For around 2 weeks, I've been getting an increasing quantity of nonspecific
reports from users of login problems on ikiwiki sites, mostly joeyh.name
and git-annex.branchable.com. A few users are still logging in
successfully, but it seems to be hitting many users; post volume has gone
down more than holidays would explain. --[[Joey]] 

It doesn't seem limited to any login method; email and password have both
been said not to work. (Openid too, but could be openid provider problem
there.)

After a few tries
I seem to have reproduced the problem with email login; I ended up at a
"Error: login failed, perhaps you need to turn on cookies?" 
page but my browser had an ikiwiki session cookie. And,
looking in the session database file, the cookie id was in there. Then I
went to "/do=prefs" in the same browser, and I was actually already 
logged in. 

That points at a problem with the "postsignin" redirect;
if the session does not get a postsignin url set, it can error out that way
despite being logged in.

Reproducing again, I posted the login form, and before clicking on the
login link, looked at the session.db -- it contained an entry for my session,
but without a postsignin url.

	$ strings sessions.db
	$D = {'_SESSION_ID' => 'xxx','_SESSION_REMOTE_ADDR' => 'yyy','_SESSION_ATIME' => 1515106022,'_SESSION_CTIME' => 1515105990};;$D

The postsignin url is certianly getting set at other times though,
and why would this have only recently started to affect lots of users?