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TLDR: When an error happens in the wsgiref's write() or close(), stack traces get inundated with irrelevant yet legitimate errors which make it hard to track down the real issue.
Couple of examples of this happening in practice:
https://stackoverflow.com/questions/28124461/how-do-i-solve-nonetype-object-is-not-callable-with-beautifulsoup-and-bottle
https://stackoverflow.com/questions/27518844/error-when-serving-html-with-wsgi
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How to reproduce: The file I've attached reproduces the error on python 3.4, 3.5 and 3.6. The handler returns a string instead of bytes, which fails an early assert in handlers.py: write(data).
BaseHandler.run() triggers, gets as far as finish_response(), which triggers the above AssertionError. It falls into the except: block which attempts to handle_error(). But before doing that, it triggers self.close(), which sets result/headers/status/environ to None, bytes_sent to 0 and headers_sent to False.
Now when handle_error() triggers, `self.headers_sent` is False because of that, which attempts to trigger finish_response() *again*. This triggers a write() which attempts sending the headers, which checks client_is_modern(), subscripting `self.environ` which at that point has already been set to None. New error, which is caught in run()'s except block and re-raised after closing the connection (again).
I probably skipped some steps because the traceback is truly a mess. I think this could be improved, if only so that it doesn't get so confusing anymore.

Looks like the error handling is broken by Issue 16220, which calls the "BaseHandler.close" method before the exception is caught for the error handler. Perhaps it is better to just close the iterator without messing with the other attributes in the exception case.
I tried various cases in Python 2.6 (without the Issue 16220 change) and the error handling seems better (not masked by double exceptions, no sending a 500 response after the start of the app’s response).
The same problem exists in Python 2, except only the later exception is reported, and the original exception is forgotten.

This may or may not be the same as what you're suggesting, Martin. But is another option to make close() a no-op if it is called a second time? Otherwise, it seems we'd need to make sure that no code path can result in close() being called twice (even during exception handling, etc).

There are actually two "close" methods in the WSGI package: ServerHandler’s implementation extends the BaseHandler implementation. Making the "close" methods do nothing if called a second time would avoid the error about "self.status" being None, but won’t help very much with other problems, such as:
* If no response has been sent when the exception happens, the first call to "ServerHandler.close" still won’t have a HTTP status code or response size to log. It is more useful in this case to log the 500 code in the second "close" call after the error page is generated, which is what happens in 2.6.
* If the response was started when the exception happens, the first call to "BaseHandler.close" will still reset "self.headers_sent", which fools "handle_error" into trying to generate a 500 response despite already having started the application response.
To be clear, what I had in mind last week was to adjust "finish_response" to look something like:
class BaseHandler:
 ...
 def finish_response(self):
 try:
 ...
 except:
 # Limited version of "close" method on exception
 if hasattr(self.result, 'close'):
 self.result.close()
 raise
 self.close() # Full "close" method when no exception
For the record: This is related to Issue 27682, specifically about handling ConnectionAbortedError and other network errors.

New changeset 7c59362a15dfce538512ff1fce4e07d33a925cfb by Berker Peksag in branch 'master':
bpo-29183: Fix double exceptions in wsgiref.handlers.BaseHandler (GH-12914)
https://github.com/python/cpython/commit/7c59362a15dfce538512ff1fce4e07d33a925cfb

New changeset f393e8eb463d60ce559982613429568c518ab8d9 by Berker Peksag (Miss Islington (bot)) in branch '3.7':
bpo-29183: Fix double exceptions in wsgiref.handlers.BaseHandler (GH-12914)
https://github.com/python/cpython/commit/f393e8eb463d60ce559982613429568c518ab8d9