Don't trip up over empty ops

services/track-changes/app/coffee/UpdateCompressor.coffee

@@ -2,6 +2,8 @@ strInject = (s1, pos, s2) -> s1[...pos] + s2 + s1[pos..]
 strRemove = (s1, pos, length) -> s1[...pos] + s1[(pos + length)..]
 
 module.exports = UpdateCompressor =
+	NOOP: "noop"
+
 	# Updates come from the doc updater in format
 	# {
 	# 	op:   [ { ... op1 ... }, { ... op2 ... } ]
@@ -19,14 +21,23 @@ module.exports = UpdateCompressor =
 	convertToSingleOpUpdates: (updates) ->
 		splitUpdates = []
 		for update in updates
-			for op in update.op
+			if update.op.length == 0
 				splitUpdates.push
-					op: op
+					op: UpdateCompressor.NOOP
 					meta:
 						start_ts: update.meta.start_ts or update.meta.ts
 						end_ts:   update.meta.end_ts   or update.meta.ts
 						user_id:  update.meta.user_id
 					v: update.v
+			else
+				for op in update.op
+					splitUpdates.push
+						op: op
+						meta:
+							start_ts: update.meta.start_ts or update.meta.ts
+							end_ts:   update.meta.end_ts   or update.meta.ts
+							user_id:  update.meta.user_id
+						v: update.v
 		return splitUpdates
 
 	concatUpdatesWithSameVersion: (updates) ->
@@ -34,12 +45,14 @@ module.exports = UpdateCompressor =
 		for update in updates
 			lastUpdate = concattedUpdates[concattedUpdates.length - 1]
 			if lastUpdate? and lastUpdate.v == update.v
-				lastUpdate.op.push update.op
+				lastUpdate.op.push update.op unless update.op == UpdateCompressor.NOOP
 			else
-				concattedUpdates.push
-					op:   [ update.op ]
+				nextUpdate =
+					op:   []
 					meta: update.meta
 					v:    update.v
+				nextUpdate.op.push update.op unless update.op == UpdateCompressor.NOOP
+				concattedUpdates.push nextUpdate
 		return concattedUpdates
 
 	compressRawUpdates: (lastPreviousUpdate, rawUpdates) ->

services/track-changes/test/acceptance/coffee/AppendingUpdatesTests.coffee

@@ -194,3 +194,35 @@ describe "Appending doc ops to the history", ->
 			expect(@updates[0].v).to.equal 3
 			expect(@updates[1].v).to.equal 4
 
+	describe "when there is a no-op update", ->
+		before (done) ->
+			@project_id = ObjectId().toString()
+			@doc_id = ObjectId().toString()
+			@user_id = ObjectId().toString()
+			oneDay = 24 * 60 * 60 * 1000
+			TrackChangesClient.pushRawUpdates @project_id, @doc_id, [{
+				op: []
+				meta: { ts: Date.now(), user_id: @user_id }
+				v: 3
+			}, {
+				op: [{ i: "foo", p: 3 }]
+				meta: { ts: Date.now() + oneDay, user_id: @user_id }
+				v: 4
+			}], (error) =>
+				throw error if error?
+				TrackChangesClient.flushAndGetCompressedUpdates @project_id, @doc_id, (error, @updates) =>
+					throw error if error?
+					done()
+
+		it "should insert the compressed no-op into mongo", ->
+			expect(@updates[0].op).to.deep.equal []
+
+
+		it "should insert the compressed next update into mongo", ->
+			expect(@updates[1].op).to.deep.equal [{
+				p: 3, i: "foo"
+			}]
+
+		it "should insert the correct version numbers into mongo", ->
+			expect(@updates[0].v).to.equal 3
+			expect(@updates[1].v).to.equal 4

