daemon: Exit on idle after txn

So...we have some bad memory leaks somewhere. We go from 14MB RSS after being
activated from `atomic host status` to 54MB after a dummy `rpm-ostree install
foo`, 80MB for a successful install, and I've seen it as high as 140MB.

Exiting on idle is what PackageKit has done forever. One problem particularly
for both of us is that libdnf and all of the libraries underneath it are used
almost exclusively by command line clients that exit when done, so memory leaks
aren't particularly important to them.

This may break Cockpit though, since it won't be able to monitor for sysroot
changes. I considered instead running txns in a subprocess, which is possible
but harder.

src/daemon/rpmostreed-daemon.c

@@ -24,6 +24,8 @@
 #include "rpmostreed-utils.h"
 
 #include <libglnx.h>
+#include <stdio.h>
+#include <systemd/sd-journal.h>
 
 /**
  * SECTION: daemon
@@ -50,6 +52,8 @@ struct _RpmostreedDaemon {
   gint64 last_message;
   guint ticker_id;
 
+  GSource *idle_exit_source;
+
   GMutex mutex;
   gint num_tasks;
 
@@ -171,6 +175,54 @@ rpmostreed_daemon_init (RpmostreedDaemon *self)
   g_mutex_init (&self->mutex);
 }
 
+static gboolean
+on_idle_exit (gpointer user_data)
+{
+  sd_journal_print (LOG_INFO, "Exiting due to idle");
+  exit (0);
+}
+
+static void
+on_active_txn_changed (GObject *object,
+                       GParamSpec *pspec,
+                       gpointer user_data)
+{
+  RpmostreedDaemon *self = user_data;
+  g_autoptr(GVariant) active_txn = NULL;
+  gboolean have_active_txn = FALSE;
+
+  g_object_get (object, "active-transaction", &active_txn, NULL);
+
+  if (active_txn)
+    {
+      const char *a, *b, *c;
+      g_variant_get (active_txn, "(&s&s&s)", &a, &b, &c);
+      if (*a)
+        have_active_txn = TRUE;
+    }
+
+  if (have_active_txn)
+    {
+      if (self->idle_exit_source)
+        {
+          g_source_destroy (self->idle_exit_source);
+          g_clear_pointer (&self->idle_exit_source, (GDestroyNotify)g_source_unref);
+        }
+    }
+  else
+    {
+      if (!self->idle_exit_source)
+        {
+          /* I think adding some randomness is a good idea, to mitigate
+           * pathological cases where someone is talking to us at the same
+           * frequency as our exit timer. */
+          self->idle_exit_source = g_timeout_source_new_seconds (10 + g_random_int_range (0, 5));
+          g_source_set_callback (self->idle_exit_source, on_idle_exit, self, NULL);
+          g_source_attach (self->idle_exit_source, NULL);
+        }
+    }
+}
+
 static gboolean
 rpmostreed_daemon_initable_init (GInitable *initable,
                                  GCancellable *cancellable,
@@ -196,6 +248,9 @@ rpmostreed_daemon_initable_init (GInitable *initable,
       goto out;
     }
 
+  g_signal_connect (rpmostreed_sysroot_get (), "notify::active-transaction",
+                    G_CALLBACK (on_active_txn_changed), self);
+
   rpmostreed_daemon_publish (self, path, FALSE, self->sysroot);
   g_dbus_connection_start_message_processing (self->connection);