(#5272) fix checkpointing against Cloudant/CouchDB 2.0

An attempt to provide a better fix for #5145. It seems like
it's safe to eagerly write a checkpoint in the event there are no
changes returned (e.g. because a filter did not generate results).

This also removes some assertions around last_seq / since values
in the tests, since they don't hold in CouchDB 2.x / Cloudant.

This at least allows the tests to pass - possibly this means we
need more tests to cover situations where a small number of docs
are returned.

packages/pouchdb-replication/src/replicate.js

@@ -308,7 +308,7 @@ function replicate(src, target, opts, returnValue, result) {
       };
 
       // update the checkpoint so we start from the right seq next time
-      if (!currentBatch && changes.last_seq > last_seq) {
+      if (!currentBatch && changes.results.length === 0) {
         writingCheckpoint = true;
         checkpointer.writeCheckpoint(changes.last_seq,
             session).then(function () {

tests/integration/test.replication.js

@@ -1568,23 +1568,20 @@ adapters.forEach(function (adapters) {
         result.ok.should.equal(true);
         result.docs_written.should.equal(0);
         result.docs_read.should.equal(0);
-        changesSince.length.should.equal(2);
-        // the returned last_seq should match the 'since'
-        // requested from remote
-        result.last_seq.should.equal(changesSince[1]);
+        changesSince.length.should.equal(2, JSON.stringify(changesSince));
+
         // the 'since' parameter should be different on the
         // next request
         changesSince[0].should.not.equal(changesSince[1]);
+
         // kick off a second replication
         return db.replicate.from(remote, replicationOpts);
       }).then(function (result) {
         result.ok.should.equal(true);
         result.docs_written.should.equal(0);
         result.docs_read.should.equal(0);
         changesSince.length.should.equal(3);
-        // the returned last_seq should match the 'since'
-        // requested from remote
-        result.last_seq.should.equal(changesSince[2]);
+
         // nothing has changed on the remote so 'since'
         // should be the same
         changesSince[1].should.equal(changesSince[2]);