services/track-changes/test/acceptance/coffee/FlushingUpdatesTests.coffee

@@ -0,0 +1,56 @@
+sinon = require "sinon"
+chai = require("chai")
+chai.should()
+expect = chai.expect
+mongojs = require "../../../app/js/mongojs"
+ObjectId = mongojs.ObjectId
+Settings = require "settings-sharelatex"
+request = require "request"
+rclient = require("redis").createClient() # Only works locally for now
+
+TrackChangesClient = require "./helpers/TrackChangesClient"
+
+describe "Flushing updates", ->
+	describe "flushing a doc's updates", ->
+		before (done) ->
+			@project_id = ObjectId().toString()
+			@doc_id = ObjectId().toString()
+			@user_id = ObjectId().toString()
+			TrackChangesClient.pushRawUpdates @project_id, @doc_id, [{
+				op: [{ i: "f", p: 3 }]
+				meta: { ts: Date.now(), user_id: @user_id }
+				v: 3
+			}], (error) =>
+				throw error if error?
+				TrackChangesClient.flushDoc @project_id, @doc_id, (error) ->
+					throw error if error?
+					done()
+
+		it "should flush the op into mongo", (done) ->
+			TrackChangesClient.getCompressedUpdates @doc_id, (error, updates) ->
+				expect(updates[0].op).to.deep.equal [{
+					p: 3, i: "f"
+				}]
+				done()
+
+	describe "flushing a project's updates", ->
+		before (done) ->
+			@project_id = ObjectId().toString()
+			@doc_id = ObjectId().toString()
+			@user_id = ObjectId().toString()
+			TrackChangesClient.pushRawUpdates @project_id, @doc_id, [{
+				op: [{ i: "f", p: 3 }]
+				meta: { ts: Date.now(), user_id: @user_id }
+				v: 3
+			}], (error) =>
+				throw error if error?
+				TrackChangesClient.flushProject @project_id, (error) ->
+					throw error if error?
+					done()
+
+		it "should flush the op into mongo", (done) ->
+			TrackChangesClient.getCompressedUpdates @doc_id, (error, updates) ->
+				expect(updates[0].op).to.deep.equal [{
+					p: 3, i: "f"
+				}]
+				done()

services/track-changes/test/unit/coffee/UpdateCompressor/UpdateCompressorTests.coffee

@@ -38,6 +38,18 @@ describe "UpdateCompressor", ->
 				v: 43
 			}]
 
+		it "should return no-op updates when the op list is empty", ->
+			expect(@UpdateCompressor.convertToSingleOpUpdates [{
+				op:   []
+				meta: { ts: @ts1, user_id: @user_id }
+				v: 42
+			}])
+			.to.deep.equal [{
+				op: @UpdateCompressor.NOOP
+				meta: { start_ts: @ts1, end_ts: @ts1, user_id: @user_id },
+				v: 42
+			}]
+
 	describe "concatUpdatesWithSameVersion", ->
 		it "should concat updates with the same version", ->
 			expect(@UpdateCompressor.concatUpdatesWithSameVersion [{
@@ -63,6 +75,18 @@ describe "UpdateCompressor", ->
 				v: 43
 			}]
 
+		it "should turn a noop into an empty op", ->
+			expect(@UpdateCompressor.concatUpdatesWithSameVersion [{
+				op:   @UpdateCompressor.NOOP
+				meta: { start_ts: @ts1, end_ts: @ts1, user_id: @user_id }
+				v: 42
+			}])
+			.to.deep.equal [{
+				op:   []
+				meta: { start_ts: @ts1, end_ts: @ts1, user_id: @user_id }
+				v: 42
+			}]
+
 	describe "compress", ->
 		describe "insert - insert", ->
 			it "should append one insert to the other", ->
@@ -239,4 +263,44 @@ describe "UpdateCompressor", ->
 					v: 43
 				}]
 
-				
+		describe "noop - insert", ->
+			it "should leave them untouched", ->
+				expect(@UpdateCompressor.compressUpdates [{
+					op: @UpdateCompressor.NOOP
+					meta: ts: @ts1, user_id: @user_id
+					v: 42
+				}, {
+					op: { p: 6, i: "bar" }
+					meta: ts: @ts1, user_id: @user_id
+					v: 43
+				}])
+				.to.deep.equal [{
+					op: @UpdateCompressor.NOOP
+					meta: start_ts: @ts1, end_ts: @ts1, user_id: @user_id
+					v: 42
+				}, {
+					op: { p: 6, i: "bar" }
+					meta: start_ts: @ts1, end_ts: @ts1, user_id: @user_id
+					v: 43
+				}]
+
+		describe "noop - delete", ->
+			it "should leave them untouched", ->
+				expect(@UpdateCompressor.compressUpdates [{
+					op: @UpdateCompressor.NOOP
+					meta: ts: @ts1, user_id: @user_id
+					v: 42
+				}, {
+					op: { p: 6, d: "bar" }
+					meta: ts: @ts1, user_id: @user_id
+					v: 43
+				}])
+				.to.deep.equal [{
+					op: @UpdateCompressor.NOOP
+					meta: start_ts: @ts1, end_ts: @ts1, user_id: @user_id
+					v: 42
+				}, {
+					op: { p: 6, d: "bar" }
+					meta: start_ts: @ts1, end_ts: @ts1, user_id: @user_id
+					v: 43
+				}]