Introduce process observer

Add a tool to easily observe process termination.

This allows to move this complexity out of the server code.

app/src/util/process.c

@@ -1,5 +1,6 @@
 #include "process.h"
 
+#include <assert.h>
 #include <libgen.h>
 #include "log.h"
 
@@ -41,3 +42,80 @@ sc_pipe_read_all(sc_pipe pipe, char *data, size_t len) {
     }
     return copied;
 }
+
+static int
+run_observer(void *data) {
+    struct sc_process_observer *observer = data;
+    sc_process_wait(observer->pid, false); // ignore exit code
+
+    sc_mutex_lock(&observer->mutex);
+    observer->terminated = true;
+    sc_cond_signal(&observer->cond_terminated);
+    sc_mutex_unlock(&observer->mutex);
+
+    if (observer->listener) {
+        observer->listener->on_terminated(observer->listener_userdata);
+    }
+
+    return 0;
+}
+
+bool
+sc_process_observer_init(struct sc_process_observer *observer, sc_pid pid,
+                         const struct sc_process_listener *listener,
+                         void *listener_userdata) {
+    // Either no listener, or on_terminated() is defined
+    assert(!listener || listener->on_terminated);
+
+    bool ok = sc_mutex_init(&observer->mutex);
+    if (!ok) {
+        return false;
+    }
+
+    ok = sc_cond_init(&observer->cond_terminated);
+    if (!ok) {
+        sc_mutex_destroy(&observer->mutex);
+        return false;
+    }
+
+    observer->pid = pid;
+    observer->listener = listener;
+    observer->listener_userdata = listener_userdata;
+    observer->terminated = false;
+
+    ok = sc_thread_create(&observer->thread, run_observer, "process_observer",
+                          observer);
+    if (!ok) {
+        sc_cond_destroy(&observer->cond_terminated);
+        sc_mutex_destroy(&observer->mutex);
+        return false;
+    }
+
+    return true;
+}
+
+bool
+sc_process_observer_timedwait(struct sc_process_observer *observer,
+                              sc_tick deadline) {
+    sc_mutex_lock(&observer->mutex);
+    bool timed_out = false;
+    while (!observer->terminated && !timed_out) {
+        timed_out = !sc_cond_timedwait(&observer->cond_terminated,
+                                       &observer->mutex, deadline);
+    }
+    bool terminated = observer->terminated;
+    sc_mutex_unlock(&observer->mutex);
+
+    return terminated;
+}
+
+void
+sc_process_observer_join(struct sc_process_observer *observer) {
+    sc_thread_join(&observer->thread, NULL);
+}
+
+void
+sc_process_observer_destroy(struct sc_process_observer *observer) {
+    sc_cond_destroy(&observer->cond_terminated);
+    sc_mutex_destroy(&observer->mutex);
+}