[PATCH 5/5] Add missing incref when creating Inferior Python object

[Simon Marchi <simon.marchi@ericsson.com>]

The test py-inferior.exp fails when using my debug build of Python 3.6.
I don't see it failing with my system's default Python, but it might be
related to the different memory allocation scheme used when doing a
build with pydebug.

The issue is that we are missing a Py_INCREF in
inferior_to_inferior_object.  The PyObject_New function initializes the
object with a refcount of 1.  If we assume that this refcount
corresponds to the reference we are keeping in the inferior data, then
we are missing an incref for the reference we are returning.  We can
also see it the other way.  If the refcount added by PyObject_New is for
the reference we are returning, then we are missing one for the inferior
data.

The counterpart for this incref is in py_free_inferior.

Here's how I can get it to crash:

  $ ./gdb -nx -ex "set debug python 1"
  (gdb) add-inferior
  Added inferior 2
  (gdb) python infs = gdb.inferiors()
  Creating Python Inferior object inf = 1
  Creating Python Inferior object inf = 2
  (gdb) remove-inferiors 2
  py_free_inferior inf = 2
  infpy_dealloc inf = <unknown>
  (gdb) python infs = None
  Fatal Python error: Objects/tupleobject.c:243 object at 0x7f9cf1a568d8 has negative ref count -1

  Current thread 0x00007f9cf1b68780 (most recent call first):
    File "<string>", line 1 in <module>
  [1]    408 abort (core dumped)  ./gdb -nx -ex "set debug python 1"

After having created the inferiors object, their refcount is 1 (which
comes from PyObject_New), but it should be two.  The gdb inferior object
has a reference and the "infs" list has a reference.

When invoking remove-inferiors, py_free_inferior gets called.  It does
the decref that corresponds to the reference that the gdb inferior
object kept.  At this moment, the refcount drops to 0 and the object
gets deallocated, even though the "infs" list still has a reference.
When we set "infs" to None, Python tries to decref the already zero
refcount and the assert triggers.

With this patch, it looks better:

  (gdb) add-inferior
  Added inferior 2
  (gdb) python infs = gdb.inferiors()
  Creating Python Inferior object inf = 1
  Creating Python Inferior object inf = 2
  (gdb) remove-inferiors 2
  py_free_inferior inf = 2
  (gdb) python infs = None
  infpy_dealloc inf = <unknown>

gdb/ChangeLog:

	* python/py-inferior.c (inferior_to_inferior_object): Manually
	increment reference count when creating the object as well.
---
 gdb/python/py-inferior.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/gdb/python/py-inferior.c b/gdb/python/py-inferior.c
index 340dddcfbd..24ef4f0ec8 100644
--- a/gdb/python/py-inferior.c
+++ b/gdb/python/py-inferior.c
@@ -227,10 +227,13 @@ inferior_to_inferior_object (struct inferior *inferior)
       inf_obj->threads = NULL;
       inf_obj->nthreads = 0;
 
+      /* PyObject_New initializes the new object with a refcount of 1.  This
+	 counts for the reference we are keeping in the inferior data.  */
       set_inferior_data (inferior, infpy_inf_data_key, inf_obj);
     }
-  else
-    Py_INCREF ((PyObject *)inf_obj);
+
+  /* We are returning a new reference.  */
+  Py_INCREF (inf_obj);
 
   return gdbpy_inf_ref (inf_obj);
 }
-- 
2.11.0