Introduce yank feature

The yank feature allows to recover from hanging qemu by "yanking"
at various parts. Other qemu systems can register themselves and
multiple yank functions. Then all yank functions for selected
instances can be called by the 'yank' out-of-band qmp command.
Available instances can be queried by a 'query-yank' oob command.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <69934ceacfd33a7dfe53db145ecc630ad39ee47c.1609167865.git.lukasstraub2@web.de>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

2 files changed, 208 insertions, 0 deletions

diff --git a/util/meson.build b/util/meson.build
index a3dfc0f966..540a605b78 100644
--- a/util/meson.build
+++ b/util/meson.build
@@ -50,6 +50,7 @@ endif
 
 if have_system
   util_ss.add(when: 'CONFIG_GIO', if_true: [files('dbus.c'), gio])
+  util_ss.add(files('yank.c'))
 endif
 
 if have_block
diff --git a/util/yank.c b/util/yank.c
new file mode 100644
index 0000000000..fc08f65209
--- /dev/null
+++ b/util/yank.c
@@ -0,0 +1,207 @@
+/*
+ * QEMU yank feature
+ *
+ * Copyright (c) Lukas Straub <lukasstraub2@web.de>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+#include "qapi/error.h"
+#include "qemu/thread.h"
+#include "qemu/queue.h"
+#include "qemu/lockable.h"
+#include "qapi/qapi-commands-yank.h"
+#include "qapi/qapi-visit-yank.h"
+#include "qapi/clone-visitor.h"
+#include "io/channel.h"
+#include "qemu/yank.h"
+
+struct YankFuncAndParam {
+    YankFn *func;
+    void *opaque;
+    QLIST_ENTRY(YankFuncAndParam) next;
+};
+
+struct YankInstanceEntry {
+    YankInstance *instance;
+    QLIST_HEAD(, YankFuncAndParam) yankfns;
+    QLIST_ENTRY(YankInstanceEntry) next;
+};
+
+typedef struct YankFuncAndParam YankFuncAndParam;
+typedef struct YankInstanceEntry YankInstanceEntry;
+
+/*
+ * This lock protects the yank_instance_list below. Because it's taken by
+ * OOB-capable commands, it must be "fast", i.e. it may only be held for a
+ * bounded, short time. See docs/devel/qapi-code-gen.txt for additional
+ * information.
+ */
+static QemuMutex yank_lock;
+
+static QLIST_HEAD(, YankInstanceEntry) yank_instance_list
+    = QLIST_HEAD_INITIALIZER(yank_instance_list);
+
+static bool yank_instance_equal(const YankInstance *a, const YankInstance *b)
+{
+    if (a->type != b->type) {
+        return false;
+    }
+
+    switch (a->type) {
+    case YANK_INSTANCE_TYPE_BLOCK_NODE:
+        return g_str_equal(a->u.block_node.node_name,
+                           b->u.block_node.node_name);
+
+    case YANK_INSTANCE_TYPE_CHARDEV:
+        return g_str_equal(a->u.chardev.id, b->u.chardev.id);
+
+    case YANK_INSTANCE_TYPE_MIGRATION:
+        return true;
+
+    default:
+        abort();
+    }
+}
+
+static YankInstanceEntry *yank_find_entry(const YankInstance *instance)
+{
+    YankInstanceEntry *entry;
+
+    QLIST_FOREACH(entry, &yank_instance_list, next) {
+        if (yank_instance_equal(entry->instance, instance)) {
+            return entry;
+        }
+    }
+    return NULL;
+}
+
+bool yank_register_instance(const YankInstance *instance, Error **errp)
+{
+    YankInstanceEntry *entry;
+
+    QEMU_LOCK_GUARD(&yank_lock);
+
+    if (yank_find_entry(instance)) {
+        error_setg(errp, "duplicate yank instance");
+        return false;
+    }
+
+    entry = g_new0(YankInstanceEntry, 1);
+    entry->instance = QAPI_CLONE(YankInstance, instance);
+    QLIST_INIT(&entry->yankfns);
+    QLIST_INSERT_HEAD(&yank_instance_list, entry, next);
+
+    return true;
+}
+
+void yank_unregister_instance(const YankInstance *instance)
+{
+    YankInstanceEntry *entry;
+
+    QEMU_LOCK_GUARD(&yank_lock);
+    entry = yank_find_entry(instance);
+    assert(entry);
+
+    assert(QLIST_EMPTY(&entry->yankfns));
+    QLIST_REMOVE(entry, next);
+    qapi_free_YankInstance(entry->instance);
+    g_free(entry);
+}
+
+void yank_register_function(const YankInstance *instance,
+                            YankFn *func,
+                            void *opaque)
+{
+    YankInstanceEntry *entry;
+    YankFuncAndParam *func_entry;
+
+    QEMU_LOCK_GUARD(&yank_lock);
+    entry = yank_find_entry(instance);
+    assert(entry);
+
+    func_entry = g_new0(YankFuncAndParam, 1);
+    func_entry->func = func;
+    func_entry->opaque = opaque;
+
+    QLIST_INSERT_HEAD(&entry->yankfns, func_entry, next);
+}
+
+void yank_unregister_function(const YankInstance *instance,
+                              YankFn *func,
+                              void *opaque)
+{
+    YankInstanceEntry *entry;
+    YankFuncAndParam *func_entry;
+
+    QEMU_LOCK_GUARD(&yank_lock);
+    entry = yank_find_entry(instance);
+    assert(entry);
+
+    QLIST_FOREACH(func_entry, &entry->yankfns, next) {
+        if (func_entry->func == func && func_entry->opaque == opaque) {
+            QLIST_REMOVE(func_entry, next);
+            g_free(func_entry);
+            return;
+        }
+    }
+
+    abort();
+}
+
+void yank_generic_iochannel(void *opaque)
+{
+    QIOChannel *ioc = QIO_CHANNEL(opaque);
+
+    qio_channel_shutdown(ioc, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+}
+
+void qmp_yank(YankInstanceList *instances,
+              Error **errp)
+{
+    YankInstanceList *tail;
+    YankInstanceEntry *entry;
+    YankFuncAndParam *func_entry;
+
+    QEMU_LOCK_GUARD(&yank_lock);
+    for (tail = instances; tail; tail = tail->next) {
+        entry = yank_find_entry(tail->value);
+        if (!entry) {
+            error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND, "Instance not found");
+            return;
+        }
+    }
+    for (tail = instances; tail; tail = tail->next) {
+        entry = yank_find_entry(tail->value);
+        assert(entry);
+        QLIST_FOREACH(func_entry, &entry->yankfns, next) {
+            func_entry->func(func_entry->opaque);
+        }
+    }
+}
+
+YankInstanceList *qmp_query_yank(Error **errp)
+{
+    YankInstanceEntry *entry;
+    YankInstanceList *ret;
+
+    ret = NULL;
+
+    QEMU_LOCK_GUARD(&yank_lock);
+    QLIST_FOREACH(entry, &yank_instance_list, next) {
+        YankInstanceList *new_entry;
+        new_entry = g_new0(YankInstanceList, 1);
+        new_entry->value = QAPI_CLONE(YankInstance, entry->instance);
+        new_entry->next = ret;
+        ret = new_entry;
+    }
+
+    return ret;
+}
+
+static void __attribute__((__constructor__)) yank_init(void)
+{
+    qemu_mutex_init(&yank_lock);
+}