Basic co_bt gdb script for coro::Task

Summary:
This is mostly a POC. It will probably fail in some cases, but it's better than nothing.
Sample output:
  (gdb) co_bt this
  0x292d70 <zero()>
  0x293f50 <one()>
  0x295050 <two()>
  0x296150 <three()>
  0x297250 <folly::coro::TaskWithExecutor<int>::start() &&::{lambda(folly::Promise<int>, folly::coro::TaskWithExecutor<int>)#1}::operator()(folly::Promise<int>, folly::coro::TaskWithExecutor<int>) const>

Reviewed By: jwiepert

Differential Revision: D13727139

fbshipit-source-id: bff98eb4f5eb2ebd73c880d3b525172782f87511

folly/experimental/coro/scripts/gdb.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+
+
+import gdb
+
+
+class CoroBacktraceCommand(gdb.Command):
+    def __init__(self):
+        super(CoroBacktraceCommand, self).__init__("co_bt", gdb.COMMAND_USER)
+
+    def invoke(self, arg, from_tty):
+        if not arg:
+            print("coroutine_handle has to be passed to 'co_bt' command")
+            return
+        coroutine_handle = gdb.parse_and_eval(arg)
+        void_star_star = gdb.lookup_type("void").pointer().pointer()
+        coroutine_frame = (
+            coroutine_handle.cast(void_star_star).dereference().cast(void_star_star)
+        )
+        while coroutine_frame < 0xFFFFFFFFFFFF:
+            print(coroutine_frame.dereference())
+            coroutine_frame = (coroutine_frame + 2).dereference().cast(void_star_star)
+
+
+def load():
+    CoroBacktraceCommand()
+
+
+def info():
+    return "Pretty printers for folly::coro"