Terminate process group after cram execution

doc/changes/11841.md

@@ -0,0 +1,4 @@
+- Kill all processes in the process group after the main process has
+  terminated; in particular this avoids background processes in cram tests to
+  stick around after the test finished (#11841, fixes #11820, @Alizter,
+  @Leonidas-from-XIV)

src/dune_engine/scheduler.ml

@@ -889,7 +889,12 @@ let wait_for_build_process t pid =
       ~on_cancel:(fun () ->
         Process_watcher.killall t.process_watcher Sys.sigkill;
         Fiber.return ())
-      (fun () -> wait_for_process t pid)
+      (fun () ->
+         let+ r = wait_for_process t pid in
+         (* [kill_process_group] on Windows only kills the pid and by this
+            time the process should've exited anyway *)
+         if not Sys.win32 then kill_process_group pid Sys.sigterm;
+         r)
   in
   ( res
   , match outcome with

test/blackbox-tests/test-cases/cram/subprocess.t

@@ -0,0 +1,29 @@
+Testing the termination of subprocesses in cram tests. We first create a dune
+project with a single cram test.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.19)
+  > EOF
+
+We create a file for tracking the PID of a subprocess.
+
+  $ cat > pid.txt
+  $ export FILE=$(realpath ./pid.txt)
+
+We create a cram test that spawns a subprocess and records its PID in the file
+we gave before.
+
+  $ cat > mycram.t <<EOF
+  >   $ sleep 5 &
+  >   $ echo \$! > $FILE
+  > EOF
+
+We can now run this test, which will record its PID in the file.
+
+  $ dune build @mycram
+
+The test finished successfully, now we make sure that the PID was correctly
+terminated.
+
+  $ ps -p $(cat $FILE) > /dev/null || echo "Process terminated"
+  Process terminated