Fixes #460 - getting SP reg on threaded apps (#463)

* Fixes #460 - getting SP reg on threaded apps

As the issue described: as we cache registers,
we might get their values wrong as we don't invalidate cache when thread is changed.

This leads to showing wrong context stack values in threaded apps.

This commit/PR adds a new memoization solution: `reset_on_prompt` which resets cache on `gdb.events.before_prompt` event.

* Fix isort

pwndbg/events.py

@@ -10,8 +10,9 @@ from __future__ import division
 from __future__ import print_function
 from __future__ import unicode_literals
 
-import functools
 import sys
+from functools import partial
+from functools import wraps
 
 import gdb
 
@@ -65,11 +66,14 @@ gdb.events.start = StartEvent()
 
 # In order to support reloading, we must be able to re-fire
 # all 'objfile' and 'stop' events.
-registered = {gdb.events.exited: [],
-              gdb.events.cont: [],
-              gdb.events.new_objfile: [],
-              gdb.events.stop: [],
-              gdb.events.start: []}
+registered = {
+    gdb.events.exited: [],
+    gdb.events.cont: [],
+    gdb.events.new_objfile: [],
+    gdb.events.stop: [],
+    gdb.events.start: [],
+    gdb.events.before_prompt: []
+}
 
 # GDB 7.9 and above only
 try:
@@ -96,7 +100,7 @@ def connect(func, event_handler, name=''):
     if debug:
         print("Connecting", func.__name__, event_handler)
 
-    @functools.wraps(func)
+    @wraps(func)
     def caller(*a):
         if debug:
             sys.stdout.write('%r %s.%s %r\n' % (name, func.__module__, func.__name__, a))
@@ -131,6 +135,9 @@ def cont(func):        return connect(func, gdb.events.cont, 'cont')
 def new_objfile(func): return connect(func, gdb.events.new_objfile, 'obj')
 def stop(func):        return connect(func, gdb.events.stop, 'stop')
 def start(func):       return connect(func, gdb.events.start, 'start')
+
+before_prompt = partial(connect, event_handler=gdb.events.before_prompt, name='before_prompt')
+
 def reg_changed(func):
     try:
         return connect(func, gdb.events.register_changed, 'reg_changed')

pwndbg/memoize.py

@@ -96,6 +96,19 @@ class reset_on_stop(memoize):
     _reset = __reset_on_stop
 
 
+class reset_on_prompt(memoize):
+    caches = []
+    kind   = 'prompt'
+
+    @staticmethod
+    @pwndbg.events.before_prompt
+    def __reset_on_prompt():
+        for obj in reset_on_prompt.caches:
+            obj.cache.clear()
+
+    _reset = __reset_on_prompt
+
+
 class reset_on_exit(memoize):
     caches = []
     kind   = 'exit'

pwndbg/regs.py

@@ -266,7 +266,7 @@ ARCH_GET_GS = 0x1004
 class module(ModuleType):
     last = {}
 
-    @pwndbg.memoize.reset_on_stop
+    @pwndbg.memoize.reset_on_prompt
     def __getattr__(self, attr):
         attr = attr.lstrip('$')
         try: