The pollers in http://hg.mozilla.org/build/buildbotcustom/file/8b1380cbf092/changes should respect etags and last-modified headers if they exist. Note that hg's rss feeds don't currently send this info (bug 566673).
Is the idea here to use a HEAD request to avoid fetching the whole log? I'm not sure how etag helps otherwise.
I don't see how this helps - we poll with &fromchange=XXXXX so usually it's an empty reply.
Maybe that wasn't the case in the past. Sounds like this is WONTFIX now, given that.
How they help on your end is if it's faster to say "if status == "304" return" (roughly, I don't know what Twisted actually puts in status) instead of having minidom parse the shell of a feed and do an empty for loop and reverse an empty list. But typically you do etags on the client side to be a good citizen, and the benefit is on the other end: if it's an expensive db operation to look at whether there's anything newer than XXXXX, but a nearly free filesystem operation to see whether a file named etags/fromchange=XXXXX-123456 exists and just return a 304 if it does, then yay etags. From a few minutes glance at the patch, it doesn't look like it's a huge load reducer on hg.m.o's side, but djc would probably have a better idea than me about that.