qmp: Fail gracefully if chardev is already in use

Trying to attach a QMP monitor to a chardev that is already in use
results in a crash because monitor_init_qmp() passes &error_abort to
qemu_chr_fe_init():

$ ./x86_64-softmmu/qemu-system-x86_64 --chardev stdio,id=foo --mon foo,mode=control --mon foo,mode=control
Unexpected error in qemu_chr_fe_init() at chardev/char-fe.c:220:
qemu-system-x86_64: --mon foo,mode=control: Device 'foo' is in use
Abgebrochen (Speicherabzug geschrieben)

Fix this by allowing monitor_init_qmp() to return an error and passing
any error in qemu_chr_fe_init() to its caller instead of aborting.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200224143008.13362-18-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/monitor/monitor.h

@@ -17,7 +17,7 @@ bool monitor_cur_is_qmp(void);
 
 void monitor_init_globals(void);
 void monitor_init_globals_core(void);
-void monitor_init_qmp(Chardev *chr, bool pretty);
+void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp);
 void monitor_init_hmp(Chardev *chr, bool use_readline);
 int monitor_init(MonitorOptions *opts, Error **errp);
 int monitor_init_opts(QemuOpts *opts, Error **errp);

monitor/monitor.c

@@ -614,6 +614,7 @@ void monitor_init_globals_core(void)
 int monitor_init(MonitorOptions *opts, Error **errp)
 {
     Chardev *chr;
+    Error *local_err = NULL;
 
     chr = qemu_chr_find(opts->chardev);
     if (chr == NULL) {
@@ -623,7 +624,7 @@ int monitor_init(MonitorOptions *opts, Error **errp)
 
     switch (opts->mode) {
     case MONITOR_MODE_CONTROL:
-        monitor_init_qmp(chr, opts->pretty);
+        monitor_init_qmp(chr, opts->pretty, &local_err);
         break;
     case MONITOR_MODE_READLINE:
         if (opts->pretty) {
@@ -636,6 +637,10 @@ int monitor_init(MonitorOptions *opts, Error **errp)
         g_assert_not_reached();
     }
 
+    if (local_err) {
+        error_propagate(errp, local_err);
+        return -1;
+    }
     return 0;
 }
 

monitor/qmp.c

@@ -395,10 +395,16 @@ static void monitor_qmp_setup_handlers_bh(void *opaque)
     monitor_list_append(&mon->common);
 }
 
-void monitor_init_qmp(Chardev *chr, bool pretty)
+void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp)
 {
     MonitorQMP *mon = g_new0(MonitorQMP, 1);
 
+    if (!qemu_chr_fe_init(&mon->common.chr, chr, errp)) {
+        g_free(mon);
+        return;
+    }
+    qemu_chr_fe_set_echo(&mon->common.chr, true);
+
     /* Note: we run QMP monitor in I/O thread when @chr supports that */
     monitor_data_init(&mon->common, true, false,
                       qemu_chr_has_feature(chr, QEMU_CHAR_FEATURE_GCONTEXT));
@@ -408,9 +414,6 @@ void monitor_init_qmp(Chardev *chr, bool pretty)
     qemu_mutex_init(&mon->qmp_queue_lock);
     mon->qmp_requests = g_queue_new();
 
-    qemu_chr_fe_init(&mon->common.chr, chr, &error_abort);
-    qemu_chr_fe_set_echo(&mon->common.chr, true);
-
     json_message_parser_init(&mon->parser, handle_qmp_command, mon, NULL);
     if (mon->common.use_io_thread) {
         /*

stubs/monitor-core.c

@@ -5,7 +5,7 @@
 
 __thread Monitor *cur_mon;
 
-void monitor_init_qmp(Chardev *chr, bool pretty)
+void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp)
 {
 }
 

tests/test-util-sockets.c

@@ -71,7 +71,7 @@ int monitor_get_fd(Monitor *mon, const char *fdname, Error **errp)
  */
 __thread Monitor *cur_mon;
 int monitor_vprintf(Monitor *mon, const char *fmt, va_list ap) { abort(); }
-void monitor_init_qmp(Chardev *chr, bool pretty) {}
+void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp) {}
 void monitor_init_hmp(Chardev *chr, bool use_readline) {}