gdb: reduce breakpoint-modified events for dprintf b/p

Consider this backtrace within GDB:

  #0  notify_breakpoint_modified (b=0x57d31d0) at ../../src/gdb/breakpoint.c:1083
  #1  0x00000000005b6406 in breakpoint_set_commands (b=0x57d31d0, commands=...) at ../../src/gdb/breakpoint.c:1523
  #2  0x00000000005c8c63 in update_dprintf_command_list (b=0x57d31d0) at ../../src/gdb/breakpoint.c:8641
  #3  0x00000000005d3c4e in dprintf_breakpoint::re_set (this=0x57d31d0) at ../../src/gdb/breakpoint.c:12476
  #4  0x00000000005d6347 in breakpoint_re_set () at ../../src/gdb/breakpoint.c:13298

Whenever breakpoint_re_set is called we re-build the commands that the
dprintf b/p will execute and store these into the breakpoint.  The
commands are re-built in update_dprintf_command_list and stored into
the breakpoint object in breakpoint_set_commands.

Now sometimes these commands can change, dprintf_breakpoint::re_set
explains one case where this can occur, and I'm sure there must be
others.  But in most cases the commands we recalculate will not
change.  This means that the breakpoint modified event which is
emitted from breakpoint_set_commands is redundant.

This commit aims to eliminate the redundant breakpoint modified events
for dprintf breakpoints.  This is done by adding a commands_equal call
to the start of breakpoint_set_commands.

The commands_equal function is a new function which compares two
command_line objects and returns true if they are identical.  Using
this function we can check if the new commands passed to
breakpoint_set_commands are identical to the breakpoint's existing
commands.  If the new commands are equal then we don't need to change
anything on the new breakpoint, and the breakpoint modified event can
be skipped.

The test for this commit stops at a dlopen() call in the inferior,
sets up a dprintf breakpoint, then uses 'next' to step over the
dlopen() call.  When the library loads GDB call breakpoint_re_set,
which calls dprintf_breakpoint::re_set.  But in this case we don't
expect the calculated command string to change, so we don't expect to
see the breakpoint modified event.

