If an error happened while processing a SAML AuthN response, or a client
ends up doing a `GET` request to `/authn_response`, then render a
customisable error page rather than a confusing error.
Turns out that figuring out a remote user id for the SAML user isn't quite as obvious as it seems. Factor it out to the SamlMappingProvider so that it's easy to control.
Fixes a bug where the default attribute maps were prioritised over
user-specified ones, resulting in incorrect mappings.
The problem is that if you call SPConfig.load() multiple times, it adds new
attribute mappers to a list. So by calling it with the default config first,
and then the user-specified config, we would always get the default mappers
before the user-specified mappers.
To solve this, let's merge the config dicts first, and then pass them to
SPConfig.
We want to assign unique mxids to saml users based on an incrementing
suffix. For that to work, we need to record the allocated mxid in a separate
table.
Make it so that most options in the config are optional, and commented out in
the generated config.
The reasons this is a good thing are as follows:
* If we decide that we should change the default for an option, we can do so,
and only those admins that have deliberately chosen to override that option
will be stuck on the old setting.
* It moves us towards a point where we can get rid of the super-surprising
feature of synapse where the default settings for the config come from the
generated yaml.
* It makes setting up a test config for unit testing an order of magnitude
easier (see forthcoming PR).
* It makes the generated config more consistent, and hopefully easier for users
to understand.
The general idea here is that config examples should just have a hash and no
extraneous whitespace, both to make it easier for people who don't understand
yaml, and to make the examples stand out from the comments.
This implements both a SAML2 metadata endpoint (at
`/_matrix/saml2/metadata.xml`), and a SAML2 response receiver (at
`/_matrix/saml2/authn_response`). If the SAML2 response matches what's been
configured, we complete the SSO login flow by redirecting to the client url
(aka `RelayState` in SAML2 jargon) with a login token.
What we don't yet have is anything to build a SAML2 request and redirect the
user to the identity provider. That is left as an exercise for the reader.
