always create .start if it doesn't exist

Summary:public
`rm -rf infer-out && infer --reactive --continue -a capture -- clang -c hello.c` used to crash.

closes #315

Differential Revision: D3109673

fb-gh-sync-id: 661bd07
fbshipit-source-id: 661bd07

infer/lib/python/infer

@@ -126,8 +126,8 @@ def main():
 
     if imported_module:
         analyze.create_results_dir(args.infer_out)
-        if not args.continue_capture:
-            analyze.reset_start_file(args.infer_out)
+        analyze.reset_start_file(args.infer_out,
+                                 touch_if_present=not args.continue_capture)
 
         utils.configure_logging(args)
         logging.info('Running command %s', ' '.join(sys.argv))

infer/lib/python/inferlib/analyze.py

@@ -175,10 +175,11 @@ def create_results_dir(results_dir):
     utils.mkdir_if_not_exists(os.path.join(results_dir, 'sources'))
 
 
-def reset_start_file(results_dir):
-    # create new empty file - this will update modified timestamp
-    open(os.path.join(results_dir, '.start'), 'w').close()
-
+def reset_start_file(results_dir, touch_if_present=False):
+    start_path = os.path.join(results_dir, '.start')
+    if (not os.path.exists(start_path)) or touch_if_present:
+        # create new empty file - this will update modified timestamp
+        open(start_path, 'w').close()
 
 def clean(infer_out):
     directories = [