python-gdb.py: enhance py-bt command

* Add py-bt-full command
* py-bt now gives an output similar to a regular Python traceback
* py-bt indicates:

  - if the garbage collector is running
  - if the thread is waiting for the GIL
  - detect PyCFunction_Call to get the name of the builtin function

Lib/test/test_gdb.py

@@ -13,6 +13,12 @@ import sysconfig
 from test import test_support
 from test.test_support import run_unittest, findfile
 
+# Is this Python configured to support threads?
+try:
+    import thread
+except ImportError:
+    thread = None
+
 def get_gdb_version():
     try:
         proc = subprocess.Popen(["gdb", "-nx", "--version"],
@@ -728,20 +734,133 @@ $''')
 class PyBtTests(DebuggerTests):
     @unittest.skipIf(python_is_optimized(),
                      "Python was compiled with optimizations")
-    def test_basic_command(self):
+    def test_bt(self):
         'Verify that the "py-bt" command works'
         bt = self.get_stack_trace(script=self.get_sample_script(),
                                   cmds_after_breakpoint=['py-bt'])
         self.assertMultilineMatches(bt,
                                     r'''^.*
-#[0-9]+ Frame 0x[0-9a-f]+, for file .*gdb_sample.py, line 7, in bar \(a=1, b=2, c=3\)
+Traceback \(most recent call first\):
+  File ".*gdb_sample.py", line 10, in baz
+    print\(42\)
+  File ".*gdb_sample.py", line 7, in bar
     baz\(a, b, c\)
-#[0-9]+ Frame 0x[0-9a-f]+, for file .*gdb_sample.py, line 4, in foo \(a=1, b=2, c=3\)
+  File ".*gdb_sample.py", line 4, in foo
     bar\(a, b, c\)
-#[0-9]+ Frame 0x[0-9a-f]+, for file .*gdb_sample.py, line 12, in <module> \(\)
+  File ".*gdb_sample.py", line 12, in <module>
     foo\(1, 2, 3\)
 ''')
 
