[PATCH v3 1/5] [gdb/python] Introduce py_{none, true, false, notimplemented} functions

[Tom de Vries <tdevries@suse.de>]

I came across this code:
...
   if (c)
     return v;
   else
     Py_RETURN_NONE;
...
and realized I couldn't easily rewrite it into "return c ? v : ...".

Probably something like this would work:
...
  return c ? v : ([] { Py_RETURN_NONE; } ());
...
but it made me wonder if we can rid of Py_RETURN_NONE.

The only point of it seems to be to increase the reference count on the
Py_None object for older python versions.

Add a refcount-safe wrapper py_none (returning a gdbpy_ref), with the aim of
replacing all uses of Py_RETURN_NONE with "return py_none ().release ()".

Likewise for Py_RETURN_TRUE/Py_RETURN_FALSE/Py_RETURN_NOTIMPLEMENTED.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34145
---
 gdb/python/python-internal.h | 44 ++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/gdb/python/python-internal.h b/gdb/python/python-internal.h
index 3147156d5be..7c58309e4d4 100644
--- a/gdb/python/python-internal.h
+++ b/gdb/python/python-internal.h
@@ -1343,4 +1343,48 @@ class gdbpy_memoizing_registry_storage
 extern int eval_python_command (const char *command, int start_symbol,
 				const char *filename = nullptr);
 
+/* The following four functions are refcount-safe wrappers around
+   Py_RETURN_{NONE,TRUE,FALSE,NOTIMPLEMENTED}.
+
+   Starting with python 3.12, None, True, False and Py_NotImplemented are
+   immortal, and increasing and decreasing refcount for such objects is a
+   no-op, and consequently for the limited C API 3.12 and newer,
+   Py_RETURN_NONE is simply "return Py_None".
+
+   So for the limited C API 3.12 and newer, we could just return a
+   "PyObject *" instead.
+
+   For the limited C API 3.12 and newer, while returning gdbpy_ref<> there's
+   an imbalance (we do a Py_DECREF in gdbpy_ref_policy::decref, without
+   corresponding Py_INCREF in Py_RETURN_NONE), but this is not harmful because
+   Py_DECREF is no-op for immortal objects.  */
+
+static inline gdbpy_ref<>
+py_none ()
+{
+  auto f = [] { Py_RETURN_NONE; };
+  return gdbpy_ref<> (f ());
+}
+
+static inline gdbpy_ref<>
+py_true ()
+{
+  auto f = [] { Py_RETURN_TRUE; };
+  return gdbpy_ref<> (f ());
+}
+
+static inline gdbpy_ref<>
+py_false ()
+{
+  auto f = [] { Py_RETURN_FALSE; };
+  return gdbpy_ref<> (f ());
+}
+
+static inline gdbpy_ref<>
+py_notimplemented ()
+{
+  auto f = [] { Py_RETURN_NOTIMPLEMENTED; };
+  return gdbpy_ref<> (f ());
+}
+
 #endif /* GDB_PYTHON_PYTHON_INTERNAL_H */

base-commit: 79fcceafcbfe9aa02441445eb9444718b17330df
-- 
2.51.0