lock compile directory

app/coffee/CompileController.coffee

@@ -15,8 +15,11 @@ module.exports = CompileController =
 			request.user_id = req.params.user_id if req.params.user_id?
 			ProjectPersistenceManager.markProjectAsJustAccessed request.project_id, (error) ->
 				return next(error) if error?
-				CompileManager.doCompile request, (error, outputFiles = []) ->
-					if error instanceof Errors.FilesOutOfSyncError
+				CompileManager.doCompileWithLock request, (error, outputFiles = []) ->
+					if error instanceof Errors.AlreadyCompilingError
+						code = 423 # Http 443 Locked
+						status = "compile-in-progress"
+					else if error instanceof Errors.FilesOutOfSyncError
 						code = 409 # Http 409 Conflict
 						status = "retry"
 					else if error?.terminated

app/coffee/CompileManager.coffee

@@ -9,6 +9,7 @@ Metrics = require "./Metrics"
 child_process = require "child_process"
 DraftModeManager = require "./DraftModeManager"
 TikzManager = require "./TikzManager"
+LockManager = require "./LockManager"
 fs = require("fs")
 fse = require "fs-extra"
 os = require("os")
@@ -26,6 +27,18 @@ getCompileDir = (project_id, user_id) ->
 	Path.join(Settings.path.compilesDir, getCompileName(project_id, user_id))
 
 module.exports = CompileManager =
+
+	doCompileWithLock: (request, callback = (error, outputFiles) ->) ->
+		compileDir = getCompileDir(request.project_id, request.user_id)
+		lockFile = Path.join(compileDir, ".project-lock")
+		# use a .project-lock file in the compile directory to prevent
+		# simultaneous compiles
+		fse.ensureDir compileDir, (error) ->
+			return callback(error) if error?
+			LockManager.runWithLock lockFile, (releaseLock) ->
+				CompileManager.doCompile(request, releaseLock)
+			, callback
+
 	doCompile: (request, callback = (error, outputFiles) ->) ->
 		compileDir = getCompileDir(request.project_id, request.user_id)
 

app/coffee/Errors.coffee

@@ -12,6 +12,14 @@ FilesOutOfSyncError = (message) ->
 	return error
 FilesOutOfSyncError.prototype.__proto__ = Error.prototype
 
+AlreadyCompilingError = (message) ->
+	error = new Error(message)
+	error.name = "AlreadyCompilingError"
+	error.__proto__ = AlreadyCompilingError.prototype
+	return error
+AlreadyCompilingError.prototype.__proto__ = Error.prototype
+
 module.exports = Errors =
 	NotFoundError: NotFoundError
 	FilesOutOfSyncError: FilesOutOfSyncError
+	AlreadyCompilingError: AlreadyCompilingError

app/coffee/LockManager.coffee

@@ -0,0 +1,23 @@
+Settings = require('settings-sharelatex')
+logger = require "logger-sharelatex"
+Lockfile = require('lockfile') # from https://github.com/npm/lockfile
+Errors = require "./Errors"
+
+module.exports = LockManager =
+	LOCK_TEST_INTERVAL: 1000 # 50ms between each test of the lock
+	MAX_LOCK_WAIT_TIME: 15000 # 10s maximum time to spend trying to get the lock
+	LOCK_STALE: 5*60*1000 # 5 mins time until lock auto expires
+
+	runWithLock: (path, runner = ((releaseLock = (error) ->) ->), callback = ((error) ->)) ->
+		lockOpts =
+			wait: @MAX_LOCK_WAIT_TIME
+			pollPeriod: @LOCK_TEST_INTERVAL
+			stale: @LOCK_STALE
+		Lockfile.lock path, lockOpts, (error) ->
+			return callback new Errors.AlreadyCompilingError("compile in progress")	if error?.code is 'EEXIST'
+			return callback(error) if error?
+			runner (error1, args...) ->
+				Lockfile.unlock path, (error2) ->
+					error = error1 or error2
+					return callback(error) if error?
+					callback(null, args...)

app/coffee/ResourceWriter.coffee

@@ -78,7 +78,7 @@ module.exports = ResourceWriter =
 					should_delete = true
 					if path.match(/^output\./) or path.match(/\.aux$/) or path.match(/^cache\//) # knitr cache
 						should_delete = false
-					if path == '.project-sync-state'
+					if path == '.project-sync-state' or path == '.project-lock'
 						should_delete = false
 					if path == "output.pdf" or path == "output.dvi" or path == "output.log" or path == "output.xdv"
 						should_delete = true