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