fix(script): Prevent stalling during shutdown

If a script module is waiting on a non-tailed script to finish, polybar
cannot shut down until the script has terminted.
To remedy this, we use the same polling loop as for tailed scripts which
checks if it needs to terminate after every iteration.

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - `tray-offset-x`, `tray-offset-y`, `offset-x`, and `offset-y` were mistakenly capped below at 0 ([`#2620`](https://github.com/polybar/polybar/pull/2620))
+- `custom/script`: Polybar shutdown being stalled by hanging script ([`#2621`](https://github.com/polybar/polybar/pull/2621))
 
 ## [3.6.0] - 2022-03-01
 ### Breaking

src/adapters/script_runner.cpp

@@ -96,16 +96,35 @@ script_runner::interval script_runner::run() {
   auto cmd = command_util::make_command<output_policy::REDIRECTED>(exec);
 
   try {
-    cmd->exec(true, m_env);
+    cmd->exec(false, m_env);
   } catch (const exception& err) {
     m_log.err("script_runner: %s", err.what());
     throw modules::module_error("Failed to execute command, stopping module...");
   }
 
-  m_exit_status = cmd->get_exit_status();
   int fd = cmd->get_stdout(PIPE_READ);
   assert(fd != -1);
-  bool changed = io_util::poll_read(fd) && set_output(cmd->readline());
+
+  bool changed = false;
+
+  bool got_output = false;
+  while (!m_stopping && cmd->is_running() && !io_util::poll(fd, POLLHUP, 0)) {
+    /**
+     * For non-tailed scripts, we only use the first line. However, to ensure interruptability when the module shuts
+     * down, we still need to continue polling.
+     */
+    if (io_util::poll_read(fd, 25) && !got_output) {
+      changed = set_output(cmd->readline());
+      got_output = true;
+    }
+  }
+
+  if (m_stopping) {
+    cmd->terminate();
+    return 0s;
+  }
+
+  m_exit_status = cmd->wait();
 
   if (!changed && m_exit_status != 0) {
     clear_output();