[flow][indexing] Diff the new_available_exports_index and old_available_exports_index

Summary:
This stack aims to fix the bug that incremental update of exports will incorrectly drop some export count to zero, because the current subtract function will incorrectly remove the entire export even through it will be added back.

This diff fixes the issue by first diffing the available export index into two disjoint index for additional and removal, which are then passes to `Export_index.merge` and `Export_index.subtract`. This ensures that for the overlapping old and new available index that's supposed to cancel out, they won't incorrectly nuke the usage count entry.

Changelog: [fix] Fixed a bug that causes incorrect updates to our index that tracks usage of exports, which leads to incorrect autoimport-ranked-by-usage results.

Reviewed By: panagosg7

Differential Revision: D69694640

fbshipit-source-id: 9f777f69db78d05b49b373f68531b1d777590763

Author: SamChou19815

src/services/export/index/export_index.ml

@@ -132,6 +132,29 @@ let fold ~f ~init t =
 
 let map ~f t = SMap.map (ExportMap.map f) t
 
+let diff ~old_index ~new_index =
+  let exist_in_index export_name export_key map =
+    match SMap.find_opt export_name map with
+    | None -> false
+    | Some map -> ExportMap.mem export_key map
+  in
+  let diff_index l r =
+    SMap.fold
+      (fun export_name map acc ->
+        ExportMap.fold
+          (fun export_key _ acc ->
+            if exist_in_index export_name export_key r then
+              acc
+            else
+              let (s, k) = export_key in
+              add export_name s k acc)
+          map
+          acc)
+      l
+      SMap.empty
+  in
+  (diff_index new_index old_index, diff_index old_index new_index)
+
 let subtract old_t t =
   let (t, dead_names) =
     SMap.fold

src/services/export/index/export_index.mli

@@ -37,6 +37,10 @@ val merge : t -> t -> t
 
 val merge_export_import : t -> t -> t
 
+(* Returns tuple of two disjoint index: (addition_index, removal_index),
+ * to be passed to [merge] and `[subtract]`. *)
+val diff : old_index:t -> new_index:t -> t * t
+
 (** [subtract to_remove t] removes all of the exports in [to_remove] from [t], and
     also returns a list of keys that no longer are exported by any file. *)
 val subtract : t -> t -> t * string list

src/services/export/search/export_search.ml

@@ -67,12 +67,16 @@ let init index =
   let type_matcher = Fuzzy_path.init types in
   { index; value_matcher; type_matcher }
 
-let merge_available_exports old_index new_index { index; value_matcher; type_matcher } =
-  let (index, dead_candidates) = Export_index.subtract old_index index in
+let merge_available_exports
+    old_available_exports_index new_available_exports_index { index; value_matcher; type_matcher } =
+  let (addition_index, removal_index) =
+    Export_index.diff ~old_index:old_available_exports_index ~new_index:new_available_exports_index
+  in
+  let (index, dead_candidates) = Export_index.subtract removal_index index in
   let value_matcher = Fuzzy_path.remove_candidates value_matcher dead_candidates in
   let type_matcher = Fuzzy_path.remove_candidates type_matcher dead_candidates in
-  let index = Export_index.merge new_index index in
-  let { values; types } = partition_candidates new_index in
+  let index = Export_index.merge addition_index index in
+  let { values; types } = partition_candidates addition_index in
   let value_matcher = Fuzzy_path.add_candidates value_matcher values in
   let type_matcher = Fuzzy_path.add_candidates type_matcher types in
   { index; value_matcher; type_matcher }