Disable out-of-scope watchpoints

Currently, when a local software watchpoint goes out of scope, GDB sets
the watchpoint's disposition to `delete at next stop' and then normal
stops (i.e., stop and wait for the next GDB command). When GDB normal
stops, it automatically deletes the breakpoints with their disposition
set to `delete at next stop'.

Suppose a Python script decides not to normal stop when a local
software watchpoint goes out of scope, the watchpoint will not be
automatically deleted even when its disposition is set to
`delete at next stop'.

Since GDB single-steps the program and tests the watched expression
after each instruction, not deleting the watchpoint causes the
watchpoint to be hit many more times than it should, as reported in
PR python/29603.

This was happening because the watchpoint is not deleted or disabled
when going out of scope.

This commit fixes this issue by disabling the watchpoint when going out
of scope. It also adds a test to ensure this feature isn't regressed in
the future.

Calling `breakpoint_auto_delete' on all kinds of stops (in
`fetch_inferior_event') seem to solve this issue, but is in fact
inappropriate, since `breakpoint_auto_delete' goes over all breakpoints
instead of just going through the bpstat chain (which only contains the
breakpoints that were hit right now).

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29603
Change-Id: Ia85e670b2bcba2799219abe4b6be3b582387e383

4 files changed, 103 insertions, 0 deletions

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index dcb00bf..fdb184a 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -1844,6 +1844,7 @@ watchpoint_del_at_next_stop (struct watchpoint *w)
       w->related_breakpoint = w;
     }
   w->disposition = disp_del_at_next_stop;
+  disable_breakpoint (w);
 }
 
 /* Extract a bitfield value from value VAL using the bit parameters contained in
diff --git a/gdb/testsuite/gdb.python/py-watchpoint.c b/gdb/testsuite/gdb.python/py-watchpoint.c
new file mode 100644
index 0000000..53712287
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-watchpoint.c
@@ -0,0 +1,26 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022-2023 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/>.  */
+
+#include <stdio.h>
+
+int
+main (void)
+{
+  volatile int i = -1;
+  for (i = 0; i < 3; i++); /* main for */
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.python/py-watchpoint.exp b/gdb/testsuite/gdb.python/py-watchpoint.exp
new file mode 100644
index 0000000..9a6ef44
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-watchpoint.exp
@@ -0,0 +1,46 @@
+# Copyright (C) 2022-2023 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/>.
+
+# Check that Watchpoints are deleted after use.
+
+load_lib gdb-python.exp
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile]} {
+    return -1
+}
+
+require allow_python_tests
+
+if ![runto_main] then {
+    return
+}
+
+# For remote host testing
+set pyfile [gdb_remote_download host ${srcdir}/${subdir}/${testfile}.py]
+
+gdb_test_no_output "set can-use-hw-watchpoints 0" "don't use hardware watchpoints"
+set for_line_no [gdb_get_line_number "main for"]
+gdb_test "break $for_line_no" ".*" "set breakpoint before loop"
+gdb_test "continue" ".*" "run until reaching loop"
+gdb_test "clear" ".*" "delete the breakpoint before loop"
+gdb_test "python print(len(gdb.breakpoints()))" "1" "check default BP count"
+gdb_test "source $pyfile" ".*Python script imported.*" \
+    "import python scripts"
+gdb_test "python print(len(gdb.breakpoints()))" "2" "check modified BP count"
+gdb_test "continue" ".*" "run until program stops"
+gdb_test "python print(bpt.n)" "5" "check watchpoint hits"
+gdb_test "python print(len(gdb.breakpoints()))" "1" "check BP count"
diff --git a/gdb/testsuite/gdb.python/py-watchpoint.py b/gdb/testsuite/gdb.python/py-watchpoint.py
new file mode 100644
index 0000000..e821222
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-watchpoint.py
@@ -0,0 +1,30 @@
+# Copyright (C) 2022-2023 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/>.
+
+
+class MyBreakpoint(gdb.Breakpoint):
+    def __init__(self):
+        super().__init__("i", gdb.BP_WATCHPOINT)
+        self.n = 0
+
+    def stop(self):
+        self.n += 1
+        print("Watchpoint Hit:", self.n, flush=True)
+        return False
+
+
+bpt = MyBreakpoint()
+
+print("Python script imported")