[Merge] Use multilink files to speed up merge with buck projects.

Summary:
Merging the results directories of targets on buck projects involved creating symbolic links into buck-out.
The bulk of files are .attr files: one per procedure. Creating these links can be a bottleneck, and the merge phase can be slower than the analysis phases on projects with many procedures.
This diff introduces multilinks to speed up merge.
A multilink is a file `multilink.txt` containing a sequence of paths
```
path/to/file1.ext
path/to/file2.ext
...
```
A multilink file is a compact way to represent a link for each entry.

This diff creates a multilink file for each `attributes/dir` directory, instead of one symbolic link for each file.

Reviewed By: jberdine

Differential Revision: D4067428

fbshipit-source-id: 911f8a9

infer/src/IR/AttributesTable.re

@@ -43,7 +43,9 @@ let res_dir_attr_filename defined::defined pname => {
 /* Load the proc attribute for the defined filename if it exists,
    otherwise try to load the declared filename. */
 let load_defined_first proc_name => {
-  let attributes_file defined => res_dir_attr_filename defined::defined proc_name;
+  let attributes_file defined => Multilinks.resolve (
+    res_dir_attr_filename defined::defined proc_name
+  );
   let attr = Serialization.from_file serializer (attributes_file true);
   attr != None ? attr : Serialization.from_file serializer (attributes_file false)
 };

infer/src/IR/Ident.re

@@ -127,11 +127,6 @@ let idlist_to_idset ids => IList.fold_left (fun set id => IdentSet.add id set) I
 
 
 /** {2 Conversion between Names and Strings} */
-let module StringHash = Hashtbl.Make {
-  type t = string;
-  let equal (s1: string) (s2: string) => s1 == s2;
-  let hash = Hashtbl.hash;
-};
 
 let module NameHash = Hashtbl.Make {
   type t = name;

infer/src/backend/mergeCapture.ml

@@ -14,6 +14,8 @@ module F = Format
 
 (** Module to merge the results of capture for different buck targets. *)
 
+let use_multilinks = true
+
 (** Flag to control whether the timestamp of symbolic links
     is used to determine whether a captured directory needs to be merged. *)
 let check_timestamp_of_symlinks = true
@@ -41,12 +43,14 @@ let debug = 0
 type stats =
   {
     mutable files_linked: int;
+    mutable files_multilinked: int;
     mutable targets_merged: int;
   }
 
 let empty_stats () =
   {
     files_linked = 0;
+    files_multilinked = 0;
     targets_merged = 0;
   }
 
@@ -56,6 +60,51 @@ let link_exists s =
     true
   with Unix.Unix_error _ -> false
 
+(* Table mapping directories to multilinks.
+   Used for the hashed directories where attrbute files are stored. *)
+let multilinks_dir_table = StringHash.create 16
+
+
+(* Add a multilink for attributes to the internal per-directory table.
+   The files will be created by create_multilinks. *)
+let add_multilink_attr ~stats src dst =
+  let attr_dir = Filename.dirname dst in
+  let attr_dir_name = Filename.basename attr_dir in
+  let multilinks =
+    try
+      StringHash.find multilinks_dir_table attr_dir_name
+    with
+    | Not_found ->
+        let multilinks = match Multilinks.read ~dir:attr_dir with
+          | Some multilinks ->
+              (* incremental merge: start from the existing file on disk *)
+              multilinks
+          | None ->
+              Multilinks.create () in
+        StringHash.add multilinks_dir_table attr_dir_name multilinks;
+        multilinks in
+  Multilinks.add multilinks src;
+  stats.files_multilinked <- stats.files_multilinked + 1
+
+let create_link ~stats src dst =
+  if link_exists dst then Unix.unlink dst;
+  Unix.symlink src dst;
+  (* Set the accessed and modified time of the original file slightly in the past.  Due to
+     the coarse precision of the timestamps, it is possible for the source and destination of a
+     link to have the same modification time. When this happens, the files will be considered to
+     need re-analysis every time, indefinitely. *)
+  let near_past = Unix.gettimeofday () -. 1. in
+  Unix.utimes src near_past near_past;
+  stats.files_linked <- stats.files_linked + 1
+
+let create_multilinks () =
+  let do_dir dir multilinks =
+    let attributes_dir =
+      Filename.concat (Filename.concat Config.results_dir Config.attributes_dir_name) dir in
+    Multilinks.write multilinks ~dir:attributes_dir in
+  StringHash.iter do_dir multilinks_dir_table
+
+
 (** 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. *)
@@ -75,18 +124,9 @@ let rec slink ~stats ~skiplevels src dst =
         items
     end
   else if skiplevels > 0 then ()
-  else
+  else if use_multilinks && Filename.check_suffix dst ".attr"
-    begin
+  then add_multilink_attr ~stats src dst
-      if link_exists dst then Unix.unlink dst;
+  else create_link ~stats src dst
-      Unix.symlink src dst;
-      (* Set the accessed and modified time of the original file slightly in the past.  Due to
-         the coarse precision of the timestamps, it is possible for the source and destination of a
-         link to have the same modification time. When this happens, the files will be considered to
-         need re-analysis every time, indefinitely. *)
-      let near_past = Unix.gettimeofday () -. 1. in
-      Unix.utimes src near_past near_past;
-      stats.files_linked <- stats.files_linked + 1;
-    end
 