+    @unittest.skipIf(python_is_optimized(),
+                     "Python was compiled with optimizations")
+    def test_bt_full(self):
+        'Verify that the "py-bt-full" command works'
+        bt = self.get_stack_trace(script=self.get_sample_script(),
+                                  cmds_after_breakpoint=['py-bt-full'])
+        self.assertMultilineMatches(bt,
+                                    r'''^.*
+#[0-9]+ Frame 0x-?[0-9a-f]+, for file .*gdb_sample.py, line 7, in bar \(a=1, b=2, c=3\)
+    baz\(a, b, c\)
+#[0-9]+ Frame 0x-?[0-9a-f]+, for file .*gdb_sample.py, line 4, in foo \(a=1, b=2, c=3\)
+    bar\(a, b, c\)
+#[0-9]+ Frame 0x-?[0-9a-f]+, for file .*gdb_sample.py, line 12, in <module> \(\)
+    foo\(1, 2, 3\)
+''')
+
+    @unittest.skipUnless(thread,
+                         "Python was compiled without thread support")
+    def test_threads(self):
+        'Verify that "py-bt" indicates threads that are waiting for the GIL'
+        cmd = '''
+from threading import Thread
+
+class TestThread(Thread):
+    # These threads would run forever, but we'll interrupt things with the
+    # debugger
+    def run(self):
+        i = 0
+        while 1:
+             i += 1
+
+t = {}
+for i in range(4):
+   t[i] = TestThread()
+   t[i].start()
+
+# Trigger a breakpoint on the main thread
+print 42
+
+'''
+        # Verify with "py-bt":
+        gdb_output = self.get_stack_trace(cmd,
+                                          cmds_after_breakpoint=['thread apply all py-bt'])
+        self.assertIn('Waiting for the GIL', gdb_output)
+
+        # Verify with "py-bt-full":
+        gdb_output = self.get_stack_trace(cmd,
+                                          cmds_after_breakpoint=['thread apply all py-bt-full'])
+        self.assertIn('Waiting for the GIL', gdb_output)
+
+    @unittest.skipIf(python_is_optimized(),
+                     "Python was compiled with optimizations")
+    # Some older versions of gdb will fail with
+    #  "Cannot find new threads: generic error"
+    # unless we add LD_PRELOAD=PATH-TO-libpthread.so.1 as a workaround
+    @unittest.skipUnless(thread,
+                         "Python was compiled without thread support")
+    def test_gc(self):
+        'Verify that "py-bt" indicates if a thread is garbage-collecting'
+        cmd = ('from gc import collect\n'
+               'print 42\n'
+               'def foo():\n'
+               '    collect()\n'
+               'def bar():\n'
+               '    foo()\n'
+               'bar()\n')
+        # Verify with "py-bt":
+        gdb_output = self.get_stack_trace(cmd,
+                                          cmds_after_breakpoint=['break update_refs', 'continue', 'py-bt'],
+                                          )
+        self.assertIn('Garbage-collecting', gdb_output)
+
+        # Verify with "py-bt-full":
+        gdb_output = self.get_stack_trace(cmd,
+                                          cmds_after_breakpoint=['break update_refs', 'continue', 'py-bt-full'],
+                                          )
+        self.assertIn('Garbage-collecting', gdb_output)
+
+    @unittest.skipIf(python_is_optimized(),
+                     "Python was compiled with optimizations")
+    # Some older versions of gdb will fail with
+    #  "Cannot find new threads: generic error"
+    # unless we add LD_PRELOAD=PATH-TO-libpthread.so.1 as a workaround
+    @unittest.skipUnless(thread,
+                         "Python was compiled without thread support")
+    def test_pycfunction(self):
+        'Verify that "py-bt" displays invocations of PyCFunction instances'
+        # Tested function must not be defined with METH_NOARGS or METH_O,
+        # otherwise call_function() doesn't call PyCFunction_Call()
+        cmd = ('from time import gmtime\n'
+               'def foo():\n'
+               '    gmtime(1)\n'
+               'def bar():\n'
+               '    foo()\n'
+               'bar()\n')
+        # Verify with "py-bt":
+        gdb_output = self.get_stack_trace(cmd,
+                                          breakpoint='time_gmtime',
+                                          cmds_after_breakpoint=['bt', 'py-bt'],
+                                          )
+        self.assertIn('<built-in function gmtime', gdb_output)
+
+        # Verify with "py-bt-full":
+        gdb_output = self.get_stack_trace(cmd,
+                                          breakpoint='time_gmtime',
+                                          cmds_after_breakpoint=['py-bt-full'],
+                                          )
+        self.assertIn('#0 <built-in function gmtime', gdb_output)
+
+
 class PyPrintTests(DebuggerTests):
     @unittest.skipIf(python_is_optimized(),
                      "Python was compiled with optimizations")

Tools/gdb/libpython.py

@@ -45,6 +45,7 @@ The module also extends gdb with some python-specific commands.
 
 from __future__ import print_function, with_statement
 import gdb
+import locale
 import os
 import sys
 
@@ -76,6 +77,8 @@ Py_TPFLAGS_TYPE_SUBCLASS     = (1 << 31)
 
 MAX_OUTPUT_LEN=1024
 
+ENCODING = locale.getpreferredencoding()
+
 class NullPyObjectPtr(RuntimeError):
     pass
 
@@ -92,6 +95,18 @@ def safe_range(val):
     # threshold in case the data was corrupted
     return xrange(safety_limit(int(val)))
 
+if sys.version_info[0] >= 3:
+    def write_unicode(file, text):
+        file.write(text)
+else:
+    def write_unicode(file, text):
+        # Write a byte or unicode string to file. Unicode strings are encoded to
+        # ENCODING encoding with 'backslashreplace' error handler to avoid
+        # UnicodeEncodeError.
+        if isinstance(text, unicode):
+            text = text.encode(ENCODING, 'backslashreplace')
+        file.write(text)
+
 
 class StringTruncated(RuntimeError):
     pass
@@ -903,7 +918,12 @@ class PyFrameObjectPtr(PyObjectPtr):
         newline character'''
         if self.is_optimized_out():
             return '(frame information optimized out)'
-        with open(self.filename(), 'r') as f:
+        filename = self.filename()
+        try:
+            f = open(filename, 'r')
+        except IOError:
+            return None
+        with f:
             all_lines = f.readlines()
             # Convert from 1-based current_line_num to 0-based list offset:
             return all_lines[self.current_line_num()-1]
@@ -914,9 +934,9 @@ class PyFrameObjectPtr(PyObjectPtr):
             return
         out.write('Frame 0x%x, for file %s, line %i, in %s ('
                   % (self.as_address(),
-                     self.co_filename,
+                     self.co_filename.proxyval(visited),
                      self.current_line_num(),
-                     self.co_name))
+                     self.co_name.proxyval(visited)))
         first = True
         for pyop_name, pyop_value in self.iter_locals():
             if not first:
@@ -929,6 +949,16 @@ class PyFrameObjectPtr(PyObjectPtr):
 
         out.write(')')
 
