Add cursorSupportedByOplogTailing.

packages/mongo-livedata/mongo_driver.js

@@ -1264,9 +1264,48 @@ MongoConnection.prototype._observeChangesTailable = function (
   };
 };
 
+// Does our oplog tailing code support this cursor? For now, we are being very
+// conservative and allowing only simple queries with simple options.
+var cursorSupportedByOplogTailing = function (cursorDescription) {
+  // First, check the options.
+  var options = cursorDescription.options;
+
+  // We don't yet implement field filtering for oplog tailing (just because it's
+  // not implemented, not because there's a deep problem with implementing it).
+  if (options.fields) return false;
+
+  // This option (which are mostly used for sorted cursors) require us to figure
+  // out where a given document fits in an order to know if it's included or
+  // not, and we don't track that information when doing oplog tailing.
+  if (options.limit || options.skip) return false;
+
+  // For now, we're just dealing with equality queries: no $operators, regexps,
+  // or $and/$or/$where/etc clauses. We can expand the scope of what we're
+  // comfortable processing later.
+  return _.all(cursorDescription.selector, function (value, field) {
+    // No logical operators like $and.
+    if (field.substr(0, 1) === '$')
+      return false;
+    // We only allow scalars, not sub-documents or $operators or RegExp.
+    // XXX Date would be easy too, though I doubt anyone is doing equality
+    // lookups on dates
+    return typeof value === "string" ||
+      typeof value === "number" ||
+      typeof value === "boolean" ||
+      value === null ||
+      value instanceof Meteor.Collection.ObjectID;
+  });
+};
+
+
+
 // XXX We probably need to find a better way to expose this. Right now
 // it's only used by tests, but in fact you need it in normal
 // operation to interact with capped collections (eg, Galaxy uses it).
 MongoInternals.MongoTimestamp = MongoDB.Timestamp;
 
 MongoInternals.Connection = MongoConnection;
+
+MongoTest = {
+  cursorSupportedByOplogTailing: cursorSupportedByOplogTailing
+};

packages/mongo-livedata/oplog_tests.js

@@ -0,0 +1,32 @@
+var OplogCollection = new Meteor.Collection("oplog-" + Random.id());
+
+Tinytest.add("mongo-livedata - oplog - cursorSupportedByOplogTailing", function (test) {
+  var supported = function (expected, selector) {
+    var cursor = OplogCollection.find(selector);
+    test.equal(
+      MongoTest.cursorSupportedByOplogTailing(cursor._cursorDescription),
+      expected);
+  };
+
+  supported(true, "asdf");
+  supported(true, 1234);
+  supported(true, new Meteor.Collection.ObjectID());
+
+  supported(true, {_id: "asdf"});
+  supported(true, {_id: 1234});
+  supported(true, {_id: new Meteor.Collection.ObjectID()});
+
+  supported(true, {foo: "asdf",
+                   bar: 1234,
+                   baz: new Meteor.Collection.ObjectID(),
+                   eeney: true,
+                   miney: false,
+                   moe: null});
+
+  supported(true, {});
+
+  supported(false, {$and: [{foo: "asdf"}, {bar: "baz"}]});
+  supported(false, {foo: {x: 1}});
+  supported(false, {foo: {$gt: 1}});
+  supported(false, {foo: [1, 2, 3]});
+});

packages/mongo-livedata/package.js

@@ -35,6 +35,8 @@ Package.on_use(function (api) {
 
   // Stuff that should be exposed via a real API, but we haven't yet.
   api.export('MongoInternals', 'server');
+  // For tests only.
+  api.export('MongoTest', 'server');
 
   api.add_files('mongo_driver.js', 'server');
   api.add_files('local_collection_driver.js', ['client', 'server']);
@@ -53,4 +55,5 @@ Package.on_test(function (api) {
   api.add_files('allow_tests.js', ['client', 'server']);
   api.add_files('collection_tests.js', ['client', 'server']);
   api.add_files('observe_changes_tests.js', ['client', 'server']);
+  api.add_files('oplog_tests.js', 'server');
 });