char: chardevice hotswap

This patch adds a possibility to change a char device without a frontend
removal.

Ideally, it would have to happen transparently to a frontend, i.e.
frontend would continue its regular operation.
However, backends are not stateless and are set up by the frontends
via qemu_chr_fe_<> functions, and it's not (generally) possible to replay
that setup entirely in a backend code, as different chardevs respond
to the setup calls differently, so do frontends work differently basing
on those setup responses.
Moreover, some frontend can generally get and save the backend pointer
(qemu_chr_fe_get_driver()), and it will become invalid after backend change.

So, a frontend which would like to support chardev hotswap has to register
a "backend change" handler, and redo its backend setup there.

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <1499342940-56739-4-git-send-email-anton.nefedov@virtuozzo.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

chardev/char.c

@@ -951,6 +951,89 @@ ChardevReturn *qmp_chardev_add(const char *id, ChardevBackend *backend,
     return ret;
 }
 
+ChardevReturn *qmp_chardev_change(const char *id, ChardevBackend *backend,
+                                  Error **errp)
+{
+    CharBackend *be;
+    const ChardevClass *cc;
+    Chardev *chr, *chr_new;
+    bool closed_sent = false;
+    ChardevReturn *ret;
+
+    chr = qemu_chr_find(id);
+    if (!chr) {
+        error_setg(errp, "Chardev '%s' does not exist", id);
+        return NULL;
+    }
+
+    if (CHARDEV_IS_MUX(chr)) {
+        error_setg(errp, "Mux device hotswap not supported yet");
+        return NULL;
+    }
+
+    if (qemu_chr_replay(chr)) {
+        error_setg(errp,
+            "Chardev '%s' cannot be changed in record/replay mode", id);
+        return NULL;
+    }
+
+    be = chr->be;
+    if (!be) {
+        /* easy case */
+        object_unparent(OBJECT(chr));
+        return qmp_chardev_add(id, backend, errp);
+    }
+
+    if (!be->chr_be_change) {
+        error_setg(errp, "Chardev user does not support chardev hotswap");
+        return NULL;
+    }
+
+    cc = char_get_class(ChardevBackendKind_lookup[backend->type], errp);
+    if (!cc) {
+        return NULL;
+    }
+
+    chr_new = qemu_chardev_new(NULL, object_class_get_name(OBJECT_CLASS(cc)),
+                               backend, errp);
+    if (!chr_new) {
+        return NULL;
+    }
+    chr_new->label = g_strdup(id);
+
+    if (chr->be_open && !chr_new->be_open) {
+        qemu_chr_be_event(chr, CHR_EVENT_CLOSED);
+        closed_sent = true;
+    }
+
+    chr->be = NULL;
+    qemu_chr_fe_init(be, chr_new, &error_abort);
+
+    if (be->chr_be_change(be->opaque) < 0) {
+        error_setg(errp, "Chardev '%s' change failed", chr_new->label);
+        chr_new->be = NULL;
+        qemu_chr_fe_init(be, chr, &error_abort);
+        if (closed_sent) {
+            qemu_chr_be_event(chr, CHR_EVENT_OPENED);
+        }
+        object_unref(OBJECT(chr_new));
+        return NULL;
+    }
+
+    object_unparent(OBJECT(chr));
+    object_property_add_child(get_chardevs_root(), chr_new->label,
+                              OBJECT(chr_new), &error_abort);
+    object_unref(OBJECT(chr_new));
+
+    ret = g_new0(ChardevReturn, 1);
+    if (CHARDEV_IS_PTY(chr_new)) {
+        ret->pty = g_strdup(chr_new->filename + 4);
+        ret->has_pty = true;
+    }
+
+    return ret;
+}
+
 void qmp_chardev_remove(const char *id, Error **errp)
 {
     Chardev *chr;

include/chardev/char.h

@@ -92,6 +92,15 @@ void qemu_chr_parse_common(QemuOpts *opts, ChardevCommon *backend);
  */
 Chardev *qemu_chr_new(const char *label, const char *filename);
 
+/**
+ * @qemu_chr_change:
+ *
+ * Change an existing character backend
+ *
+ * @opts the new backend options
+ */
+void qemu_chr_change(QemuOpts *opts, Error **errp);
+
 /**
  * @qemu_chr_cleanup:
  *

qapi-schema.json

@@ -5089,6 +5089,46 @@
                                      'backend' : 'ChardevBackend' },
   'returns': 'ChardevReturn' }
 
+##
+# @chardev-change:
+#
+# Change a character device backend
+#
+# @id: the chardev's ID, must exist
+# @backend: new backend type and parameters
+#
+# Returns: ChardevReturn.
+#
+# Since: 2.10
+#
+# Example:
+#
+# -> { "execute" : "chardev-change",
+#      "arguments" : { "id" : "baz",
+#                      "backend" : { "type" : "pty", "data" : {} } } }
+# <- { "return": { "pty" : "/dev/pty/42" } }
+#
+# -> {"execute" : "chardev-change",
+#     "arguments" : {
+#         "id" : "charchannel2",
+#         "backend" : {
+#             "type" : "socket",
+#             "data" : {
+#                 "addr" : {
+#                     "type" : "unix" ,
+#                     "data" : {
+#                         "path" : "/tmp/charchannel2.socket"
+#                     }
+#                  },
+#                  "server" : true,
+#                  "wait" : false }}}}
+# <- {"return": {}}
+#
+##
+{ 'command': 'chardev-change', 'data': {'id'      : 'str',
+                                        'backend' : 'ChardevBackend' },
+  'returns': 'ChardevReturn' }
+
 ##
 # @chardev-remove:
 #