From c69db0983dab7bad9f5e233fd6286a50d419bb47 Mon Sep 17 00:00:00 2001
From: Tom Prince <tom.prince@private.storage>
Date: Wed, 14 Jul 2021 17:01:52 -0600
Subject: [PATCH] Capture correct tracebacks when using `inline_callbacks`.

When an inlineCallback completes synchronously, such as in tests, twisted will
string-ify the traceback, which means that when the exception is then thrown
into another inlineCallback function, the stack trace is lost[1]. Twisted works
around this by inspect the stack and finding the Failure instance from the
inner call, to get traceback there.

However, this only works if `Failure.throwExceptionIntoGenerator` is used.
This adjusts eliot_friendly_generator_function to use `Failure`, instead of an
exc_info tuple, when used via `inline_callabcks`.


[1] The string-ified traceback can't be passed to `.send`.
---
 eliot/_generators.py | 13 ++++++++++++-
 eliot/twisted.py     |  1 +
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/eliot/_generators.py b/eliot/_generators.py
index 1a7b0a1..9eb596e 100644
--- a/eliot/_generators.py
+++ b/eliot/_generators.py
@@ -10,6 +10,11 @@
 from contextvars import copy_context
 from weakref import WeakKeyDictionary
 
+try:
+    from twisted.python.failure import Failure
+except ImportError:
+    pass
+
 from . import log_message
 
 
@@ -95,6 +100,8 @@ def wrapper(*a, **kw):
                 def go():
                     if ok:
                         value_out = gen.send(value_in)
+                    elif wrapper._use_failure:
+                        value_out = value_in.throwExceptionIntoGenerator(gen)
                     else:
                         value_out = gen.throw(*value_in)
                     # We have obtained a value from the generator.  In
@@ -130,9 +137,13 @@ def go():
                     # contextmanager.  But @contextmanager extremely
                     # conveniently eats it for us!  Thanks, @contextmanager!
                     ok = False
-                    value_in = exc_info()
+                    if wrapper._use_failure:
+                        value_in = Failure()
+                    else:
+                        value_in = exc_info()
                 else:
                     ok = True
 
     wrapper.debug = False
+    wrapper._use_failure = False
     return wrapper
diff --git a/eliot/twisted.py b/eliot/twisted.py
index 5f83094..37f3396 100644
--- a/eliot/twisted.py
+++ b/eliot/twisted.py
@@ -262,6 +262,7 @@ def inline_callbacks(original, debug=False):
     function.
     """
     f = eliot_friendly_generator_function(original)
+    f._use_failure = True
     if debug:
         f.debug = True
     return inlineCallbacks(f)