Implement a Fuzzy CI to catch regressions

This commit implements a Fuzzy CI to catch end-to-end regressions in Merlin
by checking the responses of a set of ocamlmerlin queries. That set of
queries is determined by a deterministic but randomly chosen set of file
location samples on the Irmin code base. The CI passes automatically if the
responses on all samples remain the same before and after the PR. If there's a
diff in the responses, the CI fails and makes the diff available as a GH action
artifact.

There's a high-level description of the CI in much more detail on
https://github.com/ocaml/merlin/wiki/Merlin-Fuzzy-CI.

Co-authored-by: Enguerrand Decorne <[email protected]>

.github/fuzzy-ci-helpers/create_diff.ml

@@ -0,0 +1,45 @@
+type read_result = Success | Eof
+
+let read_line_opt () = try Some (read_line ()) with End_of_file -> None
+let delimiter1 = Sys.argv.(1)
+let delimiter2 = Sys.argv.(2)
+let diff_file = Sys.argv.(3)
+
+let rec loop () =
+  let rec stdin_to_file ~until file_oc =
+    match read_line_opt () with
+    | None -> Eof
+    | Some line ->
+        if String.equal line until then Success
+        else (
+          output_string file_oc (line ^ "\n");
+          stdin_to_file ~until file_oc)
+  in
+  let label1 = read_line_opt () in
+  let tmp1 = Filename.temp_file "tmp1" "" in
+  let oc = open_out tmp1 in
+  let read_result1 = stdin_to_file ~until:delimiter1 oc in
+  close_out oc;
+  let label2 = read_line_opt () in
+  let tmp2 = Filename.temp_file "tmp2" "" in
+  let oc = open_out tmp2 in
+  let read_result2 = stdin_to_file ~until:delimiter2 oc in
+  close_out oc;
+  match (label1, read_result1, label2, read_result2) with
+  | Some label1, Success, Some label2, Success ->
+      let diff_cmd =
+        Printf.sprintf
+          "diff -U 5 --label=\"%s\" --label=\"%s\" \"%s\" \"%s\" >> %s" label1
+          label2 tmp1 tmp2 diff_file
+      in
+      let _ = Sys.command diff_cmd in
+      loop ()
+  | Some _, Success, Some _, Eof ->
+      raise (Failure "EOF before reaching delimiter2 when reading second JSON.")
+  | Some _, Success, None, _ ->
+      raise (Failure "EOF before reaching delimiter2 when second label.")
+  | Some _, Eof, _, _ ->
+      raise (Failure "EOF before reaching delimiter1 when reading first JSON.")
+  | None, _, _, _ -> ()
+
+let () = loop ()