+    def print_traceback(self):
+        if self.is_optimized_out():
+            sys.stdout.write('  (frame information optimized out)\n')
+            return
+        visited = set()
+        sys.stdout.write('  File "%s", line %i, in %s\n'
+                  % (self.co_filename.proxyval(visited),
+                     self.current_line_num(),
+                     self.co_name.proxyval(visited)))
+
 class PySetObjectPtr(PyObjectPtr):
     _typename = 'PySetObject'
 
@@ -1222,6 +1252,23 @@ class Frame(object):
             iter_frame = iter_frame.newer()
         return index
 
+    # We divide frames into:
+    #   - "python frames":
+    #       - "bytecode frames" i.e. PyEval_EvalFrameEx
+    #       - "other python frames": things that are of interest from a python
+    #         POV, but aren't bytecode (e.g. GC, GIL)
+    #   - everything else
+
+    def is_python_frame(self):
+        '''Is this a PyEval_EvalFrameEx frame, or some other important
+        frame? (see is_other_python_frame for what "important" means in this
+        context)'''
+        if self.is_evalframeex():
+            return True
+        if self.is_other_python_frame():
+            return True
+        return False
+
     def is_evalframeex(self):
         '''Is this a PyEval_EvalFrameEx frame?'''
         if self._gdbframe.name() == 'PyEval_EvalFrameEx':
@@ -1238,6 +1285,50 @@ class Frame(object):
 
         return False
 
+    def is_other_python_frame(self):
+        '''Is this frame worth displaying in python backtraces?
+        Examples:
+          - waiting on the GIL
+          - garbage-collecting
+          - within a CFunction
+         If it is, return a descriptive string
+         For other frames, return False
+         '''
+        if self.is_waiting_for_gil():
+            return 'Waiting for the GIL'
+        elif self.is_gc_collect():
+            return 'Garbage-collecting'
+        else:
+            # Detect invocations of PyCFunction instances:
+            older = self.older()
+            if older and older._gdbframe.name() == 'PyCFunction_Call':
+                # Within that frame:
+                #   "func" is the local containing the PyObject* of the
+                # PyCFunctionObject instance
+                #   "f" is the same value, but cast to (PyCFunctionObject*)
+                #   "self" is the (PyObject*) of the 'self'
+                try:
+                    # Use the prettyprinter for the func:
+                    func = older._gdbframe.read_var('func')
+                    return str(func)
+                except RuntimeError:
+                    return 'PyCFunction invocation (unable to read "func")'
+
+        # This frame isn't worth reporting:
+        return False
+
+    def is_waiting_for_gil(self):
+        '''Is this frame waiting on the GIL?'''
+        # This assumes the _POSIX_THREADS version of Python/ceval_gil.h:
+        name = self._gdbframe.name()
+        if name:
+            return ('PyThread_acquire_lock' in name
+                    and 'lock_PyThread_acquire_lock' not in name)
+
+    def is_gc_collect(self):
+        '''Is this frame "collect" within the garbage-collector?'''
+        return self._gdbframe.name() == 'collect'
+
     def get_pyop(self):
         try:
             f = self._gdbframe.read_var('f')
@@ -1267,8 +1358,22 @@ class Frame(object):
 
