gdb: Introduce RAII signal handler setter

This patch is motivated by the wait function for the record-full target,
that would install a custom signal handler for SIGINT, but could throw
an exception and never reset the SIGINT handler.  This is clearly a bad
idea, so this patch introduces the class scoped_signal_handler in a new
.h file.  The file is added to gdbsupport, even though only gdb code is
using it, because it feels like an addition that would be useful for
more than just directly gdb.

The implementation of the RAII class is based on the implementation
on gdb/utils.c. That is, it uses preprocessor ifdefs to probe for
sigaction support, and uses it if possible, defaulting to a raw call to
signal only if sigaction isn't supported. sigaction is preferred based
on the "portability" section of the manual page for the signal function.

There are 3 places where this class can just be dropped in,
gdb/record-full.c, gdb/utils.c and gdb/extension.c. This third place
already had a specialized RAII signal handler setter, but it is
substituted for the new general purpose one.

Approved-By: Tom Tromey <tom@tromey.com>

4 files changed, 79 insertions, 43 deletions

diff --git a/gdb/extension.c b/gdb/extension.c
index ec1aa13..47f5691 100644
--- a/gdb/extension.c
+++ b/gdb/extension.c
@@ -33,6 +33,7 @@
 #include "guile/guile.h"
 #include <array>
 #include "inferior.h"
+#include "gdbsupport/scoped_signal_handler.h"
 
 static script_sourcer_func source_gdb_script;
 static objfile_script_sourcer_func source_gdb_objfile_script;
@@ -297,28 +298,6 @@ ext_lang_auto_load_enabled (const struct extension_language_defn *extlang)
 }
 
 
-/* RAII class used to temporarily return SIG to its default handler.  */
-
-template<int SIG>
-struct scoped_default_signal
-{
-  scoped_default_signal ()
-  { m_old_sig_handler = signal (SIG, SIG_DFL); }
-
-  ~scoped_default_signal ()
-  { signal (SIG, m_old_sig_handler); }
-
-  DISABLE_COPY_AND_ASSIGN (scoped_default_signal);
-
-private:
-  /* The previous signal handler that needs to be restored.  */
-  sighandler_t m_old_sig_handler;
-};
-
-/* Class to temporarily return SIGINT to its default handler.  */
-
-using scoped_default_sigint = scoped_default_signal<SIGINT>;
-
 /* Functions that iterate over all extension languages.
    These only iterate over external extension languages, not including
    GDB's own extension/scripting language, unless otherwise indicated.  */
@@ -334,7 +313,7 @@ ext_lang_initialization (void)
       if (extlang->ops != nullptr
 	  && extlang->ops->initialize != NULL)
 	{
-	  scoped_default_sigint set_sigint_to_default_handler;
+	  scoped_signal_handler<SIGINT> set_sigint_to_default_handler (SIG_DFL);
 	  extlang->ops->initialize (extlang);
 	}
     }
diff --git a/gdb/record-full.c b/gdb/record-full.c
index d48718a..7d7959f 100644
--- a/gdb/record-full.c
+++ b/gdb/record-full.c
@@ -39,6 +39,7 @@
 #include "infrun.h"
 #include "gdbsupport/gdb_unlinker.h"
 #include "gdbsupport/byte-vector.h"
+#include "gdbsupport/scoped_signal_handler.h"
 #include "async-event.h"
 #include "top.h"
 #include "valprint.h"
@@ -1159,6 +1160,7 @@ record_full_wait_1 (struct target_ops *ops,
 {
   scoped_restore restore_operation_disable
     = record_full_gdb_operation_disable_set ();
+  scoped_signal_handler<SIGINT> interrupt_handler (record_full_sig_handler);
 
   if (record_debug)
     gdb_printf (gdb_stdlog,
@@ -1179,7 +1181,6 @@ record_full_wait_1 (struct target_ops *ops,
     }
 
   record_full_get_sig = 0;
-  signal (SIGINT, record_full_sig_handler);
 
   record_full_stop_reason = TARGET_STOPPED_BY_NO_REASON;
 
