A quote from the Python 3 documentation:

 | When interactive, stdout and stderr streams are line-buffered.
 | Otherwise, they are block-buffered like regular text files.

As a result, if an exception occurred and PyErr_Print() was called, its output
could be buffered but not printed to the log for a while (ultimately, until
the interpreter finalization).  If the application process crashed shortly,
the backtrace was completely lost.

Buffering can be disabled by redefining the sys.stderr stream object.
However, interference with standard environment objects was deemed undesirable.
Instead, sys.stderr.flush() is called every time after printing exceptions.
A potential advantage here is that lines from backtraces won't be mixed
with other lines in the log.

PyCallable_Check() doesn't produce errors.

The needless call was introduced in fdd6ed28e3b9.

PyObject_HasAttrString() is just a wrapper over PyObject_GetAttrString(),
while PyObject_CallMethod() calls it as the first step.  As a result,
PyObject_GetAttrString() was called twice if close() was present.

To get rid of PyObject_HasAttrString() while keeping the same behaviour,
the PyObject_CallMethod() call has been decomposed into separate calls of
PyObject_GetAttrString() and PyObject_CallFunction().

On success, PyObject_CallMethod() returns a new reference to
the result of the call, which previously got lost.

Also, error logging on failure was added.

The issue was introduced by b0148ec28c4d.

According to the documentation, PyObject_GetIter():

 | Raises TypeError and returns NULL if the object cannot be iterated.

Previously, this exception wasn't printed or cleared and remained unhandled.

According to the documentation, PyIter_Next():

 | If there are no remaining values, returns NULL with no exception set.
 | If an error occurs while retrieving the item, returns NULL and passes
 | along the exception.

Previously, this exception wasn't properly handled and the response was
finalized as successful.

This issue was introduced in b0148ec28c4d.

A check for PyErr_Occurred() located in the code below might print this
traceback or occasionally catch an exception from one of the two response
close() calls.

Albeit that exceptions from the close() calls also need to be catched,
it's clear that this particular check wasn't supposed to do so.  This is
another issue and it will be fixed later.

Keepalive connection is disabled if upstream response length
differs from specified in the "Content-Length" field value.