3 files changed, 196 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.mi/mi-dprintf-modified-lib.c b/gdb/testsuite/gdb.mi/mi-dprintf-modified-lib.c
new file mode 100644
index 0000000..70fc328
--- /dev/null
+++ b/gdb/testsuite/gdb.mi/mi-dprintf-modified-lib.c
@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2025 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
+foo (void)
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.mi/mi-dprintf-modified.c b/gdb/testsuite/gdb.mi/mi-dprintf-modified.c
new file mode 100644
index 0000000..7a41adbac
--- /dev/null
+++ b/gdb/testsuite/gdb.mi/mi-dprintf-modified.c
@@ -0,0 +1,55 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2025 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 <stdlib.h>
+
+#ifdef __WIN32__
+#include <windows.h>
+#define dlopen(name, mode) LoadLibrary (TEXT (name))
+#ifdef _WIN32_WCE
+# define dlsym(handle, func) GetProcAddress (handle, TEXT (func))
+#else
+# define dlsym(handle, func) GetProcAddress (handle, func)
+#endif
+#define dlclose(handle) FreeLibrary (handle)
+#else
+#include <dlfcn.h>
+#endif
+
+#include <assert.h>
+
+int
+main (void)
+{
+  int res;
+  void *handle;
+  int (*func) (void);
+  int val = 0;
+
+  handle = dlopen (SHLIB_NAME, RTLD_LAZY);		/* Break here.  */
+  assert (handle != NULL);
+
+  func = (int (*)(void)) dlsym (handle, "foo");
+  assert (func != NULL);
+
+  val += func ();
+
+  res = dlclose (handle);
+  assert (res == 0);
+
+  return val;
+}
diff --git a/gdb/testsuite/gdb.mi/mi-dprintf-modified.exp b/gdb/testsuite/gdb.mi/mi-dprintf-modified.exp
new file mode 100644
index 0000000..c3e1bdf
--- /dev/null
+++ b/gdb/testsuite/gdb.mi/mi-dprintf-modified.exp
@@ -0,0 +1,119 @@
+# Copyright 2025 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 GDB doesn't emit a 'breakpoint-modified' notification for
+# dprintf breakpoints when the dprintf commands haven't changed.
+#
+# GDB use to emit a 'breakpoint-modified' dprintf breakpoints each
+# time the dprintf_breakpoint::re_set function was called as this
+# would re-cacluate the dprintf command string, even though in most
+# cases the calculated string was no different from the previous
+# value.
+#
+# Then GDB got smarter and could recognise that the string had not
+# changed, and so would skip the 'breakpoint-modified' notification.
+#
+# This test stops at a dlopen() call in the inferior and creates a
+# dprintf breakpoint.  Then we 'next' over the dlopen() which triggers
+# a call to the ::re_set() functions.  We check that there is no
+# 'breakpoint-modified' event emitted for the dprintf breakpoint.
+
+load_lib mi-support.exp
+set MIFLAGS "-i=mi"
+
+standard_testfile .c -lib.c
+
+# Build the library.
+set libname ${testfile}-lib
+set libfile [standard_output_file $libname]
+if { [build_executable "build shlib" $libfile $srcfile2 {debug shlib}] == -1} {
+    return
+}
+
+# Build the executable.
+set opts [list debug shlib_load additional_flags=-DSHLIB_NAME=\"${libname}\"]
+if { [build_executable "build exec" $binfile $srcfile $opts] == -1} {
+    return
+}
+
+# The line number of the dlopen() call.
+set bp_line [gdb_get_line_number "Break here" $srcfile]
+
+# Start the inferior.
+mi_clean_restart $binfile
+mi_runto_main
+
+# Place a breakpoint at the dlopen() line.
+mi_create_breakpoint $srcfile:$bp_line "set breakpoint at dlopen call" \
+    -disp keep -func main -file "\[^\r\n\]+/$srcfile" -line $bp_line
+
+# And run to the breakpoint.
+mi_execute_to "exec-continue" "breakpoint-hit" main "" ".*/$srcfile" \
+    $bp_line { "" "disp=\"keep\"" } "run to breakpoint"
+
+# Cleanup breakpoints.
+mi_delete_breakpoints
+
+# Setup a dprintf breakpoint.
+mi_gdb_test "-dprintf-insert --function main \"in main\"" \
+    "\\^done,bkpt={.*}" "dprintf at main"
+
+set bpnum [mi_get_valueof "/d" "\$bpnum" "INVALID" \
+	       "get number for dprintf breakpoint"]
+
+# Use 'next' to step over loading the shared library.
+mi_gdb_test "220-exec-next" ".*" "next over dlopen"
+
+# Now wait for the 'stopped' notification.  While we wait we should
+# see a 'library-loaded' notification for the loading of the shared
+# library.
+#
+# In older versions of GDB we would also see a 'breakpoint-modified'
+# notification for the dprintf breakpoint, but newer versions of GDB
+# are smart enough to not emit this unnecessary notification.
+set bp_re [mi_make_breakpoint -number $bpnum \
+	       -type dprintf -disp keep -enabled y -func main]
+set saw_bp_modified false
+set saw_lib_load false
+set saw_stopped false
+gdb_test_multiple "" "wait for 'next' to complete" {
+    -re "^=library-loaded,id=\[^\r\n\]+\r\n" {
+	set saw_lib_load true
+	exp_continue
+    }
+
+    -re "^=breakpoint-modified,$bp_re\r\n" {
+	set saw_bp_modified true
+	exp_continue
+    }
+
+    -re "^\\*stopped,reason=\"end-stepping-range\",\[^\r\n\]+\r\n" {
+	set saw_stopped true
+	exp_continue
+    }
+
+    -re "^$mi_gdb_prompt$" {
+	gdb_assert { $saw_lib_load } \
+	    "$gdb_test_name, library was loaded"
+	gdb_assert { $saw_stopped } \
+	    "$gdb_test_name, saw stopped message"
+	gdb_assert { !$saw_bp_modified } \
+	    "$gdb_test_name, no breakpoint-modified"
+    }
+
+    -re "^\[^\r\n\]+\r\n" {
+	exp_continue
+    }
+}