 (** Determine if the destination should link to the source.
     To check if it was linked before, check if all the captured source files
@@ -164,9 +204,11 @@ let process_merge_file deps_file =
   Option.may
     (fun lines -> IList.iter process_line lines)
     (read_file deps_file);
+  create_multilinks ();
   L.stdout "Captured results merged.@.";
   L.stdout "Targets merged: %d@." stats.targets_merged;
-  L.stdout "Files linked: %d@." stats.files_linked
+  L.stdout "Files linked: %d@." stats.files_linked;
+  L.stdout "Files multilinked: %d@." stats.files_multilinked
 
 
 let merge_captured_targets () =

infer/src/base/Multilinks.re

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2015 - present Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+open! Utils;
+
+let module F = Format;
+
+let module L = Logging;
+
+let multilink_file_name = "multilink.txt";
+
+type t = StringHash.t string;
+
+let add multilinks fname => StringHash.replace multilinks (Filename.basename fname) fname;
+
+let create () :t => StringHash.create 1;
+
+/* Cache of multilinks files read from disk */
+let multilink_files_cache = StringHash.create 1;
+
+let reset_cache () => StringHash.reset multilink_files_cache;
+
+let read dir::dir :option t => {
+  let multilink_fname = Filename.concat dir multilink_file_name;
+  switch (Utils.read_file multilink_fname) {
+  | None => None
+  | Some lines =>
+    let links = create ();
+    IList.iter (fun line => StringHash.add links (Filename.basename line) line) lines;
+    StringHash.add multilink_files_cache dir links;
+    Some links
+  }
+};
+
+/* Write a multilink file in the given directory */
+let write multilinks dir::dir => {
+  let fname = Filename.concat dir multilink_file_name;
+  let outc = open_out fname;
+  StringHash.iter (fun _ src => output_string outc (src ^ "\n")) multilinks;
+  close_out outc
+};
+
+let lookup dir::dir =>
+  try (Some (StringHash.find multilink_files_cache dir)) {
+  | Not_found => read dir::dir
+  };
+
+let resolve fname => {
+  let fname_s = DB.filename_to_string fname;
+  if (Sys.file_exists fname_s) {
+    fname
+  } else {
+    let base = Filename.basename fname_s;
+    let dir = Filename.dirname fname_s;
+    switch (lookup dir::dir) {
+    | None => fname
+    | Some links =>
+      try (DB.filename_from_string (StringHash.find links base)) {
+      | Not_found => fname
+      }
+    }
+  }
+};

infer/src/base/Multilinks.rei

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2015 - present Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+open! Utils;
+
+let module F = Format;
+
+let module L = Logging;
+
+
+/** In-memory representation of multilink files. */
+type t;
+
+
+/** Add a link. */
+let add: t => string => unit;
+
+
+/** Create a new multilink. */
+let create: unit => t;
+
+
+/** Name of the multilink file.
+    A multilink file is recognized by its file name. */
+let multilink_file_name: string;
+
+
+/** Read a multilink file from disk. */
+let read: dir::string => option t;
+
+
+/** Resolve a filename following multilinks.
+    The cache is updated if a new multilinks file is read. */
+let resolve: DB.filename => DB.filename;
+
+
+/** Reset the cache of multilink files */
+let reset_cache: unit => unit;
+
+
+/** Write a multilink file in the given directory */
+let write: t => dir::string => unit;

infer/src/base/Utils.ml

@@ -107,6 +107,14 @@ module IntSet =
     let compare = int_compare
   end)
 
+(** Hash table over strings *)
+module StringHash = Hashtbl.Make (
+  struct
+    type t = string
+    let equal (s1: string) (s2: string) = s1 = s2
+    let hash = Hashtbl.hash
+  end)
+
 (** Set of strings *)
 module StringSet = Set.Make(String)
 

infer/src/base/Utils.mli

@@ -90,6 +90,9 @@ val int_of_bool : bool -> int
 (** Set of integers *)
 module IntSet : Set.S with type elt = int
 
+(** Hash table over strings *)
+module StringHash : Hashtbl.S with type key = string
+
 (** Set of strings *)
 module StringSet : Set.S with type elt = string