write files incrementally

app/coffee/CompileController.coffee

@@ -15,7 +15,10 @@ module.exports = CompileController =
 			ProjectPersistenceManager.markProjectAsJustAccessed request.project_id, (error) ->
 				return next(error) if error?
 				CompileManager.doCompile request, (error, outputFiles = []) ->
-					if error?.terminated
+					if error?.message is "invalid state"
+						code = 409 # Http 409 Conflict
+						status = "retry"
+					else if error?.terminated
 						status = "terminated"
 					else if error?.validate
 						status = "validation-#{error.validate}"

app/coffee/CompileManager.coffee

@@ -31,7 +31,7 @@ module.exports = CompileManager =
 
 		timer = new Metrics.Timer("write-to-disk")
 		logger.log project_id: request.project_id, user_id: request.user_id, "syncing resources to disk"
-		ResourceWriter.syncResourcesToDisk request.project_id, request.resources, compileDir, (error) ->
+		ResourceWriter.syncResourcesToDisk request, compileDir, (error) ->
 			if error?
 				logger.err err:error, project_id: request.project_id, user_id: request.user_id, "error writing resources to disk"
 				return callback(error) 

app/coffee/RequestParser.coffee

@@ -31,6 +31,12 @@ module.exports = RequestParser =
 			response.check = @_parseAttribute "check",
 				compile.options.check,
 				type: "string"
+			response.incremental = @_parseAttribute "incremental",
+				compile.options.incremental,
+				type: "string"
+			response.state = @_parseAttribute "state",
+				compile.options.state,
+				type: "string"
 
 			if response.timeout > RequestParser.MAX_TIMEOUT
 				response.timeout = RequestParser.MAX_TIMEOUT

app/coffee/ResourceWriter.coffee

@@ -11,7 +11,39 @@ settings = require("settings-sharelatex")
 parallelFileDownloads = settings.parallelFileDownloads or 1
 
 module.exports = ResourceWriter =
-	syncResourcesToDisk: (project_id, resources, basePath, callback = (error) ->) ->
+
+	syncResourcesToDisk: (request, basePath, callback = (error) ->) ->
+		if request.incremental?
+			ResourceWriter.checkState request.incremental, basePath, (error, ok) ->
+				logger.log state: request.state, result:ok, "checked state on incremental request"
+				return callback new Error("invalid state") if not ok
+				ResourceWriter.saveIncrementalResourcesToDisk request.project_id, request.resources, basePath, callback
+		else
+			@saveAllResourcesToDisk request.project_id, request.resources, basePath, (error) ->
+				return callback(error) if error?
+				ResourceWriter.storeState request.state, basePath, callback
+
+	storeState: (state, basePath, callback) ->
+		logger.log state:state, basePath:basePath, "writing state"
+		fs.writeFile Path.join(basePath, ".resource-state"), state, {encoding: 'ascii'}, callback
+
+	checkState: (state, basePath, callback) ->
+		fs.readFile Path.join(basePath, ".resource-state"), {encoding:'ascii'}, (err, oldState) ->
+			logger.log state:state, oldState: oldState, basePath:basePath, err:err, "checking state"
+			if state is oldState
+				return callback(null, true)
+			else
+				return callback(null, false)
+
+	saveIncrementalResourcesToDisk: (project_id, resources, basePath, callback = (error) ->) ->
+		@_createDirectory basePath, (error) =>
+			return callback(error) if error?
+			jobs = for resource in resources
+				do (resource) =>
+					(callback) => @_writeResourceToDisk(project_id, resource, basePath, callback)
+			async.parallelLimit jobs, parallelFileDownloads, callback
+
+	saveAllResourcesToDisk: (project_id, resources, basePath, callback = (error) ->) ->
 		@_createDirectory basePath, (error) =>
 			return callback(error) if error?
 			@_removeExtraneousFiles resources, basePath, (error) =>