hgweb: use a capped reader for WSGI input stream
Per PEP 3333, the input stream from WSGI should respect EOF and
prevent reads past the end of the request body. However, not all
WSGI servers guarantee this. Notably, our BaseHTTPServer based
built-in HTTP server doesn't. Instead, it exposes the raw socket
and you can read() from it all you want, getting the connection in
a bad state by doing so.
We have a "cappedreader" utility class that proxies a file object
and prevents reading past a limit.
This commit converts the WSGI input stream into a capped reader when
the input length is advertised via Content-Length headers.
"cappedreader" only exposes a read() method. PEP 3333 states that
the input stream MUST also support readline(), readlines(hint), and
__iter__(). However, since our WSGI application code only calls
read() and since we're not manipulating the stream exposed by the
WSGI server, we're not violating the spec here.
Differential Revision: https://phab.mercurial-scm.org/D2768