Assume that Wget has retrieved a document from the host A, which
hasn't closed the connection in accordance with Wget's keep-alive
request.
Then Wget needs to connect to host B, which is really the same as A
because the provider uses DNS-based virtual hosts. Is it OK to reuse
the connection to A to talk to B? Specifically, Wget does this:
0. Assume Wget has a file descriptor, old_fd, connected to A.
Connection to B is needed to get a URL `http://B/...'.
1. Resolve B, getting an array of IP addresses that it resolves to.
This would need to be done anyway, so no additional overhead is
introduced.
2. Use getpeername(old_fd) to determine the address of the endpoint
old_fd is connected to.
3. If old_fd's peer address is one of the addresses that B resolves
to, determined in step 1, use old_fd to communicate with B.
Since Wget uses the `Host' header to specify the virtual host to
talk to, everything works. (IP-based virtual hosts are not a
problem because they have different addresses in the first
place.)
I cannot decide if this is totally evil or perfectly fine. On the one
hand, persistent connections are (AFAIK) supposed to be a mere
optimization, and should not modify the semantics of the
communication. So if I have the connection to the endpoint, I should
be able to reuse it. But on the other hand, a server might decide to
connect a file descriptor to a handler for a specific virtual host,
which would be unable to serve anything else. FWIW, it works fine
with Apache.
