Node: register exit hook automatically

Part of #314.

README.md

@@ -216,17 +216,11 @@ and install it:
     ]
     ```
 
-3. If your tests will be running on Node,
-[call this function](https://github.com/aantron/bisect-starter-bsb/blob/master/hello.re#L2)
+3. If you are using Jest, [call this
+function](https://github.com/aantron/bisect-starter-bsb/blob/master/hello.re#L2)
 somewhere in your tester, which will have Node write a file like
 `bisect0123456789.coverage` when the tester exits:
 
-    ```reason
-    Bisect.Runtime.write_coverage_data_on_exit();
-    ```
-
-    If you are using Jest, try
-
     ```reason
     afterAll(Bisect.Runtime.write_coverage_data);
     ```
@@ -238,7 +232,7 @@ somewhere in your tester, which will have Node write a file like
     ```
 
     This returns binary coverage data in a `string option`, which you should
-    upload or otherwise get out of the browser, and write into an `.coverage`
+    upload or otherwise get out of the browser, and write into a `.coverage`
     file yourself.
 
 4. Build in development with `BISECT_ENABLE=yes`, run tests, and generate the

src/runtime/bucklescript/runtime.ml

@@ -4,10 +4,6 @@
 
 
 
-let register_file
-    ~bisect_file:_ ~bisect_silent:_ file ~point_count ~point_definitions =
-  Bisect_common.register_file file ~point_count ~point_definitions
-
 let get_coverage_data =
   Bisect_common.runtime_data_to_string
 
@@ -29,14 +25,25 @@ let write_coverage_data () =
     in
     create_file 100
 
+let reset_coverage_data =
+  Bisect_common.reset_counters
+
 let node_at_exit = [%bs.raw {|
   function (callback) {
-    process.on("exit", callback);
+    if (typeof process !== 'undefined' && typeof process.on !== 'undefined')
+      process.on("exit", callback);
   }
 |}]
 
+let exit_hook_added = ref false
+
 let write_coverage_data_on_exit () =
-  node_at_exit write_coverage_data
+  if not !exit_hook_added then begin
+    node_at_exit (fun () -> write_coverage_data (); reset_coverage_data ());
+    exit_hook_added := true
+  end
 
-let reset_coverage_data =
-  Bisect_common.reset_counters
+let register_file
+    ~bisect_file:_ ~bisect_silent:_ file ~point_count ~point_definitions =
+  write_coverage_data_on_exit ();
+  Bisect_common.register_file file ~point_count ~point_definitions

test/bucklescript/expected

@@ -1 +1 @@
-Coverage: 2/2 (100.00%)
+Coverage: 1/1 (100.00%)

test/bucklescript/hello.re

@@ -1,4 +1,3 @@
 let () = {
-  Bisect.Runtime.write_coverage_data_on_exit();
   print_endline("Hello, world!");
-}
+};