[PATCH] Make gdb.selected_thread().inferior return a new reference

[Maksim Dzabraev <dzabraew@gmail.com>]

thpy_get_inferior function should return a new reference to the
existing inferior object, and therefore should increment its refcount.

Fixed bug lokks like this.
If multiple time call gdb.selected_thread ().inferior, gdb throws exception:

(gdb) pi gdb.selected_thread().inferior
<gdb.Inferior object at 0x7f1952bea698>
(gdb) pi gdb.selected_thread().inferior
Python Exception <type 'exceptions.AttributeError'> 'NoneType' object
has no attribute 'inferior':
Error while executing Python code.
(gdb) info threads
  Id   Target Id         Frame
* 1    Thread 0x7f54f0474740 (LWP 584) "mc" 0x00007f54ef055c33 in
---
 gdb/ChangeLog                             | 6 ++++++
 gdb/python/py-infthread.c                 | 1 +
 gdb/testsuite/ChangeLog                   | 6 ++++++
 gdb/testsuite/gdb.python/py-infthread.exp | 9 +++++++++
 4 files changed, 22 insertions(+)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 9fe632985e..4583a78fe5 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2017-10-28  Maksim Dzabraev  <dzabraew@gmail.com>
+
+	PR python/21213
+	* python/py-infthread.c (thpy_get_inferior): Increment reference
+	of inferior before returning it.
+
 2017-10-27  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* unittests/common-utils-selftests.c (format): Add
diff --git a/gdb/python/py-infthread.c b/gdb/python/py-infthread.c
index 626c15cedb..31d576f7c7 100644
--- a/gdb/python/py-infthread.c
+++ b/gdb/python/py-infthread.c
@@ -162,6 +162,7 @@ thpy_get_inferior (PyObject *self, void *ignore)
   thread_object *thread_obj = (thread_object *) self;
 
   THPY_REQUIRE_VALID (thread_obj);
+  Py_INCREF (thread_obj->inf_obj);
 
   return thread_obj->inf_obj;
 }
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 0254a28550..72334fd3e2 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2017-10-28 Maksim Dzabraev <dzabraew@gmail.com>
+
+	PR python/21213
+	* gdb.python/py-infthread.exp: Test calling
+	InferiorThread.inferior () multiple times in a row.
+
 2017-10-27  Keith Seitz  <keiths@redhat.com>
 
 	* gdb.opt/inline-break.exp (break_info_1): New procedure.
diff --git a/gdb/testsuite/gdb.python/py-infthread.exp b/gdb/testsuite/gdb.python/py-infthread.exp
index 0711d6994e..69fc9709b2 100644
--- a/gdb/testsuite/gdb.python/py-infthread.exp
+++ b/gdb/testsuite/gdb.python/py-infthread.exp
@@ -51,6 +51,15 @@ gdb_test "python print(seen_a_thread)" "True"
 
 # Test basic gdb.Inferior attributes and methods.
 
+# Make sure that InferiorThread.inferior returns a new reference (see PR 21213).
+
+gdb_py_test_silent_cmd "python gdb.selected_thread ().inferior" "test InferiorThread.inferior 1" 1
+gdb_py_test_silent_cmd "python gdb.selected_thread ().inferior" "test InferiorThread.inferior 2" 1
+gdb_test_no_output "python import gc; gc.collect()" "call Python garbage collection"
+gdb_py_test_silent_cmd "python gdb.selected_thread ().inferior" "test InferiorThread.inferior 3" 1
+gdb_py_test_silent_cmd "python gdb.selected_thread ().inferior" "test InferiorThread.inferior 4" 1
+
+
 gdb_py_test_silent_cmd "python t0 = gdb.selected_thread ()" "test gdb.selected_thread" 1
 gdb_test "python print (t0)" "\\<gdb.InferiorThread object at 0x\[\[:xdigit:\]\]+>" "verify InferiorThread object"
 gdb_test "python print ('result = %s' % t0.num)" " = 1" "test InferiorThread.num"
-- 
2.13.0