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

@@ -538,10 +538,37 @@
 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;

@@ -558,24 +585,11 @@
         (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;
-                }
-        }
+        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.