Allow pretty-printer 'children' method to return strings

A user noticed that, while a pretty-printer can return Python strings
from its "children" method, this does not really work for MI.  I
tracked this down to my_value_of_variable calling into
c_value_of_variable, which specially handles arrays and structures --
not using the actual contents of the string.

Now, this part of MI seems bad to me, but rather than change that,
this applies the fix to only dynamic varobjs, which is the only
scenario where a string like this can really be returned.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=18282
Reviewed-by: Keith Seitz <keiths@redhat.com>

gdb/testsuite/gdb.python/py-varobj.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/>.  */
+
+struct test {
+  int x;
+};
+
+struct test tval = {23};
+
+int main () {
+  return 0;
+}

gdb/testsuite/gdb.python/py-varobj.exp

@@ -0,0 +1,49 @@
+# Copyright (C) 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/>.
+
+# Some varobj tests involving pretty-printers
+
+load_lib mi-support.exp
+set MIFLAGS "-i=mi"
+
+require allow_python_tests
+
+standard_testfile
+
+if {[gdb_compile "$srcdir/$subdir/$srcfile" $binfile executable {debug}] != ""} {
+    return -1
+}
+
+mi_clean_restart $binfile
+
+set pyfile [gdb_remote_download host ${srcdir}/${subdir}/${testfile}.py]
+mi_gdb_test "source ${pyfile}" \
+  ".*\\^done" \
+  "load python file"
+
+mi_gdb_test "-enable-pretty-printing" \
+  "\\^done" \
+  "-enable-pretty-printing"
+
+mi_gdb_test "set python print-stack full" \
+  ".*\\^done" \
+  "set python print-stack full"
+
+mi_runto_main
+
+mi_gdb_test "-var-create tval * tval" \
+   "\\^done.*"
+
+mi_gdb_test "-var-list-children --all-values tval" \
+    ".*value=.*flicker.*"

gdb/testsuite/gdb.python/py-varobj.py

@@ -0,0 +1,35 @@
+# Copyright (C) 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/>.
+
+import gdb.printing
+
+
+class TestPrinter:
+    def __init__(self, val):
+        self.val = val
+
+    def to_string(self):
+        return "map"
+
+    def children(self):
+        yield "1", "flicker"
+
+
+def str_lookup_function(val):
+    lookup_tag = val.type.tag
+    if lookup_tag == "test":
+        return TestPrinter(val)
+
+
+gdb.printing.register_pretty_printer(None, str_lookup_function)