gdb, breakpoint: add a destructor to the watchpoint struct

Make sure to unlink the related breakpoint when the watchpoint instance
is deleted.  This prevents having a wp-related breakpoint that is
linked to a NULL watchpoint (e.g.  the watchpoint instance is being
deleted when the 'watch' command fails).  With the below scenario,
having such a left out breakpoint will lead to a GDB hang, and this
is due to an infinite loop when deleting all inferior breakpoints.

Scenario:
	(gdb) set can-use-hw-watchpoints 0
	(gdb) awatch <SCOPE VAR>
	Can't set read/access watchpoint when hardware watchpoints are disabled.
	(gdb) rwatch <SCOPE VAR>
	Can't set read/access watchpoint when hardware watchpoints are disabled.
	(gdb) <continue the program until the end>
	>> HANG <<

Signed-off-by: Mohamed Bouhaouel <mohamed.bouhaouel@intel.com>
Reviewed-by: Bruno Larsen <blarsen@redhat.com>

gdb/breakpoint.c

@@ -9817,6 +9817,20 @@ break_range_command (const char *arg, int from_tty)
   install_breakpoint (false, std::move (br), true);
 }
 
+/* See breakpoint.h.  */
+
+watchpoint::~watchpoint ()
+{
+  /* Make sure to unlink the destroyed watchpoint from the related
+     breakpoint ring.  */
+
+  breakpoint *bpt = this;
+  while (bpt->related_breakpoint != this)
+    bpt = bpt->related_breakpoint;
+
+  bpt->related_breakpoint = this->related_breakpoint;
+}
+
 /*  Return non-zero if EXP is verified as constant.  Returned zero
     means EXP is variable.  Also the constant detection may fail for
     some constant expressions and in such case still falsely return

gdb/breakpoint.h

@@ -1000,6 +1000,9 @@ struct watchpoint : public breakpoint
   void print_recreate (struct ui_file *fp) const override;
   bool explains_signal (enum gdb_signal) override;
 
+  /* Destructor for WATCHPOINT.  */
+  ~watchpoint ();
+
   /* String form of exp to use for displaying to the user (malloc'd),
      or NULL if none.  */
   gdb::unique_xmalloc_ptr<char> exp_string;

gdb/testsuite/gdb.base/scope-hw-watch-disable.c

@@ -0,0 +1,26 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2023 Free Software Foundation, Inc.
+
+   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/>.  */
+
+int
+main ()
+{
+  int a = 0, b = 0;
+  b = a;
+  a = b + 10;
+
+  return 0;
+}

gdb/testsuite/gdb.base/scope-hw-watch-disable.exp

@@ -0,0 +1,40 @@
+# Copyright 2023 Free Software Foundation, Inc.
+
+# 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/>.
+
+# Test that GDB displays the correct error message when hardware watchpoints
+# are not supported or explicitly disabled.  Test also that GDB terminates
+# successfully after several attempts to insert a hardware watchpoint.
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" ${testfile} ${srcfile}]} {
+    return -1
+}
+
+gdb_test_no_output "set can-use-hw-watchpoints 0"
+
+if {![runto_main]} {
+    return -1
+}
+
+gdb_test "awatch a" \
+    "Can't set read/access watchpoint when hardware watchpoints are disabled." \
+    "unsuccessful attempt to create an access watchpoint"
+gdb_test "rwatch b" \
+    "Can't set read/access watchpoint when hardware watchpoints are disabled." \
+    "unsuccessful attempt to create a read watchpoint"
+
+# The program continues until termination.
+gdb_continue_to_end