Fix issue in merge of captured buck targets where .start file was overwritten.

Summary:public
The merge option -m would link every file in a captured results dir.
This includes the .start file, which would compromise the subsequent checks for modified files during reactive analysis.
Now only link files inside directories inside the results dir (don't link any files in the first 2 levels).

Reviewed By: jberdine

Differential Revision: D3155819

fb-gh-sync-id: 8ad180f
fbshipit-source-id: 8ad180f

infer/src/backend/mergeCapture.ml

@@ -57,9 +57,9 @@ let link_exists s =
 (** Create symbolic links recursively from the destination to the source.
     Replicate the structure of the source directory in the destination,
     with files replaced by links to the source. *)
-let rec slink ~stats src dst =
+let rec slink ~stats ~skiplevels src dst =
   if debug >=3
-  then L.stderr "slink src:%s dst:%s@." src dst;
+  then L.stderr "slink src:%s dst:%s skiplevels:%d@." src dst skiplevels;
   if Sys.is_directory src
   then
     begin
@@ -67,9 +67,12 @@ let rec slink ~stats src dst =
       then Unix.mkdir dst 0o700;
       let items = Sys.readdir src in
       Array.iter
-        (fun item -> slink ~stats (Filename.concat src item) (Filename.concat dst item))
+        (fun item ->
+           slink ~stats ~skiplevels:(skiplevels - 1)
+             (Filename.concat src item) (Filename.concat dst item))
         items
     end
+  else if skiplevels > 0 then ()
   else
     begin
       if link_exists dst then Unix.unlink dst;
@@ -148,8 +151,9 @@ let process_merge_file deps_file =
     match Str.split_delim (Str.regexp (Str.quote "\t")) line with
     | target :: _ :: target_results_dir :: _ ->
         let infer_out_src = Filename.concat (Filename.dirname (buck_out ())) target_results_dir in
+        let skiplevels = 2 in (** Don't link toplevel files, definitely not .start *)
         if should_link ~target ~target_results_dir ~stats infer_out_src infer_out_dst
-        then slink ~stats infer_out_src infer_out_dst
+        then slink ~stats ~skiplevels infer_out_src infer_out_dst
     | _ ->
         () in
   Option.may