src: introduce an abstraction for running event loops

We want a way to easily run a private GMainContext in a
thread, with correct synchronization between startup
and shutdown of the thread.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

po/POTFILES.in

@@ -241,6 +241,7 @@
 @SRCDIR@/src/util/virerror.c
 @SRCDIR@/src/util/virerror.h
 @SRCDIR@/src/util/virevent.c
+@SRCDIR@/src/util/vireventthread.c
 @SRCDIR@/src/util/virfcp.c
 @SRCDIR@/src/util/virfdstream.c
 @SRCDIR@/src/util/virfile.c

src/libvirt_private.syms

@@ -1989,6 +1989,11 @@ virEventGLibRegister;
 virEventGLibRunOnce;
 
 
+# util/vireventthread.h
+virEventThreadGetContext;
+virEventThreadNew;
+
+
 # util/virfcp.h
 virFCIsCapableRport;
 virFCReadRportValue;

src/util/Makefile.inc.am

@@ -63,6 +63,8 @@ UTIL_SOURCES = \
 	util/vireventglib.h \
 	util/vireventglibwatch.c \
 	util/vireventglibwatch.h \
+	util/vireventthread.c \
+	util/vireventthread.h \
 	util/virfcp.c \
 	util/virfcp.h \
 	util/virfdstream.c \

src/util/vireventthread.c

@@ -0,0 +1,190 @@
+/*
+ * vireventthread.c: thread running a dedicated GMainLoop
+ *
+ * Copyright (C) 2020 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see
+ * <http://www.gnu.org/licenses/>.
+ */
+
+#include <config.h>
+
+#include "vireventthread.h"
+#include "virthread.h"
+#include "virerror.h"
+
+struct _virEventThread {
+    GObject parent;
+
+    GThread *thread;
+    GMainContext *context;
+    GMainLoop *loop;
+};
+
+G_DEFINE_TYPE(virEventThread, vir_event_thread, G_TYPE_OBJECT)
+
+#define VIR_FROM_THIS VIR_FROM_EVENT
+
+static void
+vir_event_thread_finalize(GObject *object)
+{
+    virEventThread *evt = VIR_EVENT_THREAD(object);
+
+    if (evt->thread) {
+        g_main_loop_quit(evt->loop);
+        g_thread_unref(evt->thread);
+    }
+
+    g_main_loop_unref(evt->loop);
+    g_main_context_unref(evt->context);
+
+    G_OBJECT_CLASS(vir_event_thread_parent_class)->finalize(object);
+}
+
+
+static void
+vir_event_thread_init(virEventThread *evt)
+{
+    evt->context = g_main_context_new();
+    evt->loop = g_main_loop_new(evt->context, FALSE);
+}
+
+
+static void
+vir_event_thread_class_init(virEventThreadClass *klass)
+{
+    GObjectClass *obj = G_OBJECT_CLASS(klass);
+
+    obj->finalize = vir_event_thread_finalize;
+}
+
+
+typedef struct {
+    GCond cond;
+    GMutex lock;
+    bool running;
+
+    GMainContext *context;
+    GMainLoop *loop;
+} virEventThreadData;
+
+
+static void
+virEventThreadDataFree(virEventThreadData *data)
+{
+    g_main_loop_unref(data->loop);
+    g_main_context_unref(data->context);
+
+    g_mutex_clear(&data->lock);
+    g_cond_clear(&data->cond);
+
+    g_free(data);
+}
+
+
+static gboolean
+virEventThreadNotify(void *opaque)
+{
+    virEventThreadData *data = opaque;
+
+    g_mutex_lock(&data->lock);
+    data->running = TRUE;
+    g_mutex_unlock(&data->lock);
+    g_cond_signal(&data->cond);
+
+    return G_SOURCE_REMOVE;
+}
+
+
+static void *
+virEventThreadWorker(void *opaque)
+{
+    virEventThreadData *data = opaque;
+    g_autoptr(GSource) running = g_idle_source_new();
+
+    g_source_set_callback(running, virEventThreadNotify, data, NULL);
+
+    g_source_attach(running, data->context);
+
+    g_main_loop_run(data->loop);
+
+    virEventThreadDataFree(data);
+
+    return NULL;
+}
+
+
+static int
+virEventThreadStart(virEventThread *evt, const char *name)
+{
+    g_autoptr(GError) gerr = NULL;
+    g_autofree char *thname = NULL;
+    size_t maxname = virThreadMaxName();
+    virEventThreadData *data;
+
+    if (maxname)
+        thname = g_strndup(name, maxname);
+    else
+        thname = g_strdup(name);
+
+    if (evt->thread) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("Event thread is already running"));
+        return -1;
+    }
+
+    data = g_new0(virEventThreadData, 1);
+    data->loop = g_main_loop_ref(evt->loop);
+    data->context = g_main_context_ref(evt->context);
+    g_mutex_init(&data->lock);
+    g_cond_init(&data->cond);
+
+    evt->thread = g_thread_try_new(thname,
+                                   virEventThreadWorker,
+                                   data,
+                                   &gerr);
+    if (!evt->thread) {
+        virEventThreadDataFree(data);
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("Unable to start event thread: %s"),
+                       gerr->message);
+        return -1;
+    }
+
+    g_mutex_lock(&data->lock);
+    while (!data->running)
+        g_cond_wait(&data->cond, &data->lock);
+    g_mutex_unlock(&data->lock);
+
+    return 0;
+}
+
+
+virEventThread *
+virEventThreadNew(const char *name)
+{
+    g_autoptr(virEventThread) evt = VIR_EVENT_THREAD(g_object_new(VIR_TYPE_EVENT_THREAD, NULL));
+
+    if (virEventThreadStart(evt, name) < 0)
+        return NULL;
+
+    return g_steal_pointer(&evt);
+}
+
+
+GMainContext *
+virEventThreadGetContext(virEventThread *evt)
+{
+    return evt->context;
+}

src/util/vireventthread.h

@@ -0,0 +1,31 @@
+/*
+ * vireventthread.h: thread running a dedicated GMainLoop
+ *
+ * Copyright (C) 2020 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see
+ * <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "internal.h"
+#include <glib-object.h>
+
+#define VIR_TYPE_EVENT_THREAD vir_event_thread_get_type()
+G_DECLARE_FINAL_TYPE(virEventThread, vir_event_thread, VIR, EVENT_THREAD, GObject);
+
+virEventThread *virEventThreadNew(const char *name);
+
+GMainContext *virEventThreadGetContext(virEventThread *evt);