Request for /hst:platform try to match site hosts instead of the cms host

The problem is as follows:

When going to the channel mngr overview and opening a channel, a call is made to an HST page composer rest endpoint (in the platform webapp) that sets the 'rendering host' on the CmsSessionContext : This way, when a request hits the HST site webapp over the host of the CMS, the HST in the site webapp 'knows' which site to render (it uses the host stored in the CmsSessionContext instead of the host from the request).

The CmsSessionContext is however shared between all webapps, meaning that a request that hits the CMS webapp will use the same 'rendering host', but this is typically a host something like www.example.com, which obviously cannot be matched in the /hst:platform configuration. As a result, after opening a channel, the CMS starts to malfunction.

Note that the above only happens when the site host is different than the cms host. For local development where both site and cms are typically accessed over localhost, it thus cannot be observed.

Local reproduction on 13.0.0

• to your hosts file add dev.example.com 127.0.0.1
• to /hst:myproject add the hostgroup 'test', add the host config com/example/com and copy the hst:root from localhost
• to /hst:platform add the hostgroup 'test' and add 127.0.0.1 and copy hst:root from localhost

After the above, go to http://127.0.0.1:8080/cms . Go to channel mgr, after opening a channel, lots of things start to fail and logs show something like

23.01.2019 10:24:19 WARN  http-nio-8080-exec-6 [HstDelegateeFilterBean.doFilter:251] 'Request{ method='GET', scheme='http', host='dev.example.com:8080', requestURI='/cms/', queryString='1-1.IBehaviorListener.0-root-pinger&_=1548234309825'}' can not be matched to a host. Skip HST Filter and request processing.