Let TokenBasedRememberMeServices2 tolerate Authentication.principal not UserDetails

[jglick]

While experimenting with tests of SSO logins, I came across a case where clicking Remember me caused a CCE because the Authentication had a principal of type String rather than the UserDetails this class currently expects. Using a debugger I found that the String-based token was created here:

at org.springframework.security.authentication.AbstractAuthenticationToken.setDetails(AbstractAuthenticationToken.java:99)
at org.springframework.security.web.authentication.UsernamePasswordAuthenticationFilter.setDetails(UsernamePasswordAuthenticationFilter.java:126)
at org.springframework.security.web.authentication.UsernamePasswordAuthenticationFilter.attemptAuthentication(UsernamePasswordAuthenticationFilter.java:84)
at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:231)
at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:221)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:99)
at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:97)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:99)
at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:117)
at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:87)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:63)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:99)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:111)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:172)
at …

I think this arose because the test was actually mistaken at that point, trying to use the /login form when the configured security realm was not username/password-based. Probably UsernamePasswordAuthenticationFilter would not normally be used in this way. At any rate, the Javadoc for Object getPrincipal() says

In the case of an authentication request with username and password, this would be the username.
…
Many of the authentication providers will create a UserDetails object as the principal.

implying that String and UserDetails are the expected types (but that there might be others!); and so it would be best to tolerate anything.

Testing done

Prevented a CCE in the context of original experiments (closed source). Interactive sanity test logging in to built-in security realm.

Proposed changelog entries

• Avoiding a ClassCastException from TokenBasedRememberMeServices2 (not known to occur in realistic environments).

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[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
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