zed to learn that my HTTP server of choice, nginx, doesn’t validate the Content-Length it receives from its upstreams and is simply unaware when the response it received from an upstream server is chopped off. If your response specifies a content length but closes the connection without delivering enough bytes, nginx will simply stall the request for a long time without closing the connection downstream, even though it has no hope of receiving additional data to push downstream. I tried this both with proxy_pass and uwsgi_pass, but I’m quite confident it’s true for other backends (fastcgi_pass, scgi_pass, etc). This is bad, but not as bad as the case where you want an nginx module to manipulate your content, removing existing content length/transfer encoding and applying its own (the gzip module indeed does that). If a backend error occurs while content-length-oblivious-nginx is altering the data, the content altering module will apply what it applies to the bytes it received, add new content-length/transfer-encoding, assuring everyone the response is OK, and entice user-agents or even proxies to enter the almost-never-recover bad cache scenario I described in the previews paragraph. Ouch!
The proper way to fix this, IMHO, is that nginx simply must start looking at the upstream’s content length (or transfer encoding, once nginx starts using chunked responses with its upstreams). Part of the reason I’m writing this post is that Maxim Dounin, venerable nginx comitter and an OK chap overall, told me he doesn’t consider this a top priority at the moment, but I humbly disagree with his assessment of how serious the issue is. Until such a time as nginx is fixed about this, I think you must disable all content-manipulating nginx modules and instead handle all message length affecting work in your upstream (compression, addition, etc). This is what I opted to do with my django based web app, I replaced nginx’s gzip module with Django’s GZipMiddleware. It’s a terrible shame though. It’s doing the job of nginx for it, probably in a lesser fashion than how nginx could, it violates a must not clause in Python’s WSGI PEP333, and I have empiric proof that Tim Berners-Lee chokes a kitten every time you do it.
But what’s the alternative? Risk invisibly cached corrupt data for an undetermined length of time? Ditch nginx, which I think is the best HTTP server on this planet despite this debacle? Nah. Both are unacceptable.