libvchan: create xenstore entries in one transaction

This will prevent race when client waits for server with xs_watch - all
entries should appear at once.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

diff --git a/tools/libvchan/init.c b/tools/libvchan/init.c
index 0b3759a05619cefe3617baa9adfde8a28a389418..ba5a6eb29e83af763ecdc68030c3093e6d1052c9 100644 (file)
--- a/tools/libvchan/init.c
+++ b/tools/libvchan/init.c
@@ -250,6 +250,7 @@ static int init_xs_srv(struct libxenvchan *ctrl, int domain, const char* xs_base
        char buf[64];
        char ref[16];
        char* domid_str = NULL;
+       xs_transaction_t xs_trans = NULL;
        xs = xs_domain_open();
        if (!xs)
                goto fail;
@@ -265,21 +266,31 @@ static int init_xs_srv(struct libxenvchan *ctrl, int domain, const char* xs_base
        perms[1].id = domain;
        perms[1].perms = XS_PERM_READ;
 
+retry_transaction:
+       xs_trans = xs_transaction_start(xs);
+       if (!xs_trans)
+               goto fail_xs_open;
+
        snprintf(ref, sizeof ref, "%d", ring_ref);
        snprintf(buf, sizeof buf, "%s/ring-ref", xs_base);
-       if (!xs_write(xs, 0, buf, ref, strlen(ref)))
+       if (!xs_write(xs, xs_trans, buf, ref, strlen(ref)))
                goto fail_xs_open;
-       if (!xs_set_permissions(xs, 0, buf, perms, 2))
+       if (!xs_set_permissions(xs, xs_trans, buf, perms, 2))
                goto fail_xs_open;
 
        snprintf(ref, sizeof ref, "%d", ctrl->event_port);
        snprintf(buf, sizeof buf, "%s/event-channel", xs_base);
-       if (!xs_write(xs, 0, buf, ref, strlen(ref)))
+       if (!xs_write(xs, xs_trans, buf, ref, strlen(ref)))
                goto fail_xs_open;
-       if (!xs_set_permissions(xs, 0, buf, perms, 2))
+       if (!xs_set_permissions(xs, xs_trans, buf, perms, 2))
                goto fail_xs_open;
 
-       ret = 0;
+       if (!xs_transaction_end(xs, xs_trans, 0)) {
+               if (errno == EAGAIN)
+                       goto retry_transaction;
+       } else {
+               ret = 0;
+       }
  fail_xs_open:
        free(domid_str);
        xs_daemon_close(xs);