Comments and safety belts from glasser's review.

packages/autoupdate/autoupdate_client.js

@@ -1,16 +1,16 @@
 // Subscribe to the `meteor_autoupdate_clientVersions` collection,
 // which contains the set of acceptable client versions.
 //
-// A "hard code push" occurs when the current client version is not in
+// A "hard code push" occurs when the running client version is not in
 // the set of acceptable client versions (or the server updates the
-// collection, and the current client version is no longer in the
-// set).
+// collection, there is a published client version marked `current` and
+// the running client version is no longer in the set).
 //
 // When the `reload` package is loaded, a hard code push causes
 // the browser to reload, so that it will load the latest client
 // version from the server.
 //
-// A "soft code push" represents the situation when the current client
+// A "soft code push" represents the situation when the running client
 // version is in the set of acceptable versions, but there is a newer
 // version available on the server.
 //
@@ -24,7 +24,7 @@
 
 // The client version of the client code currently running in the
 // browser.
-var autoUpdateVersion = __meteor_runtime_config__.autoUpdateVersion;
+var autoUpdateVersion = __meteor_runtime_config__.autoUpdateVersion || "unknown";
 
 
 // The collection of acceptable client versions.

packages/autoupdate/autoupdate_server.js

@@ -45,10 +45,12 @@ AutoUpdate = {};
 AutoUpdate.autoUpdateVersion = null;
 
 Meteor.startup(function () {
+  // Allow people to override AutoUpdate.autoUpdateVersion before
+  // startup. Tests do this.
   if (AutoUpdate.autoUpdateVersion === null)
     AutoUpdate.autoUpdateVersion =
       process.env.AUTOUPDATE_VERSION ||
-      process.env.SERVER_ID ||
+      process.env.SERVER_ID || // XXX COMPAT 0.6.6
       WebApp.clientHash;
 
   // Make autoUpdateVersion available on the client.
@@ -63,12 +65,17 @@ Meteor.publish(
     // Using `autoUpdateVersion` here is safe because we can't get a
     // subscription before webapp starts listening, and it doesn't do
     // that until the startup hooks have run.
-    self.added(
-      "meteor_autoupdate_clientVersions",
-      AutoUpdate.autoUpdateVersion,
-      {current: true}
-    );
-    self.ready();
+    if (AutoUpdate.autoUpdateVersion) {
+      self.added(
+        "meteor_autoupdate_clientVersions",
+        AutoUpdate.autoUpdateVersion,
+        {current: true}
+      );
+      self.ready();
+    } else {
+      // huh? shouldn't happen. Just error the sub.
+      self.error(new Meteor.Error(500, "AutoUpdate.autoUpdateVersion not set"));
+    }
   },
   {is_auto: true}
 );

packages/livedata/livedata_connection.js

@@ -181,9 +181,9 @@ var Connection = function (url, options) {
     }
 
     if (msg === null || !msg.msg) {
-      // DEPRECATED. ignore the old welcome message for back compat.
-      // Remove this 'if' once the server stops sending welcome messages
-      // (stream_server.js).
+      // XXX COMPAT WITH 0.6.6. ignore the old welcome message for back
+      // compat.  Remove this 'if' once the server stops sending welcome
+      // messages (stream_server.js).
       if (! (msg && msg.server_id))
         Meteor._debug("discarding invalid livedata message", msg);
       return;

packages/livedata/stream_server.js

@@ -65,10 +65,10 @@ StreamServer = function () {
     });
     self.open_sockets.push(socket);
 
-    // DEPRECATED. Send the old style welcome message, which will force
-    // old clients to reload. Remove this once we're not concerned about
-    // people upgrading from a pre-0.6.7 release. Also, remove the
-    // clause in the client that ignores the welcome message
+    // XXX COMPAT WITH 0.6.6. Send the old style welcome message, which
+    // will force old clients to reload. Remove this once we're not
+    // concerned about people upgrading from a pre-0.6.7 release. Also,
+    // remove the clause in the client that ignores the welcome message
     // (livedata_connection.js)
     socket.send(JSON.stringify({server_id: "0"}));