Add RAII class for blocking gdb signals

This adds configury support and an RAII class that can be used to
temporarily block signals that are used by gdb.  (This class is not
used in this patch, but it split out for easier review.)

The idea of this patch is that these signals should only be delivered
to the main thread.  So, when creating a background thread, they are
temporarily blocked; the blocked state is inherited by the new thread.

The sigprocmask man page says:

    The use of sigprocmask() is unspecified in a multithreaded
    process; see pthread_sigmask(3).

This patch changes gdb to use pthread_sigmask when appropriate, by
introducing a convenience define.

I've updated gdbserver as well, because I had to touch gdbsupport, and
because the threading patches will make it link against the thread
library.

I chose not to touch the NTO code, because I don't know anything about
that platform and because I cannot test it.

Finally, this modifies an existing spot in the Guile layer to use the
new facility.

gdb/ChangeLog
2019-11-26  Tom Tromey  <tom@tromey.com>

	* gdbsupport/signals-state-save-restore.c (original_signal_mask):
	Remove comment.
	(save_original_signals_state, restore_original_signals_state): Use
	gdb_sigmask.
	* linux-nat.c (block_child_signals, restore_child_signals_mask)
	(_initialize_linux_nat): Use gdb_sigmask.
	* guile/guile.c (_initialize_guile): Use block_signals.
	* Makefile.in (HFILES_NO_SRCDIR): Add gdb-sigmask.h.
	* gdbsupport/gdb-sigmask.h: New file.
	* event-top.c (async_sigtstp_handler): Use gdb_sigmask.
	* cp-support.c (gdb_demangle): Use gdb_sigmask.
	* gdbsupport/common.m4 (GDB_AC_COMMON): Check for
	pthread_sigmask.
	* configure, config.in: Rebuild.
	* gdbsupport/block-signals.h: New file.

gdb/gdbserver/ChangeLog
2019-11-26  Tom Tromey  <tom@tromey.com>

	* remote-utils.c (block_unblock_async_io): Use gdb_sigmask.
	* linux-low.c (linux_wait_for_event_filtered, linux_async): Use
	gdb_sigmask.
	* configure, config.in: Rebuild.

Change-Id: If3f37dc57dd859c226e9e4d79458a0514746e8c6

1 files changed, 82 insertions, 0 deletions

diff --git a/gdb/configure b/gdb/configure
index 50164ac..dd936d2 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -2520,6 +2520,73 @@ fi
   as_fn_set_status $ac_retval
 
 } # ac_fn_cxx_try_link
+
+# ac_fn_cxx_check_func LINENO FUNC VAR
+# ------------------------------------
+# Tests whether FUNC exists, setting the cache variable VAR accordingly
+ac_fn_cxx_check_func ()
+{
+  as_lineno=${as_lineno-"$1"} as_lineno_stack=as_lineno_stack=$as_lineno_stack
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for $2" >&5
+$as_echo_n "checking for $2... " >&6; }
+if eval \${$3+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+/* Define $2 to an innocuous variant, in case <limits.h> declares $2.
+   For example, HP-UX 11i <limits.h> declares gettimeofday.  */
+#define $2 innocuous_$2
+
+/* System header to define __stub macros and hopefully few prototypes,
+    which can conflict with char $2 (); below.
+    Prefer <limits.h> to <assert.h> if __STDC__ is defined, since
+    <limits.h> exists even on freestanding compilers.  */
+
+#ifdef __STDC__
+# include <limits.h>
+#else
+# include <assert.h>
+#endif
+
+#undef $2
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char $2 ();
+/* The GNU C library defines this for functions which it implements
+    to always fail with ENOSYS.  Some functions are actually named
+    something starting with __ and the normal name is an alias.  */
+#if defined __stub_$2 || defined __stub___$2
+choke me
+#endif
+
+int
+main ()
+{
+return $2 ();
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_link "$LINENO"; then :
+  eval "$3=yes"
+else
+  eval "$3=no"
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+fi
+eval ac_res=\$$3
+	       { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_res" >&5
+$as_echo "$ac_res" >&6; }
+  eval $as_lineno_stack; ${as_lineno_stack:+:} unset as_lineno
+
+} # ac_fn_cxx_check_func
 cat >config.log <<_ACEOF
 This file contains any messages produced by compilers while
 running configure, to aid debugging if configure makes a mistake.
@@ -14263,6 +14330,21 @@ rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
 fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $gdb_cv_cxx_std_thread" >&5
 $as_echo "$gdb_cv_cxx_std_thread" >&6; }
+
+    # This check must be here, while LIBS includes any necessary
+    # threading library.
+    for ac_func in pthread_sigmask
+do :
+  ac_fn_cxx_check_func "$LINENO" "pthread_sigmask" "ac_cv_func_pthread_sigmask"
+if test "x$ac_cv_func_pthread_sigmask" = xyes; then :
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_PTHREAD_SIGMASK 1
+_ACEOF
+
+fi
+done
+
+
     LIBS="$save_LIBS"
     CXXFLAGS="$save_CXXFLAGS"
   fi