@@ -1468,8 +1469,6 @@ record_full_wait_1 (struct target_ops *ops,
 	}
     }
 
-  signal (SIGINT, handle_sigint);
-
   return inferior_ptid;
 }
 
diff --git a/gdb/utils.c b/gdb/utils.c
index bd3a1f1..6f2055e 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -19,6 +19,7 @@
 
 #include <ctype.h>
 #include "gdbsupport/gdb_wait.h"
+#include "gdbsupport/scoped_signal_handler.h"
 #include "event-top.h"
 #include "gdbthread.h"
 #include "fnmatch.h"
@@ -3468,18 +3469,7 @@ wait_to_die_with_timeout (pid_t pid, int *status, int timeout)
   if (timeout > 0)
     {
 #ifdef SIGALRM
-#if defined (HAVE_SIGACTION) && defined (SA_RESTART)
-      struct sigaction sa, old_sa;
-
-      sa.sa_handler = sigalrm_handler;
-      sigemptyset (&sa.sa_mask);
-      sa.sa_flags = 0;
-      sigaction (SIGALRM, &sa, &old_sa);
-#else
-      sighandler_t ofunc;
-
-      ofunc = signal (SIGALRM, sigalrm_handler);
-#endif
+      scoped_signal_handler<SIGALRM> alarm_restore (sigalrm_handler);
 
       alarm (timeout);
 #endif
@@ -3488,11 +3478,6 @@ wait_to_die_with_timeout (pid_t pid, int *status, int timeout)
 
 #ifdef SIGALRM
       alarm (0);
-#if defined (HAVE_SIGACTION) && defined (SA_RESTART)
-      sigaction (SIGALRM, &old_sa, NULL);
-#else
-      signal (SIGALRM, ofunc);
-#endif
 #endif
     }
   else
diff --git a/gdbsupport/scoped_signal_handler.h b/gdbsupport/scoped_signal_handler.h
new file mode 100644
index 0000000..3dffd79
--- /dev/null
+++ b/gdbsupport/scoped_signal_handler.h
@@ -0,0 +1,73 @@
+/* RAII class to install a separate handler for a given signal
+
+   Copyright (C) 2024 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 SCOPED_SIGNAL_HANDLER_H
+#define SCOPED_SIGNAL_HANDLER_H
+
+#include <signal.h>
+
+#undef HAVE_SIGACTION
+
+/* RAII class to set a signal handler for a scope, that will take care of
+   unsetting the handler when the scope is left.
+   This class will try to use sigaction whenever available, following the
+   recommendation on the man page for signal, and only fallback to signal
+   if necessary.  */
+template <int SIG>
+class scoped_signal_handler
+{
+public:
+  scoped_signal_handler (sighandler_t handler)
+  {
+#if defined (HAVE_SIGACTION)
+    struct sigaction act;
+
+    act.sa_handler = handler;
+    sigemptyset (&act.sa_mask);
+    act.sa_flags = 0;
+    sigaction (SIG, &act, &m_prev_handler);
+#else
+    /* The return of the function call is the previous signal handler, or
+       SIG_ERR if the function doesn't succeed.  */
+    m_prev_handler = signal (SIG, handler);
+    /* According to the GNU libc manual, the only way signal fails is if
+       the signum given is invalid, so we should be safe to assert.  */
+    gdb_assert (m_prev_handler != SIG_ERR);
+#endif
+  }
+
+  ~scoped_signal_handler ()
+  {
+#if defined (HAVE_SIGACTION)
+    sigaction (SIG, &m_prev_handler, nullptr);
+#else
+    signal (SIG, m_prev_handler);
+#endif
+  }
+
+  DISABLE_COPY_AND_ASSIGN (scoped_signal_handler);
+private:
+#if defined (HAVE_SIGACTION)
+  struct sigaction m_prev_handler;
+#else
+  sighandler_t m_prev_handler;
+#endif
+};
+
+#endif /* SCOPED_SIGNAL_HANDLER_H  */