simplify tikzexternalize checks

app/coffee/CompileManager.coffee

@@ -62,10 +62,12 @@ module.exports = CompileManager =
 					callback()
 
 			createTikzFileIfRequired = (callback) ->
-				if TikzManager.needsOutputFile(request.rootResourcePath, resourceList)
-					TikzManager.injectOutputFile compileDir, request.rootResourcePath, callback
-				else
-					callback()
+				TikzManager.checkMainFile compileDir, request.rootResourcePath, resourceList, (error, usesTikzExternalize) ->
+					return callback(error) if error?
+					if usesTikzExternalize
+						TikzManager.injectOutputFile compileDir, request.rootResourcePath, callback
+					else
+						callback()
 
 			# set up environment variables for chktex
 			env = {}

app/coffee/TikzManager.coffee

@@ -1,6 +1,7 @@
 fs = require "fs"
 Path = require "path"
 ResourceWriter = require "./ResourceWriter"
+SafeReader = require "./SafeReader"
 logger = require "logger-sharelatex"
 
 # for \tikzexternalize to work the main file needs to match the
@@ -8,25 +9,21 @@ logger = require "logger-sharelatex"
 # copy of the main file as 'output.tex'.
 
 module.exports = TikzManager =
-	needsOutputFile: (rootResourcePath, resources) ->
+
+	checkMainFile: (compileDir, mainFile, resources, callback = (error, usesTikzExternalize) ->) ->
 		# if there's already an output.tex file, we don't want to touch it
 		for resource in resources
 			if resource.path is "output.tex"
-				return false
+				logger.log compileDir: compileDir, mainFile: mainFile, "output.tex already in resources"
+				return callback(null, false)
 		# if there's no output.tex, see if we are using tikz/pgf in the main file
-		for resource in resources
-			if resource.path is rootResourcePath
-				return TikzManager._includesTikz (resource)
-		# otherwise false
-		return false
-
-	_includesTikz: (resource) ->
-		# check if we are using tikz externalize
-		content = resource.content?.slice(0,65536)
-		if content?.indexOf("\\tikzexternalize") >= 0
-			return true
-		else
-			return false
+		ResourceWriter.checkPath compileDir, mainFile, (error, path) ->
+			return callback(error) if error?
+			SafeReader.readFile path, 65536, "utf8", (error, content) ->
+				return callback(error) if error?
+				usesTikzExternalize = content?.indexOf("\\tikzexternalize") >= 0
+				logger.log compileDir: compileDir, mainFile: mainFile, usesTikzExternalize:usesTikzExternalize, "checked for tikzexternalize"
+				callback null, usesTikzExternalize
 
 	injectOutputFile: (compileDir, mainFile, callback = (error) ->) ->
 		ResourceWriter.checkPath compileDir, mainFile, (error, path) ->