     @classmethod
     def get_selected_python_frame(cls):
-        '''Try to obtain the Frame for the python code in the selected frame,
-        or None'''
+        '''Try to obtain the Frame for the python-related code in the selected
+        frame, or None'''
+        frame = cls.get_selected_frame()
+
+        while frame:
+            if frame.is_python_frame():
+                return frame
+            frame = frame.older()
+
+        # Not found:
+        return None
+
+    @classmethod
+    def get_selected_bytecode_frame(cls):
+        '''Try to obtain the Frame for the python bytecode interpreter in the
+        selected GDB frame, or None'''
         frame = cls.get_selected_frame()
 
         while frame:
@@ -1283,14 +1388,38 @@ class Frame(object):
         if self.is_evalframeex():
             pyop = self.get_pyop()
             if pyop:
-                sys.stdout.write('#%i %s\n' % (self.get_index(), pyop.get_truncated_repr(MAX_OUTPUT_LEN)))
+                line = pyop.get_truncated_repr(MAX_OUTPUT_LEN)
+                write_unicode(sys.stdout, '#%i %s\n' % (self.get_index(), line))
                 if not pyop.is_optimized_out():
                     line = pyop.current_line()
-                    sys.stdout.write('    %s\n' % line.strip())
+                    if line is not None:
+                        sys.stdout.write('    %s\n' % line.strip())
             else:
                 sys.stdout.write('#%i (unable to read python frame information)\n' % self.get_index())
         else:
-            sys.stdout.write('#%i\n' % self.get_index())
+            info = self.is_other_python_frame()
+            if info:
+                sys.stdout.write('#%i %s\n' % (self.get_index(), info))
+            else:
+                sys.stdout.write('#%i\n' % self.get_index())
+
+    def print_traceback(self):
+        if self.is_evalframeex():
+            pyop = self.get_pyop()
+            if pyop:
+                pyop.print_traceback()
+                if not pyop.is_optimized_out():
+                    line = pyop.current_line()
+                    if line is not None:
+                        sys.stdout.write('    %s\n' % line.strip())
+            else:
+                sys.stdout.write('  (unable to read python frame information)\n')
+        else:
+            info = self.is_other_python_frame()
+            if info:
+                sys.stdout.write('  %s\n' % info)
+            else:
+                sys.stdout.write('  (not a python frame)\n')
 
 class PyList(gdb.Command):
     '''List the current Python source code, if any
@@ -1326,9 +1455,10 @@ class PyList(gdb.Command):
         if m:
             start, end = map(int, m.groups())
 
-        frame = Frame.get_selected_python_frame()
+        # py-list requires an actual PyEval_EvalFrameEx frame:
+        frame = Frame.get_selected_bytecode_frame()
         if not frame:
-            print('Unable to locate python frame')
+            print('Unable to locate gdb frame for python bytecode interpreter')
             return
 
         pyop = frame.get_pyop()
@@ -1346,7 +1476,13 @@ class PyList(gdb.Command):
         if start<1:
             start = 1
 
-        with open(filename, 'r') as f:
+        try:
+            f = open(filename, 'r')
+        except IOError as err:
+            sys.stdout.write('Unable to open %s: %s\n'
+                             % (filename, err))
+            return
+        with f:
             all_lines = f.readlines()
             # start and end are 1-based, all_lines is 0-based;
             # so [start-1:end] as a python slice gives us [start, end] as a
@@ -1374,7 +1510,7 @@ def move_in_stack(move_up):
         if not iter_frame:
             break
 
-        if iter_frame.is_evalframeex():
+        if iter_frame.is_python_frame():
             # Result:
             if iter_frame.select():
                 iter_frame.print_summary()
@@ -1416,6 +1552,24 @@ if hasattr(gdb.Frame, 'select'):
     PyUp()
     PyDown()
 
+class PyBacktraceFull(gdb.Command):
+    'Display the current python frame and all the frames within its call stack (if any)'
+    def __init__(self):
+        gdb.Command.__init__ (self,
+                              "py-bt-full",
+                              gdb.COMMAND_STACK,
+                              gdb.COMPLETE_NONE)
+
+
+    def invoke(self, args, from_tty):
+        frame = Frame.get_selected_python_frame()
+        while frame:
+            if frame.is_python_frame():
+                frame.print_summary()
+            frame = frame.older()
+
+PyBacktraceFull()
+
 class PyBacktrace(gdb.Command):
     'Display the current python frame and all the frames within its call stack (if any)'
     def __init__(self):
@@ -1426,10 +1580,11 @@ class PyBacktrace(gdb.Command):
 
 
     def invoke(self, args, from_tty):
+        sys.stdout.write('Traceback (most recent call first):\n')
         frame = Frame.get_selected_python_frame()
         while frame:
-            if frame.is_evalframeex():
-                frame.print_summary()
+            if frame.is_python_frame():
+                frame.print_traceback()
             frame = frame.older()
 
 PyBacktrace()