feat: GSSAPI client (with unix support)

[levkohimins]

Refactoring the code from this PR to implement ldap.GSSAPIClient interface.

// NewClientWithKeytab creates a new client from a keytab credential.
gssapi.NewClientWithKeytab(username, realm, keytabPath, krb5confPath)

// NewClientWithPassword creates a new client from a password credential.
gssapi.NewClientWithPassword(username, realm, password, krb5confPath)

// NewClientFromCCache creates a client from a populated client cache.
gssapi.NewClientFromCCache(ccachePath, krb5confPath)

[patryk4815]

works fine ;) @t2y why not merged yet?

[t2y]

I don't have permission to merge. I think @cpuschma can review it.

[cpuschma]

There are open merge conflicts that need to be resolved first but first and foremost we/I need to review the function and structs, as there already GSSAPI related functions available coming from PR #402 .

[Macmod]

My two cents...

Client from PR #449 implements GSSAPIClient from PR #402 (but is not Windows-only), therefore the "Bind" functions from PR #402 should work fine with Clients instead of SSPIClients, since they rely on the interface instead of the old concrete type.

The only functions from PR #402 that reference the old concrete type are the NewSSPIClient* functions, which allow for backwards compatibility if the user still wants to use the Windows-only implementation instead of migrating to the improved version.

IMO right now this should be enough to guarantee that this PR only adds functionality to go-ldap that will allow for lots of new features in projects that use it and should not cause any issues.

The alternative is to introduce breaking changes by removing the Windows-only types and methods and adding extra code for the interface and the bind to this PR, which does not sound ideal. Another possibility is to rename "Client" to "SSPIClient", remove the old code from PR #402, and rename other functions from this PR accordingly to match the old PR - this would be a much more sensitive operation, involving removing the dependency on alexbrainman/sspi and changing some type signatures...

[cpuschma]

My two cents...

Client from PR #449 implements GSSAPIClient from PR #402 (but is not Windows-only), therefore the "Bind" functions from PR #402 should work fine with Clients instead of SSPIClients, since they rely on the interface instead of the old concrete type.

The only functions from PR #402 that reference the old concrete type are the NewSSPIClient* functions, which allow for backwards compatibility if the user still wants to use the Windows-only implementation instead of migrating to the improved version.

IMO right now this should be enough to guarantee that this PR only adds functionality to go-ldap that will allow for lots of new features in projects that use it and should not cause any issues.

The alternative is to introduce breaking changes by removing the Windows-only types and methods and adding extra code for the interface and the bind to this PR, which does not sound ideal. Another possibility is to rename "Client" to "SSPIClient", remove the old code from PR #402, and rename other functions from this PR accordingly to match the old PR - this would be a much more sensitive operation, involving removing the dependency on alexbrainman/sspi and changing some type signatures...

The pull request as such is definitely a good addition and I am grateful for all contributions, especially because time is currently tight due to work (but the situation should ease soon). This was the reason for the first message, I haven't had time to review the PR and check if the functions and structs differ too much from the existing ones.

We can and should avoid breaking changes whenever possible. I'll review and merge the PR today, only an addition of the examples would be helpful if someone finds the time and has a system with Kerberos integration at hand.