save output files in a .cache directory

services/clsi/app/coffee/CompileManager.coffee

@@ -1,6 +1,7 @@
 ResourceWriter = require "./ResourceWriter"
 LatexRunner = require "./LatexRunner"
 OutputFileFinder = require "./OutputFileFinder"
+OutputCacheManager = require "./OutputCacheManager"
 Settings = require("settings-sharelatex")
 Path = require "path"
 logger = require "logger-sharelatex"
@@ -32,7 +33,8 @@ module.exports = CompileManager =
 
 				OutputFileFinder.findOutputFiles request.resources, compileDir, (error, outputFiles) ->
 					return callback(error) if error?
-					callback null, outputFiles
+					OutputCacheManager.saveOutputFiles outputFiles, compileDir,  (error, newOutputFiles) -> 
+						callback null, newOutputFiles
 	
 	clearProject: (project_id, _callback = (error) ->) ->
 		callback = (error) ->

services/clsi/app/coffee/OutputCacheManager.coffee

@@ -0,0 +1,49 @@
+async = require "async"
+fs = require "fs"
+fse = require "fs-extra"
+Path = require "path"
+logger = require "logger-sharelatex"
+_ = require "underscore"
+
+module.exports = OutputCacheManager =
+	CACHE_DIR: '.cache/clsi'
+
+	saveOutputFiles: (outputFiles, target, callback) ->
+		# make a target/build_id directory and
+		# copy all the output files into it
+		buildId = 'build-' + Date.now()
+		relDir = OutputCacheManager.CACHE_DIR + '/' + buildId
+		newDir = target + '/' + relDir
+		OutputCacheManager.expireOutputFiles target
+		fse.ensureDir newDir, (err) ->
+			if err?
+				callback(err, outputFiles)
+			else
+				async.mapSeries outputFiles, (file, cb) ->
+					newFile = _.clone(file)
+					newFile.path = relDir + '/' + file.path
+					src = target + '/' + file.path
+					dst = target + '/' + newFile.path
+					#console.log 'src', src, 'dst', dst
+					fs.stat src, (err, stats) ->
+						if err?
+							cb(err)
+						else if stats.isFile()
+							#console.log 'isFile: copying'
+							fse.copy src, dst, (err) ->
+								cb(err, newFile)
+						else
+							# other filetype - shouldn't happen
+							cb(new Error("output file is not a file"), file)
+				, (err, results) ->
+					if err?
+						callback err, outputFiles
+					else
+						callback(err, results)
+
+	expireOutputFiles: (target, callback) ->
+		# look in target for build dirs and delete if > N or age of mod time > T
+		cacheDir = target + '/' + OutputCacheManager.CACHE_DIR
+		fs.readdir cacheDir, (err, results) ->
+			console.log 'CACHEDIR', results
+			callback(err) if callback?