Implement a Fuzzy CI to catch ocamlmerlin regressions

.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 ()