Log more details around child process start and stop (#94)

By logging the PID, it's easier to tell sessions apart.

The log level has been reduced to log_debug,
since there are log_info messages that better convey the state transitions in the applications.

Exit code and signal number are no longer mixed up in the log.
The new log messages look like this:

* killall -SIGINT  dockerd → "exited with exit code 0"
* killall -SIGTERM dockerd → "exited with exit code 0"
* killall -SIGABRT dockerd → "exited with exit code 2"
* killall -SIGKILL dockerd → "was killed by signal 9"

Co-authored-by: Mattias Axelsson <mattiaax@axis.com>

app/dockerdwrapperwithcompose.c

@@ -592,6 +592,7 @@ static bool start_dockerd(const struct settings* settings, struct app_state* app
         set_status_parameter(STATUS_NOT_STARTED);
         goto end;
     }
+    log_debug("Child process dockerd (%d) was started.", dockerd_process_pid);
 
     // Watch the child process.
     g_child_watch_add(dockerd_process_pid, dockerd_process_exited_callback, app_state);
@@ -633,7 +634,7 @@ static bool stop_dockerd(void) {
         goto end;
     }
 
-    log_info("Sending SIGTERM to dockerd.");
+    log_debug("Sending SIGTERM to dockerd (%d).", dockerd_process_pid);
     bool sigterm_successfully_sent = kill(dockerd_process_pid, SIGTERM) == 0;
     if (!sigterm_successfully_sent) {
         log_error("Failed to send SIGTERM to child. Error: %s", strerror(errno));
@@ -651,25 +652,39 @@ static bool stop_dockerd(void) {
         goto end;
     }
 
-    // SIGTERM failed, let's try SIGKILL
+    log_debug("Sending SIGKILL to dockerd (%d).", dockerd_process_pid);
     killed = kill(dockerd_process_pid, SIGKILL) == 0;
     if (!killed)
         log_error("Failed to send SIGKILL to child. Error: %s", strerror(errno));
 
+    log_info("Stopped dockerd.");
 end:
     return killed;
 }
 
+static void log_child_process_exit_cause(const char* name, GPid pid, int status) {
+    GError* error = NULL;
+    char msg[128];
+    const char* end = msg + sizeof(msg);
+    char* ptr = msg + g_snprintf(msg, end - msg, "Child process %s (%d)", name, pid);
+
+    if (g_spawn_check_wait_status(status, &error) || error->domain == G_SPAWN_EXIT_ERROR)
+        g_snprintf(ptr, end - ptr, " exited with exit code %d", error ? error->code : 0);
+    else if (error->domain == G_SPAWN_ERROR && error->code == G_SPAWN_ERROR_FAILED)
+        g_snprintf(ptr, end - ptr, " was killed by signal %d", status);
+    else
+        g_snprintf(ptr, end - ptr, " terminated in an unexpected way: %s", error->message);
+    g_clear_error(&error);
+    log_debug("%s", msg);
+}
+
 /**
  * @brief Callback called when the dockerd process exits.
  */
 static void dockerd_process_exited_callback(GPid pid, gint status, gpointer app_state_void_ptr) {
+    log_child_process_exit_cause("dockerd", pid, status);
+
     struct app_state* app_state = app_state_void_ptr;
-    GError* error = NULL;
-    if (!g_spawn_check_wait_status(status, &error)) {
-        log_error("Dockerd process exited with error: %d", status);
-        g_clear_error(&error);
-    }
 
     dockerd_process_pid = -1;
     g_spawn_close_pid(pid);