Add a tricky test to verify disowning an in-flight job

There's some logic in fish to prevent blowing up when an under-construction
job is disowned. Add a test for it.

src/fish_test_helper.cpp

@@ -46,6 +46,12 @@ static void print_stdout_stderr() {
     fflush(nullptr);
 }
 
+static void print_pid_then_sleep() {
+    fprintf(stdout, "%d\n", getpid());
+    fflush(nullptr);
+    usleep(1000000 / .5);  //.5 secs
+}
+
 int main(int argc, char *argv[]) {
     if (argc <= 1) {
         fprintf(stderr, "No commands given.\n");
@@ -60,6 +66,8 @@ int main(int argc, char *argv[]) {
             sigint_parent();
         } else if (!strcmp(argv[i], "print_stdout_stderr")) {
             print_stdout_stderr();
+        } else if (!strcmp(argv[i], "print_pid_then_sleep")) {
+            print_pid_then_sleep();
         } else {
             fprintf(stderr, "%s: Unknown command: %s\n", argv[0], argv[i]);
             return EXIT_FAILURE;

tests/checks/disown-parent.fish

@@ -0,0 +1,14 @@
+# RUN: env fish_test_helper=%fish_test_helper %fish %s
+
+# Ensure that a job which attempts to disown itself does not explode.
+# Here fish_test_helper is the process group leader; we attempt to disown
+# its pid within a pipeline containing it.
+
+function disowner
+    read -l pid
+    echo Disown $pid
+    disown $pid
+end
+$fish_test_helper print_pid_then_sleep | disowner
+
+# CHECK: Disown {{\d+}}