gdb/python: add gdb.InferiorThread.__repr__() method

Add a gdb.InferiorThread.__repr__() method.  Before this patch we
would see output like this:

  (gdb) pi
  >>> gdb.selected_thread()
  <gdb.InferiorThread object at 0x7f4dcc49b970>

After this patch, we now see:

  (gdb) pi
  >>> gdb.selected_thread()
  <gdb.InferiorThread id=1.2 target-id="Thread 0x7ffff7da1700 (LWP 458134)">

More verbose, but, I hope, more useful.

If the gdb.InferiorThread becomes invalid, then we will see:

  (gdb) pi
  >>> invalid_thread_variable
  <gdb.InferiorThread (invalid)>

Which is inline with how other invalid objects are displayed.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/python/py-infthread.c

diff --git a/gdb/python/py-infthread.c b/gdb/python/py-infthread.c
index 632984d..b5887c7 100644
--- a/gdb/python/py-infthread.c
+++ b/gdb/python/py-infthread.c
@@ -337,6 +337,23 @@ thpy_thread_handle (PyObject *self, PyObject *args)
   return object;
 }
 
+/* Implement repr() for gdb.InferiorThread.  */
+
+static PyObject *
+thpy_repr (PyObject *self)
+{
+  thread_object *thread_obj = (thread_object *) self;
+
+  if (thread_obj->thread == nullptr)
+    return gdb_py_invalid_object_repr (self);
+
+  thread_info *thr = thread_obj->thread;
+  return PyUnicode_FromFormat ("<%s id=%s target-id=\"%s\">",
+			       Py_TYPE (self)->tp_name,
+			       print_full_thread_id (thr),
+			       target_pid_to_str (thr->ptid).c_str ());
+}
+
 /* Return a reference to a new Python object representing a ptid_t.
    The object is a tuple containing (pid, lwp, tid). */
 PyObject *
@@ -456,7 +473,7 @@ PyTypeObject thread_object_type =
   0,				  /*tp_getattr*/
   0,				  /*tp_setattr*/
   0,				  /*tp_compare*/
-  0,				  /*tp_repr*/
+  thpy_repr,			  /*tp_repr*/
   0,				  /*tp_as_number*/
   0,				  /*tp_as_sequence*/
   0,				  /*tp_as_mapping*/

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

diff --git a/gdb/testsuite/gdb.python/py-inferior.exp b/gdb/testsuite/gdb.python/py-inferior.exp
index 6fbcdd6..5a221f8 100644
--- a/gdb/testsuite/gdb.python/py-inferior.exp
+++ b/gdb/testsuite/gdb.python/py-inferior.exp
@@ -78,7 +78,9 @@
     "Inferior.connection_num equals Inferior.connection.num"
 gdb_test "python print ('result = %s' % i0.pid)" " = \[0-9\]+" "test Inferior.pid"
 gdb_test "python print ('result = %s' % i0.was_attached)" " = False" "test Inferior.was_attached"
-gdb_test "python print (i0.threads ())" "\\(<gdb.InferiorThread object at 0x\[\[:xdigit:\]\]+>,\\)" "test Inferior.threads"
+gdb_test "python print (i0.threads ())" \
+    "\\(<gdb.InferiorThread id=${decimal}\\.${decimal} target-id=\"\[^\r\n\]*\">,\\)" \
+    "test Inferior.threads"
 
 gdb_test "python print (i0.progspace)" "<gdb.Progspace object at $hex>"
 gdb_test "python print (i0.progspace == gdb.progspaces()\[0\])" "True"
@@ -89,11 +91,23 @@
 gdb_continue_to_breakpoint "cont to check_threads" ".*pthread_barrier_wait.*"
 gdb_test "python print (len (i0.threads ()))" "\r\n9" "test Inferior.threads 2"
 
+# Grab the last thread from the list.  This thread object will become
+# invalid when the corresponding thread exits.
+gdb_test_no_output "python last_thread = i0.threads()\[-1\]"
+gdb_test "python print(last_thread)" \
+    "<gdb.InferiorThread id=${decimal}\\.${decimal} target-id=\"\[^\r\n\]*\">" \
+    "test repr of a valid thread"
+
 # Proceed to the next test.
 
 gdb_breakpoint [gdb_get_line_number "Break here."]
 gdb_continue_to_breakpoint "cont to Break here." ".*Break here\..*"
 
+# Check the repr() for an invalid gdb.InferiorThread object.
+gdb_test "python print(last_thread)" \
+    "<gdb.InferiorThread \\(invalid\\)>" \
+    "test repr of an invalid thread"
+
 # Test memory read and write operations.
 
 gdb_py_test_silent_cmd "python addr = gdb.selected_frame ().read_var ('str')" \

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

diff --git a/gdb/testsuite/gdb.python/py-infthread.exp b/gdb/testsuite/gdb.python/py-infthread.exp
index 3b07d84..3ae5a45 100644
--- a/gdb/testsuite/gdb.python/py-infthread.exp
+++ b/gdb/testsuite/gdb.python/py-infthread.exp
@@ -57,7 +57,8 @@
 
 
 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 (t0)" \
+    "<gdb.InferiorThread id=${decimal}\\.${decimal} target-id=\"\[^\r\n\]*\">"
 gdb_test "python print ('result = %s' % t0.num)" " = 1" "test InferiorThread.num"
 gdb_test "python print ('result = %s' % t0.global_num)" " = 1" "test InferiorThread.global_num"
 gdb_test "python print ('result = %s' % str (t0.ptid))" " = \\(\[0-9\]+, \[0-9\]+, \[0-9\]+\\)" "test InferiorThread.ptid"