OS-4470 lxbrand unblocking signals in new threads must be atomic

*** 538,547 ****
--- 538,574 ----
  set_setcontext_enforcement(int on)
  {
          setcontext_enforcement = on;
  }
  
+ /*
+  * The LX brand emulation library implements an operation that is analogous to
+  * setcontext(), but takes a different path in to the kernel.  So that it can
+  * correctly restore a signal mask, we expose just the signal mask handling
+  * part of the regular setcontext() routine as a private interface.
+  */
+ void
+ setcontext_sigmask(ucontext_t *ucp)
+ {
+         ulwp_t *self = curthread;
+ 
+         if (ucp->uc_flags & UC_SIGMASK) {
+                 block_all_signals(self);
+                 delete_reserved_signals(&ucp->uc_sigmask);
+                 self->ul_sigmask = ucp->uc_sigmask;
+                 if (self->ul_cursig) {
+                         /*
+                          * We have a deferred signal present.
+                          * The signal mask will be set when the
+                          * signal is taken in take_deferred_signal().
+                          */
+                         ASSERT(self->ul_critical + self->ul_sigdefer != 0);
+                         ucp->uc_flags &= ~UC_SIGMASK;
+                 }
+         }
+ }
+ 
  #pragma weak _setcontext = setcontext
  int
  setcontext(const ucontext_t *ucp)
  {
          ulwp_t *self = curthread;
*** 558,581 ****
          (void) memcpy(&uc, ucp, sizeof (uc));
  
          /*
           * Restore previous signal mask and context link.
           */
!         if (uc.uc_flags & UC_SIGMASK) {
!                 block_all_signals(self);
!                 delete_reserved_signals(&uc.uc_sigmask);
!                 self->ul_sigmask = uc.uc_sigmask;
!                 if (self->ul_cursig) {
!                         /*
!                          * We have a deferred signal present.
!                          * The signal mask will be set when the
!                          * signal is taken in take_deferred_signal().
!                          */
!                         ASSERT(self->ul_critical + self->ul_sigdefer != 0);
!                         uc.uc_flags &= ~UC_SIGMASK;
!                 }
!         }
          self->ul_siglink = uc.uc_link;
  
          /*
           * We don't know where this context structure has been.
           * Preserve the curthread pointer, at least.
--- 585,595 ----
          (void) memcpy(&uc, ucp, sizeof (uc));
  
          /*
           * Restore previous signal mask and context link.
           */
!         setcontext_sigmask(&uc);
          self->ul_siglink = uc.uc_link;
  
          /*
           * We don't know where this context structure has been.
           * Preserve the curthread pointer, at least.