Allow a pretty-printer without a to_string method

PR python/16047 points out that, while the documentation says that the
to_string method is optional for a pretty-printer, the code disagrees
and throws an exception.  This patch fixes the problem.  varobj is
already ok here.

Tested on x86-64 Fedora 26.

gdb/ChangeLog
2018-09-08  Tom Tromey  <tom@tromey.com>

	PR python/16047:
	* python/py-prettyprint.c (pretty_print_one_value): Check for
	to_string method.

gdb/testsuite/ChangeLog
2018-09-08  Tom Tromey  <tom@tromey.com>

	PR python/16047:
	* gdb.python/py-prettyprint.py (pp_int_typedef3): New class.
	(register_pretty_printers): Register new printer.
	* gdb.python/py-prettyprint.exp (run_lang_tests): Add int_type3
	test.
	* gdb.python/py-prettyprint.c (int_type3): New typedef.
	(an_int_type3): New global.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2018-09-08  Tom Tromey  <tom@tromey.com>
+
+	PR python/16047:
+	* python/py-prettyprint.c (pretty_print_one_value): Check for
+	to_string method.
+
 2018-09-08  Joel Brobecker  <brobecker@adacore.com>
 
 	* ada-lang.c (resolve_subexp): Pass correct OPLEN in call to

gdb/python/py-prettyprint.c

@@ -195,18 +195,23 @@ pretty_print_one_value (PyObject *printer, struct value **out_value)
   *out_value = NULL;
   TRY
     {
-      result.reset (PyObject_CallMethodObjArgs (printer, gdbpy_to_string_cst,
-						NULL));
-      if (result != NULL)
+      if (!PyObject_HasAttr (printer, gdbpy_to_string_cst))
+	result = gdbpy_ref<>::new_reference (Py_None);
+      else
 	{
-	  if (! gdbpy_is_string (result.get ())
-	      && ! gdbpy_is_lazy_string (result.get ())
-	      && result != Py_None)
+	  result.reset (PyObject_CallMethodObjArgs (printer, gdbpy_to_string_cst,
+						    NULL));
+	  if (result != NULL)
 	    {
-	      *out_value = convert_value_from_python (result.get ());
-	      if (PyErr_Occurred ())
-		*out_value = NULL;
-	      result = NULL;
+	      if (! gdbpy_is_string (result.get ())
+		  && ! gdbpy_is_lazy_string (result.get ())
+		  && result != Py_None)
+		{
+		  *out_value = convert_value_from_python (result.get ());
+		  if (PyErr_Occurred ())
+		    *out_value = NULL;
+		  result = NULL;
+		}
 	    }
 	}
     }

gdb/testsuite/ChangeLog

@@ -1,3 +1,13 @@
+2018-09-08  Tom Tromey  <tom@tromey.com>
+
+	PR python/16047:
+	* gdb.python/py-prettyprint.py (pp_int_typedef3): New class.
+	(register_pretty_printers): Register new printer.
+	* gdb.python/py-prettyprint.exp (run_lang_tests): Add int_type3
+	test.
+	* gdb.python/py-prettyprint.c (int_type3): New typedef.
+	(an_int_type3): New global.
+
 2018-09-08  Joel Brobecker  <brobecker@adacore.com>
 
 	* gdb.ada/expr_with_funcall: New testcase.

gdb/testsuite/gdb.python/py-prettyprint.c

@@ -271,10 +271,12 @@ bug_14741()
    when looking for a printer.  */
 typedef int int_type;
 typedef int_type int_type2;
+typedef int_type int_type3;
 
 int an_int = -1;
 int_type an_int_type = 1;
 int_type2 an_int_type2 = 2;
+int_type3 an_int_type3 = 3;
 
 int
 main ()

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

@@ -127,6 +127,10 @@ proc run_lang_tests {exefile lang} {
     gdb_test "print (int_type) an_int_type2" " = type=int_type, val=2"
     gdb_test "print (int_type2) an_int_type2" " = type=int_type2, val=2"
 
+    # PR python/16047: it is ok for a pretty printer not to have a
+    # to_string method.
+    gdb_test "print (int_type3) an_int_type2" " = {s = 27}"
+
     gdb_continue_to_end
 }
 

gdb/testsuite/gdb.python/py-prettyprint.py

@@ -253,6 +253,15 @@ class pp_int_typedef (object):
     def to_string(self):
         return "type=%s, val=%s" % (self.val.type, int(self.val))
 
+class pp_int_typedef3 (object):
+    "A printer without a to_string method"
+
+    def __init__(self, val):
+        self.val = val
+
+    def children(self):
+        yield 's', 27
+
 def lookup_function (val):
     "Look-up and return a pretty-printer that can print val."
 
@@ -362,6 +371,7 @@ def register_pretty_printers ():
 
     typedefs_pretty_printers_dict[re.compile ('^int_type$')] = pp_int_typedef
     typedefs_pretty_printers_dict[re.compile ('^int_type2$')] = pp_int_typedef
+    typedefs_pretty_printers_dict[re.compile ('^int_type3$')] = pp_int_typedef3
 
 # Dict for struct types with typedefs fully stripped.
 pretty_printers_dict = {}