hgweb: refactor fake file object proxy for archiving
Python's zip file writer operates on a file object. When doing work,
it periodically calls write(), flush(), and tell() on that object.
In WSGI contexts, the start_response function returns a write()
function. That's a function to write data, not a full file object.
So, when the archival code was first introduced in
2b03c6733efa in
2006, someone invented a proxy "tellable" type that wrapped a file
object like object and kept track of write count so it could
implement tell() and satisfy zipfile's needs.
When our archival code runs, it attempts to tell() the destination
and if that fails, converts it to a "tellable" instance. Our WSGI
application passes the "wsgirequest" instance to the archival
function. It fails the tell() test and is converted to a "tellable."
It's worth noting that "wsgirequest" implements flush(), so
"tellable" doesn't.
This hackery all seems very specific to the WSGI code. So this commit
moves the "tellable" type and the conversion of the destination file
object into the WSGI code. There's a chance some other caller may be
passing a file object like object that doesn't implement tell(). But
I doubt it.
As part of the refactor, our new type implements flush() and doesn't
implement __getattr__. Given the intended limited use of this type,
I want things to fail fast if there is an attempt to access attributes
because I think it is important to document which attributes are being
used for what purposes.
Differential Revision: https://phab.mercurial-scm.org/D2791