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);
+}

app/src/util/process.h

@@ -4,6 +4,7 @@
 #include "common.h"
 
 #include <stdbool.h>
+#include "util/thread.h"
 
 #ifdef _WIN32
 
@@ -32,6 +33,34 @@
 
 #endif
 
+struct sc_process_listener {
+    void (*on_terminated)(void *userdata);
+};
+
+/**
+ * Tool to observe process termination
+ *
+ * To keep things simple and multiplatform, it runs a separate thread to wait
+ * for process termination (without closing the process to avoid race
+ * conditions).
+ *
+ * It allows a caller to block until the process is terminated (with a
+ * timeout), and to be notified asynchronously from the observer thread.
+ *
+ * The process is not owned by the observer (the observer will never close it).
+ */
+struct sc_process_observer {
+    sc_pid pid;
+
+    sc_mutex mutex;
+    sc_cond cond_terminated;
+    bool terminated;
+
+    sc_thread thread;
+    const struct sc_process_listener *listener;
+    void *listener_userdata;
+};
+
 enum sc_process_result {
     SC_PROCESS_SUCCESS,
     SC_PROCESS_ERROR_GENERIC,
@@ -106,4 +135,42 @@ sc_pipe_read_all(sc_pipe pipe, char *data, size_t len);
 void
 sc_pipe_close(sc_pipe pipe);
 
+/**
+ * Start observing process
+ *
+ * The listener is optional. If set, its callback will be called from the
+ * observer thread once the process is terminated.
+ */
+bool
+sc_process_observer_init(struct sc_process_observer *observer, sc_pid pid,
+                         const struct sc_process_listener *listener,
+                         void *listener_userdata);
+
+/**
+ * Wait for process termination until a deadline
+ *
+ * Return true if the process is already terminated. Return false if the
+ * process terminatation has not been detected yet (however, it may have
+ * terminated in the meantime).
+ *
+ * To wait without timeout/deadline, just use sc_process_wait() instead.
+ */
+bool
+sc_process_observer_timedwait(struct sc_process_observer *observer,
+                              sc_tick deadline);
+
+/**
+ * Join the observer thread
+ */
+void
+sc_process_observer_join(struct sc_process_observer *observer);
+
+/**
+ * Destroy the observer
+ *
+ * This does not close the associated process.
+ */
+void
+sc_process_observer_destroy(struct sc_process_observer *observer);
+
 #endif