Improve test coverage for SnapshotRequest.onConnectionStateChanged

This change improves `SnapshotRequest` test coverage to 100%.

Our current test for checking that a dropped connection works didn't
actually fail properly when removing `this.sent = false` from
`SnapshotRequest._onConnectionStateChanged`, so an improved test has
been written that use middleware to wait until the request has
definitely been sent, before dropping the connection and reconnecting.

It also adds a previously uncovered test case for checking that the
connection cannot send the same request multiple times, by using a
similar setup to the rewritten dropped connection case.

This change also adds some documentation recording why we might want to
reset `this.sent = false`, because just looking at the code it's
unclear why we would ever want to do that.

Author: Alec Gibson

lib/client/snapshot-request.js

@@ -56,9 +56,12 @@ SnapshotRequest.prototype.send = function () {
 };
 
 SnapshotRequest.prototype._onConnectionStateChanged = function () {
-  if (this.connection.canSend && !this.sent) {
-    this.send();
-  } else if (!this.connection.canSend) {
+  if (this.connection.canSend) {
+    if (!this.sent) this.send();
+  } else {
+    // If the connection can't send, then we've had a disconnection, and even if we've already sent
+    // the request previously, we need to re-send it over this reconnected client, so reset the
+    // sent flag to false.
     this.sent = false;
   }
 };

test/client/snapshot-request.js

@@ -214,13 +214,34 @@ describe('SnapshotRequest', function () {
     it('can drop its connection and reconnect, and the callback is just called once', function (done) {
       var connection = backend.connect();
 
-      connection.fetchSnapshot('books', 'don-quixote', function (error) {
-        if (error) return done(error);
-        done();
+      var connectionInterrupted = false;
+      backend.use(backend.MIDDLEWARE_ACTIONS.readSnapshots, function (request, callback) {
+        if (!connectionInterrupted) {
+          connection.close();
+          backend.connect(connection);
+          connectionInterrupted = true;
+        }
+
+        callback();
+      });
+
+      connection.fetchSnapshot('books', 'don-quixote', done);
+    });
+
+    it('cannot send the same request twice over a connection', function (done) {
+      var connection = backend.connect();
+
+      var hasResent = false;
+      backend.use(backend.MIDDLEWARE_ACTIONS.readSnapshots, function (request, callback) {
+        if (!hasResent) {
+          connection._snapshotRequests[1]._onConnectionStateChanged();
+          hasResent = true;
+        }
+
+        callback();
       });
 
-      connection.close();
-      backend.connect(connection);
+      connection.fetchSnapshot('books', 'don-quixote', done);
     });
 
     describe('readSnapshots middleware', function () {