Should processOAuthCallback throw when previous cookie signature is invalid?

[andrew-nowak]

ref:

pan-domain-authentication/pan-domain-auth-play_2-8/src/main/scala/com/gu/pandomainauth/action/Actions.scala

Lines 156 to 163 in a579631

	val existingAuth = CookieUtils.parseCookieData(c.value, settings.publicKey)
	logger.debug("user re-authed, merging auth data")
	claimedAuth.copy(
	authenticatingSystem = system,
	authenticatedIn = existingAuth.authenticatedIn ++ Set(system),
	multiFactor = checkMultifactor(claimedAuth)
	)

guardian/grid#3865

When panda gets the oauth callback, it tries to parse any previous panda cookie and merge the authorised systems. But if for some reason the previous panda cookie was signed with a different private key, then the signature won't match and an exception will be thrown preventing a new cookie from being minted (even if that cookie is expired, you'll be stuck in this situation until you clear your cookie manually).

I suspect it's not a common problem as the panda keys don't change, we only noticed on the Grid when testing our alternative (local dev) oauth provider.