bigfile/virtmem: When restoring SIGSEGV, don't change procmask for other signals

We factored out SIGSEGV block/restore from fileh_dirty_writeout() to all
functions in cb7a7055 (bigfile/virtmem: Block/restore SIGSEGV in
non-pagefault-handling function). The restoration however just sets
whole thread sigmask.

It could be possible that between block/restore calls procmask for other
signals could be changed, and this way - setting procmask directly - we
will overwrite them.

So be careful, and when restoring SIGSEGV mask, touch mask bit for only
that signal.

( we need xsigismember helper to get this done, which is also introduced
  in this patch )

bigfile/virtmem.c

@@ -64,7 +64,12 @@ static void sigsegv_block(sigset_t *save_sigset)
 
 static void sigsegv_restore(const sigset_t *save_sigset)
 {
-    xpthread_sigmask(SIG_SETMASK, save_sigset, NULL);
+    int how = xsigismember(save_sigset, SIGSEGV) ? SIG_BLOCK : SIG_UNBLOCK;
+    sigset_t mask_segv;
+
+    xsigemptyset(&mask_segv);
+    xsigaddset(&mask_segv, SIGSEGV);
+    xpthread_sigmask(how, &mask_segv, NULL);
 }
 
 

include/wendelin/utils.h

@@ -76,6 +76,7 @@ void xftruncate(int fd, off_t len);
 
 void xsigemptyset(sigset_t *set);
 void xsigaddset(sigset_t *set, int sig);
+int  xsigismember(const sigset_t *set, int sig);
 void xpthread_sigmask(int how, const sigset_t *set, sigset_t *oldset);
 
 #endif

lib/utils.c

@@ -123,6 +123,16 @@ void xsigaddset(sigset_t *set, int sig)
         BUGe();
 }
 
+int xsigismember(const sigset_t *set, int sig)
+{
+    int ret;
+    ret = sigismember(set, sig);
+    if (ret == -1)
+        BUGe();
+
+    return ret;
+}
+
 /* pthread_sigmask() should not fail on any correct usage */
 void xpthread_sigmask(int how, const sigset_t *set, sigset_t *oldset)
 {