vnc: implement shared flag handling.

VNC clients send a shared flag in the client init message.  Up to now
qemu completely ignores this.  This patch implements shared flag
handling.  It comes with three policies:  By default qemu behaves as one
would expect:  Asking for a exclusive access grants exclusive access to
the client connecting.  There is also a desktop sharing mode which
disallows exclusive connects (so one forgetting -shared wouldn't drop
everybody else) and a compatibility mode which mimics the traditional
(but non-conforming) qemu behavior.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

qemu-options.hx

@@ -1095,6 +1095,19 @@ This can be really helpful to save bandwidth when playing videos. Disabling
 adaptive encodings allows to restore the original static behavior of encodings
 like Tight.
 
+@item share=[allow-exclusive|force-shared|ignore]
+
+Set display sharing policy.  'allow-exclusive' allows clients to ask
+for exclusive access.  As suggested by the rfb spec this is
+implemented by dropping other connections.  Connecting multiple
+clients in parallel requires all clients asking for a shared session
+(vncviewer: -shared switch).  This is the default.  'force-shared'
+disables exclusive client access.  Useful for shared desktop sessions,
+where you don't want someone forgetting specify -shared disconnect
+everybody else.  'ignore' completely ignores the shared flag and
+allows everybody connect unconditionally.  Doesn't conform to the rfb
+spec but is traditional qemu behavior.
+
 @end table
 ETEXI
 

ui/vnc.c

@@ -47,6 +47,29 @@ static DisplayChangeListener *dcl;
 
 static int vnc_cursor_define(VncState *vs);
 
+static void vnc_set_share_mode(VncState *vs, VncShareMode mode)
+{
+#ifdef _VNC_DEBUG
+    static const char *mn[] = {
+        [0]                           = "undefined",
+        [VNC_SHARE_MODE_CONNECTING]   = "connecting",
+        [VNC_SHARE_MODE_SHARED]       = "shared",
+        [VNC_SHARE_MODE_EXCLUSIVE]    = "exclusive",
+        [VNC_SHARE_MODE_DISCONNECTED] = "disconnected",
+    };
+    fprintf(stderr, "%s/%d: %s -> %s\n", __func__,
+            vs->csock, mn[vs->share_mode], mn[mode]);
+#endif
+
+    if (vs->share_mode == VNC_SHARE_MODE_EXCLUSIVE) {
+        vs->vd->num_exclusive--;
+    }
+    vs->share_mode = mode;
+    if (vs->share_mode == VNC_SHARE_MODE_EXCLUSIVE) {
+        vs->vd->num_exclusive++;
+    }
+}
+
 static char *addr_to_string(const char *format,
                             struct sockaddr_storage *sa,
                             socklen_t salen) {
@@ -997,6 +1020,7 @@ static void vnc_disconnect_start(VncState *vs)
 {
     if (vs->csock == -1)
         return;
+    vnc_set_share_mode(vs, VNC_SHARE_MODE_DISCONNECTED);
     qemu_set_fd_handler2(vs->csock, NULL, NULL, NULL, NULL);
     closesocket(vs->csock);
     vs->csock = -1;
@@ -2054,8 +2078,67 @@ static int protocol_client_msg(VncState *vs, uint8_t *data, size_t len)
 static int protocol_client_init(VncState *vs, uint8_t *data, size_t len)
 {
     char buf[1024];
+    VncShareMode mode;
     int size;
 
