1,92 @@
+/* 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/. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  E10SUtils: "resource://gre/modules/E10SUtils.sys.mjs",
+});
+
+var SessionMigrationInternal = {
+  /**
+   * Convert the original session restore state into a minimal state. It will
+   * only contain:
+   * - open windows
+   *   - with tabs
+   *     - with history entries with only title, url, triggeringPrincipal
+   *     - with pinned state
+   *     - with tab group info (hidden + group id)
+   *     - with selected tab info
+   *   - with selected window info
+   *
+   * The complete state is then wrapped into the "about:welcomeback" page as
+   * form field info to be restored when restoring the state.
+   */
+  convertState(aStateObj) {
+    let state = {
+      selectedWindow: aStateObj.selectedWindow,
+      _closedWindows: [],
+    };
+    state.windows = aStateObj.windows.map(function(oldWin) {
+      var win = { extData: {} };
+      win.tabs = oldWin.tabs.map(function(oldTab) {
+        var tab = {};
+        // Keep only titles, urls and triggeringPrincipals for history entries
+        tab.entries = oldTab.entries.map(function(entry) {
+          return {
+            url: entry.url,
+            triggeringPrincipal_base64: entry.triggeringPrincipal_base64,
+            title: entry.title,
+          };
+        });
+        tab.index = oldTab.index;
+        tab.hidden = oldTab.hidden;
+        tab.pinned = oldTab.pinned;
+        return tab;
+      });
+      win.selected = oldWin.selected;
+      win._closedTabs = [];
+      return win;
+    });
+    let url = "about:welcomeback";
+    let formdata = { id: { sessionData: state }, url };
+    let entry = {
+      url,
+      triggeringPrincipal_base64: lazy.E10SUtils.SERIALIZED_SYSTEMPRINCIPAL,
+    };
+    return { windows: [{ tabs: [{ entries: [entry], formdata }] }] };
+  },
+  /**
+   * Asynchronously read session restore state (JSON) from a path
+   */
+  readState(aPath) {
+    return IOUtils.readJSON(aPath, { decompress: true });
+  },
+  /**
+   * Asynchronously write session restore state as JSON to a path
+   */
+  writeState(aPath, aState) {
+    return IOUtils.writeJSON(aPath, aState, {
+      compress: true,
+      tmpPath: `${aPath}.tmp`,
+    });
+  },
+};
+
+export var SessionMigration = {
+  /**
+   * Migrate a limited set of session data from one path to another.
+   */
+  migrate(aFromPath, aToPath) {
+    return (async function() {
+      let inState = await SessionMigrationInternal.readState(aFromPath);
+      let outState = SessionMigrationInternal.convertState(inState);
+      // Unfortunately, we can't use SessionStore's own SessionFile to
+      // write out the data because it has a dependency on the profile dir
+      // being known. When the migration runs, there is no guarantee that
+      // that's true.
+      await SessionMigrationInternal.writeState(aToPath, outState);
+    })();
+  },
+};