Introduce alternate_signal_stack RAII class

This introduces a new RAII class that temporarily installs an
alternate signal stack (on systems that have sigaltstack); then
changes the one gdb use of sigaltstack to use this class instead.

This will be used in a later patch, when creating new threads that may
want to handle SIGSEGV.

gdb/ChangeLog
2019-09-30  Tom Tromey  <tom@tromey.com>

	* main.c (setup_alternate_signal_stack): Remove.
	(captured_main_1): Use gdb::alternate_signal_stack.
	* gdbsupport/alt-stack.h: New file.

3 files changed, 78 insertions, 24 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 1d834af..395a44f 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,11 @@
 2019-09-30  Tom Tromey  <tom@tromey.com>
 
+	* main.c (setup_alternate_signal_stack): Remove.
+	(captured_main_1): Use gdb::alternate_signal_stack.
+	* gdbsupport/alt-stack.h: New file.
+
+2019-09-30  Tom Tromey  <tom@tromey.com>
+
 	* gdbsupport/common.m4 (GDB_AC_COMMON): Check for
 	pthread_sigmask.
 	* configure: Rebuild.
diff --git a/gdb/gdbsupport/alt-stack.h b/gdb/gdbsupport/alt-stack.h
new file mode 100644
index 0000000..1708fb4
--- /dev/null
+++ b/gdb/gdbsupport/alt-stack.h
@@ -0,0 +1,70 @@
+/* Temporarily install an alternate signal stack
+
+   Copyright (C) 2019 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDBSUPPORT_ALT_STACK_H
+#define GDBSUPPORT_ALT_STACK_H
+
+#include <signal.h>
+
+namespace gdb
+{
+
+/* Try to set up an alternate signal stack for SIGSEGV handlers.
+   This allows us to handle SIGSEGV signals generated when the
+   normal process stack is exhausted.  If this stack is not set
+   up (sigaltstack is unavailable or fails) and a SIGSEGV is
+   generated when the normal stack is exhausted then the program
+   will behave as though no SIGSEGV handler was installed.  */
+class alternate_signal_stack
+{
+public:
+  alternate_signal_stack ()
+  {
+#ifdef HAVE_SIGALTSTACK
+    m_stack.reset ((char *) xmalloc (SIGSTKSZ));
+
+    stack_t stack;
+    stack.ss_sp = m_stack.get ();
+    stack.ss_size = SIGSTKSZ;
+    stack.ss_flags = 0;
+
+    sigaltstack (&stack, &m_old_stack);
+#endif
+  }
+
+  ~alternate_signal_stack ()
+  {
+#ifdef HAVE_SIGALTSTACK
+    sigaltstack (&m_old_stack, nullptr);
+#endif
+  }
+
+  DISABLE_COPY_AND_ASSIGN (alternate_signal_stack);
+
+private:
+
+#ifdef HAVE_SIGALTSTACK
+  gdb::unique_xmalloc_ptr<char> m_stack;
+  stack_t m_old_stack;
+#endif
+};
+
+}
+
+#endif /* GDBSUPPORT_ALT_STACK_H */
diff --git a/gdb/main.c b/gdb/main.c
index 7fab8ff..7b75dc1 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -48,6 +48,7 @@
 #include <vector>
 #include "gdbsupport/pathstuff.h"
 #include "cli/cli-style.h"
+#include "gdbsupport/alt-stack.h"
 
 /* The selected interpreter.  This will be used as a set command
    variable, so it should always be malloc'ed - since
@@ -290,29 +291,6 @@ get_init_files (std::string *system_gdbinit,
   *local_gdbinit = localinit;
 }
 
-/* Try to set up an alternate signal stack for SIGSEGV handlers.
-   This allows us to handle SIGSEGV signals generated when the
-   normal process stack is exhausted.  If this stack is not set
-   up (sigaltstack is unavailable or fails) and a SIGSEGV is
-   generated when the normal stack is exhausted then the program
-   will behave as though no SIGSEGV handler was installed.  */
-
-static void
-setup_alternate_signal_stack (void)
-{
-#ifdef HAVE_SIGALTSTACK
-  stack_t ss;
-
-  /* FreeBSD versions older than 11.0 use char * for ss_sp instead of
-     void *.  This cast works with both types.  */
-  ss.ss_sp = (char *) xmalloc (SIGSTKSZ);
-  ss.ss_size = SIGSTKSZ;
-  ss.ss_flags = 0;
-
-  sigaltstack(&ss, NULL);
-#endif
-}
-
 /* Call command_loop.  */
 
 /* Prevent inlining this function for the benefit of GDB's selftests
@@ -859,7 +837,7 @@ captured_main_1 (struct captured_main_args *context)
   save_original_signals_state (quiet);
 
   /* Try to set up an alternate signal stack for SIGSEGV handlers.  */
-  setup_alternate_signal_stack ();
+  gdb::alternate_signal_stack signal_stack;
 
   /* Initialize all files.  */
   gdb_init (gdb_program_name);