This change:

• Now dump the Python traceback of all threads when Py_UNREACHABLE() is called: call Py_FatalError()
• Call DebugBreak() on Windows in debug mode (Py_FatalError)
• Should make Python binary a few bytes smaller in debug mode: replace fputs()+abort() calls with a single call to Py_UNREACHABLE()
• Keep the short version of the xkcd joke in debug mode

I didn't keep the long version of the joke since RANDALL_WAS_HERE compilation flag is not documented anyway. If someone wants to keep it, I suggest to document it in Misc/SpecialBuilds.txt. Moreover, I dislike untested code :-/ I'm also fine with merging the two xkcd texts into a single longer one if you prefer.

Recently, @serhiy-storchaka fixed "from future import barry_as_FLUFL": this easteregg doesn't need to rebuild Python, so I'm fine with keep it.

You can test this change by (1) applying this change:

diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
index a40eb421f5..a85c3fbaae 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -1552,6 +1552,7 @@ builtin_len(PyObject *module, PyObject *obj)
         assert(PyErr_Occurred());
         return NULL;
     }
+    if (res == 123456789) { Py_UNREACHABLE(); }
     return PyLong_FromSsize_t(res);
 }
 

(2) run this script:

class HaveLength:
    def __len__(self):
        return 123456789

def func(obj):
    print("len", len(obj))

def main():
    func(HaveLength())

main()

Output with this change (in debug mode):

$ ./python x.py 
Fatal Python error: We've reached an unreachable state. Anything is possible.
The limits were in our heads all along. Follow your dreams.
https://xkcd.com/2200
Python runtime state: initialized

Current thread 0x00007f81b54ec740 (most recent call first):
  File "/home/vstinner/python/master/x.py", line 6 in func
  File "/home/vstinner/python/master/x.py", line 9 in main
  File "/home/vstinner/python/master/x.py", line 11 in <module>
Aborted (core dumped)

Output without this change (in debug mode):

$ ./python x.py
ERROR:

We've reached an unreachable state. Anything is possible.
The limits were in our heads all along. Follow your dreams.

https://xkcd.com/2200
Aborted (core dumped)

This main enhancement is that you now get the Python traceback where the bug occurred.

This change fix https://bugs.python.org/issue38205

@zware: @serhiy-storchaka prefers to keep a macro rather converting it to a function: https://bugs.python.org/issue38205#msg352782 What's the opinion?

I'm somewhat in favor of keeping it as a macro just for complete backwards compatibility, even if it's implemented as a private _Py_Unreachable function. I will note that about the only third-party usage of Py_UNREACHABLE that comes up in a quick GitHub search is in py3c which checks #if defined(Py_UNREACHABLE), which may not play nicely with a function.

I wrote PR #16290 which fix https://bugs.python.org/issue38205 but keeps Py_UNREACHABLE() as a macro.

@vstinner I think you can close this one and merge PR #16290

It was decided to merge PR #16290 instead.