+    mode = data[0] ? VNC_SHARE_MODE_SHARED : VNC_SHARE_MODE_EXCLUSIVE;
+    switch (vs->vd->share_policy) {
+    case VNC_SHARE_POLICY_IGNORE:
+        /*
+         * Ignore the shared flag.  Nothing to do here.
+         *
+         * Doesn't conform to the rfb spec but is traditional qemu
+         * behavior, thus left here as option for compatibility
+         * reasons.
+         */
+        break;
+    case VNC_SHARE_POLICY_ALLOW_EXCLUSIVE:
+        /*
+         * Policy: Allow clients ask for exclusive access.
+         *
+         * Implementation: When a client asks for exclusive access,
+         * disconnect all others. Shared connects are allowed as long
+         * as no exclusive connection exists.
+         *
+         * This is how the rfb spec suggests to handle the shared flag.
+         */
+        if (mode == VNC_SHARE_MODE_EXCLUSIVE) {
+            VncState *client;
+            QTAILQ_FOREACH(client, &vs->vd->clients, next) {
+                if (vs == client) {
+                    continue;
+                }
+                if (client->share_mode != VNC_SHARE_MODE_EXCLUSIVE &&
+                    client->share_mode != VNC_SHARE_MODE_SHARED) {
+                    continue;
+                }
+                vnc_disconnect_start(client);
+            }
+        }
+        if (mode == VNC_SHARE_MODE_SHARED) {
+            if (vs->vd->num_exclusive > 0) {
+                vnc_disconnect_start(vs);
+                return 0;
+            }
+        }
+        break;
+    case VNC_SHARE_POLICY_FORCE_SHARED:
+        /*
+         * Policy: Shared connects only.
+         * Implementation: Disallow clients asking for exclusive access.
+         *
+         * Useful for shared desktop sessions where you don't want
+         * someone forgetting to say -shared when running the vnc
+         * client disconnect everybody else.
+         */
+        if (mode == VNC_SHARE_MODE_EXCLUSIVE) {
+            vnc_disconnect_start(vs);
+            return 0;
+        }
+        break;
+    }
+    vnc_set_share_mode(vs, mode);
+
     vs->client_width = ds_get_width(vs->ds);
     vs->client_height = ds_get_height(vs->ds);
     vnc_write_u16(vs, vs->client_width);
@@ -2562,6 +2645,7 @@ static void vnc_connect(VncDisplay *vd, int csock, int skipauth)
 
     vnc_client_cache_addr(vs);
     vnc_qmp_event(vs, QEVENT_VNC_CONNECTED);
+    vnc_set_share_mode(vs, VNC_SHARE_MODE_CONNECTING);
 
     vs->vd = vd;
     vs->ds = vd->ds;
@@ -2755,6 +2839,7 @@ int vnc_display_open(DisplayState *ds, const char *display)
 
     if (!(vs->display = strdup(display)))
         return -1;
+    vs->share_policy = VNC_SHARE_POLICY_ALLOW_EXCLUSIVE;
 
     options = display;
     while ((options = strchr(options, ','))) {
@@ -2810,6 +2895,19 @@ int vnc_display_open(DisplayState *ds, const char *display)
             vs->lossy = true;
         } else if (strncmp(options, "non-adapative", 13) == 0) {
             vs->non_adaptive = true;
+        } else if (strncmp(options, "share=", 6) == 0) {
+            if (strncmp(options+6, "ignore", 6) == 0) {
+                vs->share_policy = VNC_SHARE_POLICY_IGNORE;
+            } else if (strncmp(options+6, "allow-exclusive", 15) == 0) {
+                vs->share_policy = VNC_SHARE_POLICY_ALLOW_EXCLUSIVE;
+            } else if (strncmp(options+6, "force-shared", 12) == 0) {
+                vs->share_policy = VNC_SHARE_POLICY_FORCE_SHARED;
+            } else {
+                fprintf(stderr, "unknown vnc share= option\n");
+                g_free(vs->display);
+                vs->display = NULL;
+                return -1;
+            }
         }
     }
 

ui/vnc.h

@@ -122,9 +122,24 @@ struct VncSurface
     DisplaySurface *ds;
 };
 
+typedef enum VncShareMode {
+    VNC_SHARE_MODE_CONNECTING = 1,
+    VNC_SHARE_MODE_SHARED,
+    VNC_SHARE_MODE_EXCLUSIVE,
+    VNC_SHARE_MODE_DISCONNECTED,
+} VncShareMode;
+
+typedef enum VncSharePolicy {
+    VNC_SHARE_POLICY_IGNORE = 1,
+    VNC_SHARE_POLICY_ALLOW_EXCLUSIVE,
+    VNC_SHARE_POLICY_FORCE_SHARED,
+} VncSharePolicy;
+
 struct VncDisplay
 {
     QTAILQ_HEAD(, VncState) clients;
+    int num_exclusive;
+    VncSharePolicy share_policy;
     QEMUTimer *timer;
     int timer_interval;
     int lsock;
@@ -250,6 +265,7 @@ struct VncState
     int last_y;
     int client_width;
     int client_height;
+    VncShareMode share_mode;
 
     uint32_t vnc_encoding;