hmp: Fail gracefully if chardev is already in use

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

$ ./x86_64-softmmu/qemu-system-x86_64 --chardev stdio,id=foo --mon foo --mon foo
QEMU 4.2.50 monitor - type 'help' for more information
(qemu) Unexpected error in qemu_chr_fe_init() at chardev/char-fe.c:220:
qemu-system-x86_64: --mon foo: Device 'foo' is in use
Abgebrochen (Speicherabzug geschrieben)

Fix this by allowing monitor_init_hmp() 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: <20200224143008.13362-19-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

chardev/char.c

@@ -737,7 +737,13 @@ Chardev *qemu_chr_new_noreplay(const char *label, const char *filename,
 
     if (qemu_opt_get_bool(opts, "mux", 0)) {
         assert(permit_mux_mon);
-        monitor_init_hmp(chr, true);
+        monitor_init_hmp(chr, true, &err);
+        if (err) {
+            error_report_err(err);
+            object_unparent(OBJECT(chr));
+            chr = NULL;
+            goto out;
+        }
     }
 
 out:

gdbstub.c

@@ -3367,7 +3367,7 @@ int gdbserver_start(const char *device)
         /* Initialize a monitor terminal for gdb */
         mon_chr = qemu_chardev_new(NULL, TYPE_CHARDEV_GDB,
                                    NULL, NULL, &error_abort);
-        monitor_init_hmp(mon_chr, false);
+        monitor_init_hmp(mon_chr, false, &error_abort);
     } else {
         qemu_chr_fe_deinit(&s->chr, true);
         mon_chr = s->mon_chr;

include/monitor/monitor.h

@@ -18,7 +18,7 @@ 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, Error **errp);
-void monitor_init_hmp(Chardev *chr, bool use_readline);
+void monitor_init_hmp(Chardev *chr, bool use_readline, Error **errp);
 int monitor_init(MonitorOptions *opts, Error **errp);
 int monitor_init_opts(QemuOpts *opts, Error **errp);
 void monitor_cleanup(void);

monitor/hmp.c

@@ -1399,12 +1399,16 @@ static void monitor_readline_flush(void *opaque)
     monitor_flush(&mon->common);
 }
 
-void monitor_init_hmp(Chardev *chr, bool use_readline)
+void monitor_init_hmp(Chardev *chr, bool use_readline, Error **errp)
 {
     MonitorHMP *mon = g_new0(MonitorHMP, 1);
 
+    if (!qemu_chr_fe_init(&mon->common.chr, chr, errp)) {
+        g_free(mon);
+        return;
+    }
+
     monitor_data_init(&mon->common, false, false, false);
-    qemu_chr_fe_init(&mon->common.chr, chr, &error_abort);
 
     mon->use_readline = use_readline;
     if (mon->use_readline) {

monitor/monitor.c

@@ -631,7 +631,7 @@ int monitor_init(MonitorOptions *opts, Error **errp)
             warn_report("'pretty' is deprecated for HMP monitors, it has no "
                         "effect and will be removed in future versions");
         }
-        monitor_init_hmp(chr, true);
+        monitor_init_hmp(chr, true, &local_err);
         break;
     default:
         g_assert_not_reached();

stubs/monitor.c

@@ -9,7 +9,7 @@ int monitor_get_fd(Monitor *mon, const char *name, Error **errp)
     return -1;
 }
 
-void monitor_init_hmp(Chardev *chr, bool use_readline)
+void monitor_init_hmp(Chardev *chr, bool use_readline, Error **errp)
 {
 }
 

tests/test-util-sockets.c

@@ -72,7 +72,7 @@ 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, Error **errp) {}
-void monitor_init_hmp(Chardev *chr, bool use_readline) {}
+void monitor_init_hmp(Chardev *chr, bool use_readline, Error **errp) {}
 
 
 static void test_socket_fd_pass_name_good(void)