libxl: Fix carefd lock leak in save callout

If libxl_pipe fails we leave the carefd locked, which translates to
the atfork lock remaining held.  This would probably cause the process
to deadlock shortly afterwards.

Of course libxl_pipe is very unlikely to fail unless things are
already going very badly.  This bug has not been observed anywhere as
far as we are aware.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: George Dunlap <george.dunlap@eu.citrix.com>
(cherry picked from commit 7eb73add5d)
(cherry picked from commit 4bb3a17449)

tools/libxl/libxl_save_callout.c

@@ -185,7 +185,11 @@ static void run_helper(libxl__egc *egc, libxl__save_helper_state *shs,
     for (childfd=0; childfd<2; childfd++) {
         /* Setting up the pipe for the child's fd childfd */
         int fds[2];
-        if (libxl_pipe(CTX,fds)) { rc = ERROR_FAIL; goto out; }
+        if (libxl_pipe(CTX,fds)) {
+            rc = ERROR_FAIL;
+            libxl__carefd_unlock();
+            goto out;
+        }
         int childs_end = childfd==0 ? 0 /*read*/  : 1 /*write*/;
         int our_end    = childfd==0 ? 1 /*write*/ : 0 /*read*/;
         childs_pipes[childfd] = libxl__carefd_record(CTX, fds[childs_end]);