manager: rework error handling and logging in manager_reload()

let's clean up error handling and logging in manager_reload() a bit.
Specifically: make sure we log about every error we might encounter at
least and at most once.

When we encounter an error before the "point of no return" then log at
LOG_ERR about it and propagate it. Otherwise, eat it up, but warn about
it and proceed, it's the best we can do.

src/core/main.c

@@ -1698,10 +1698,7 @@ static int invoke_main_loop(
                         if (saved_log_target >= 0)
                                 manager_override_log_target(m, saved_log_target);
 
-                        r = manager_reload(m);
-                        if (r < 0)
-                                log_warning_errno(r, "Failed to reload, ignoring: %m");
-
+                        (void) manager_reload(m);
                         break;
                 }
 

src/core/manager.c

@@ -3453,37 +3453,42 @@ static void manager_flush_finished_jobs(Manager *m) {
 }
 
 int manager_reload(Manager *m) {
-        int r, q;
-        _cleanup_fclose_ FILE *f = NULL;
         _cleanup_fdset_free_ FDSet *fds = NULL;
+        _cleanup_fclose_ FILE *f = NULL;
+        int r;
 
         assert(m);
 
         r = manager_open_serialization(m, &f);
         if (r < 0)
-                return r;
-
-        m->n_reloading++;
-        bus_manager_send_reloading(m, true);
+                return log_error_errno(r, "Failed to create serialization file: %m");
 
         fds = fdset_new();
-        if (!fds) {
-                m->n_reloading--;
-                return -ENOMEM;
-        }
+        if (!fds)
+                return log_oom();
+
+        /* We are officially in reload mode from here on */
+        m->n_reloading++;
 
         r = manager_serialize(m, f, fds, false);
         if (r < 0) {
-                m->n_reloading--;
-                return r;
+                log_error_errno(r, "Failed to serialize manager: %m");
+                goto fail;
         }
 
         if (fseeko(f, 0, SEEK_SET) < 0) {
-                m->n_reloading--;
-                return -errno;
+                r = log_error_errno(errno, "Failed to seek to beginning of serialization: %m");
+                goto fail;
         }
 
-        /* From here on there is no way back. */
+        /* 💀 This is the point of no return, from here on there is no way back. 💀 */
+
+        bus_manager_send_reloading(m, true);
+
+        /* Start by flushing out all jobs and units, all generated units, all runtime environments, all dynamic users
+         * and everything else that is worth flushing out. We'll get it all back from the serialization — if we need
+         * it.*/
+
         manager_clear_jobs_and_units(m);
         lookup_paths_flush_generator(&m->lookup_paths);
         lookup_paths_free(&m->lookup_paths);
@@ -3493,45 +3498,33 @@ int manager_reload(Manager *m) {
         m->gid_refs = hashmap_free(m->gid_refs);
 
         r = lookup_paths_init(&m->lookup_paths, m->unit_file_scope, 0, NULL);
+        if (r < 0)
+                log_warning_errno(r, "Failed to initialize path lookup table, ignoring: %m");
 
-        q = manager_run_environment_generators(m);
-        if (q < 0 && r >= 0)
-                r = q;
+        (void) manager_run_environment_generators(m);
+        (void) manager_run_generators(m);
 
-        /* Find new unit paths */
-        q = manager_run_generators(m);
-        if (q < 0 && r >= 0)
-                r = q;
+        r = lookup_paths_reduce(&m->lookup_paths);
+        if (r < 0)
+                log_warning_errno(r, "Failed ot reduce unit file paths, ignoring: %m");
 
-        lookup_paths_reduce(&m->lookup_paths);
         manager_build_unit_path_cache(m);
 
-        /* First, enumerate what we can from all config files */
+        /* First, enumerate what we can from kernel and suchlike */
         manager_enumerate(m);
 
         /* Second, deserialize our stored data */
-        q = manager_deserialize(m, f, fds);
-        if (q < 0) {
-                log_error_errno(q, "Deserialization failed: %m");
-
-                if (r >= 0)
-                        r = q;
-        }
+        r = manager_deserialize(m, f, fds);
+        if (r < 0)
+                log_warning_errno(r, "Deserialization failed, proceeding anyway: %m");
 
+        /* We don't need the serialization anymore */
         f = safe_fclose(f);
 
-        /* Re-register notify_fd as event source */
-        q = manager_setup_notify(m);
-        if (q < 0 && r >= 0)
-                r = q;
-
-        q = manager_setup_cgroups_agent(m);
-        if (q < 0 && r >= 0)
-                r = q;
-
-        q = manager_setup_user_lookup_fd(m);
-        if (q < 0 && r >= 0)
-                r = q;
+        /* Re-register notify_fd as event source, and set up other sockets/communication channels we might need */
+        (void) manager_setup_notify(m);
+        (void) manager_setup_cgroups_agent(m);
+        (void) manager_setup_user_lookup_fd(m);
 
         /* Third, fire things up! */
         manager_coldplug(m);
@@ -3545,6 +3538,7 @@ int manager_reload(Manager *m) {
 
         exec_runtime_vacuum(m);
 
+        /* Consider the reload process complete now. */
         assert(m->n_reloading > 0);
         m->n_reloading--;
 
@@ -3556,14 +3550,19 @@ int manager_reload(Manager *m) {
         manager_catchup(m);
 
         /* Sync current state of bus names with our set of listening units */
-        q = manager_enqueue_sync_bus_names(m);
-        if (q < 0 && r >= 0)
-                r = q;
+        (void) manager_enqueue_sync_bus_names(m);
 
         if (!MANAGER_IS_RELOADING(m))
                 manager_flush_finished_jobs(m);
 
         m->send_reloading_done = true;
+        return 0;
+
+fail:
+        /* Fail the call. Note that we hit this only if we fail before the point of no return, i.e. when the error is
+         * still something that can be handled. */
+        assert(m->n_reloading > 0);
+        m->n_reloading--;
 
         return r;
 }