test(utils): add SleeperThread reassignment regression

We needed a regression test for reassignment safety after lifecycle fixes.

I added a subprocess test that reassigns SleeperThread workers and verifies the
process exits normally instead of terminating.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

test/utils/meson.build

@@ -13,6 +13,7 @@ test_src = files(
     '../../src/config.cpp',
     'JsonParser.cpp',
     'SafeSignal.cpp',
+    'sleeper_thread.cpp',
     'css_reload_helper.cpp',
     '../../src/util/css_reload_helper.cpp',
 )

test/utils/sleeper_thread.cpp

@@ -0,0 +1,42 @@
+#if __has_include(<catch2/catch_test_macros.hpp>)
+#include <catch2/catch_test_macros.hpp>
+#else
+#include <catch2/catch.hpp>
+#endif
+
+#include <chrono>
+#include <sys/wait.h>
+#include <thread>
+#include <unistd.h>
+
+#include "util/sleeper_thread.hpp"
+
+namespace waybar::util {
+SafeSignal<bool>& prepare_for_sleep() {
+  static SafeSignal<bool> signal;
+  return signal;
+}
+}  // namespace waybar::util
+
+namespace {
+int run_reassignment_regression() {
+  waybar::util::SleeperThread thread;
+  thread = [] { std::this_thread::sleep_for(std::chrono::milliseconds(10)); };
+  thread = [] { std::this_thread::sleep_for(std::chrono::milliseconds(1)); };
+  return 0;
+}
+}  // namespace
+
+TEST_CASE("SleeperThread reassignment does not terminate process", "[util][sleeper_thread]") {
+  const auto pid = fork();
+  REQUIRE(pid >= 0);
+
+  if (pid == 0) {
+    _exit(run_reassignment_regression());
+  }
+
+  int status = -1;
+  REQUIRE(waitpid(pid, &status, 0) == pid);
+  REQUIRE(WIFEXITED(status));
+  REQUIRE(WEXITSTATUS(status) == 0);
+}