add an optimisation pass for the cached output files

app/coffee/OutputFileOptimiser.coffee

@@ -0,0 +1,30 @@
+fs = require "fs"
+Path = require "path"
+spawn = require("child_process").spawn
+logger = require "logger-sharelatex"
+
+module.exports = OutputFileOptimiser =
+
+	optimiseFile: (src, dst, callback = (error) ->) ->
+		if src.match(/\.pdf$/)
+			OutputFileOptimiser.optimisePDF src, dst, callback
+		else
+			callback (null)
+
+	optimisePDF: (src, dst, callback = (error) ->) ->
+		tmpOutput = dst + '.opt'
+		args = ["--linearize", src, tmpOutput]
+		logger.log args: args, "running qpdf command"
+
+		proc = spawn("qpdf", args)
+		stdout = ""
+		proc.stdout.on "data", (chunk) ->
+			stdout += chunk.toString()	
+		proc.on "error", callback	
+		proc.on "close", (code) ->
+			if code != 0
+				logger.warn {directory, code}, "qpdf returned error"
+				return callback null
+			fs.rename tmpOutput, dst, (err) ->
+				# could log an error here
+				callback null