split hash sets facts into smaller chunks

Summary:
The hack db has this predicate which represents a set of hashes of the input files, in no particular order. It's arbitrarily split into 15 facts (... because there are 15 indexing shards).

```
www.hack.light> :stat hack.IndexerInputsHash
hack.IndexerInputsHash.6
  count: 15
  size:  129201564 (123.22 MiB) 0.1334%
```

Each of these fact is about 10MB.  We've seen query to this predicate failing with an allocation failure error from the server. I'm guessing that making the facts smaller than the pagination limit will enable the server to paginate these results.

The `glean query` CLI has a  page limit of

```
 - --page-bytes: Default is 1,000,000 (1
  MB)
```

This diff constructs the hashes as <1MB facts.

Reviewed By: jjuliamolin

Differential Revision: D92045104

fbshipit-source-id: fd031f98adc120bcb4793f7291616a4039877068

Author: Philippe Bidinger

hphp/hack/src/typing/write_symbol_info/entrypoint.ml

@@ -76,15 +76,43 @@ let write_facts_file out_dir ?(global = false) files json_chunks =
    indexing run. There are
    - symbol hash facts for incrementality (empty if gen_sym_hash isn't set)
    - the namespace aliases defined in .hhconfig *)
+
+let bytes_per_hash = 16
+
+(* input hashes are stored in facts, we try to keep the max size
+   of such  fact to a reasonable size (800KB), ideally less than
+   Glean pagination limit *)
+let max_fact_bytes = 800 * 1024 (* 800KB *)
+
+let max_hashes_per_fact = max_fact_bytes / bytes_per_hash
+
 let gen_global_facts ns ~ownership ~shard_name all_hashes =
-  let fa = Fact_acc.init ~ownership in
   let list_hashes = Set.to_list all_hashes in
-  if ownership then Fact_acc.set_ownership_unit fa (Some ".hhconfig");
-  List.fold ns ~init:fa ~f:(fun fa (from, to_) ->
-      Add_fact.global_namespace_alias fa ~from ~to_ |> snd)
-  |> Add_fact.indexerInputsHash shard_name list_hashes
-  |> snd
-  |> Fact_acc.to_json
+  (* Split hashes into chunks *)
+  let hash_chunks = List.chunks_of list_hashes ~length:max_hashes_per_fact in
+  let num_chunks = List.length hash_chunks in
+  (* Generate facts for each chunk *)
+  List.concat_mapi hash_chunks ~f:(fun i chunk_hashes ->
+      let fa = Fact_acc.init ~ownership in
+      (* Only include namespace aliases in the first shard *)
+      let fa =
+        if i = 0 then begin
+          if ownership then Fact_acc.set_ownership_unit fa (Some ".hhconfig");
+          List.fold ns ~init:fa ~f:(fun fa (from, to_) ->
+              Add_fact.global_namespace_alias fa ~from ~to_ |> snd)
+        end else
+          fa
+      in
+      let chunk_shard_name =
+        if num_chunks = 1 then
+          shard_name
+        else
+          Printf.sprintf "%s%03d" shard_name i
+      in
+      fa
+      |> Add_fact.indexerInputsHash chunk_shard_name chunk_hashes
+      |> snd
+      |> Fact_acc.to_json)
 
 let write_json
     (ctx : Provider_context.t)