The Admin → Login-Groups UI feature and its corresponding login-group
command solve a common problem with Fossil: you’ve created multiple
repositories that some subset of users need access to, and you
don’t want to redundantly administer the user credentials for each
repository.
Restrictions
This feature ties changes to the “user” table in one repo to that in
one or more other repos, keyed by the user’s name. The interactions are
non-obvious because although the goal is for the end result to “just
work,” there are practical security and administration matters that
complicate things:
Login group handling only works between joined repositories for the subset of users with the same name.
If you’re logged in on one repo in a group, any other repo in that group that has a matching user record will accept your valid login.
When you set up a login group between two repos, the user tables aren’t merged, so even though you may have users that appear in both, they will retain their initial passwords, credentials, and so forth.
The same is true after the login group is created: changes you make to the user table in one repo in the group only propagate to the other repos in the group when you check the “Apply changes to all repositories“ box in the “Scope” section of the user edit screen. Otherwise, user changes remain local to the repo you made them on.
Login groups only affect the HTTP interfaces. Contrast things like
ssh://clones, where unless you go out of your way to force them to run over one of the HTTP interfaces that pays attention to Fossil’s RBAC system, login groups aren’t consulted.
Interactions
These restrictions combine in subtle and interesting ways. Examples:
#1 and #2: If you are logged into repo C as “charlie” and then try to visit joined repo A where “charlie” doesn’t exist, your valid login on C won’t get you into A.
#2 and #3: If “alice” exists in both of these same repos, logging in on A gets her into C, but if she has different user capabilities on each from the time before the two repos joined the login group, her caps on A don’t apply to C, nor vice versa.
Let us say F is a forum-only repo, and W is a wiki-only repo, and that Alice has forum-posting rights on F and wiki-editing rights on W. If both repos are joined by a login group, Alice can log in on F and then access W without logging in on it separately, but she cannot then post a forum message on W even though she could on F.
#3 and #4: If you change the caps for user “alice” on one repo in a group and tell Fossil to apply the changes to all repos in the group, the new caps will overwrite those on the other repos, not merge with them.
To extend the practical example from the prior point, let us say you wish to grant Alice the “write unversioned” capability on both F and W. If you check that single user cap box on F plus the “apply to all” option, then “Apply Changes,” she will end up with forum + unversioned caps on repo W, losing her wiki-editing caps in the process.
If you want user caps to differ on each repo, you must administer them separately even if there is a common subset of caps between all repos in the group for that user. Remember: selecting the “apply to all” box calls for an overwrite operation, not a merge.
#4 and #1: If you make a change to an existing user “bob” in repo B and select the “apply to all” option, it will only affect other repos in the group that have a user “bob” configured.
But, if you are instead creating user “bob” for the first time and select that option, that user will be created in all repos. The same is true of user deletion: that destructive action will propagate through the group if you request it.
#5 and #1: If you have a user “daisy” on both repos A and B in a login group, logging in over the web to A doesn’t let you push changes into B over SSH. Without the workaround linked above, SSH only pays attention to the operating system’s user authentication system, not Fossil’s.
Inversely, if Daisy successfully logs in over SSH to repo B, she gains no access to any of the other repos in that group. She needs at least one valid login over HTTP to one of the group’s repos.
Discussion
The end result of all of this is that you can have a subset of users with credentials only on repo A, a different subset only on B, and a third subset common to both. The only thing selecting which case applies is restriction #1 above.
Login groups have names. A repo can be in only one of these named login groups at a time.
Trust in login groups is transitive within a single server. Consider this sequence:
$ cd /path/to/A/checkout
$ fossil login-group join --name G ~/museum/B.fossil
$ cd /path/to/C/checkout
$ fossil login-group join ~/museum/B.fossil
That creates login group G joining repo A to B, then joins C to B. Although we didn’t explicitly tie C to A, a successful login on C gets you into both A and B, within the restrictions set out above.
Changes are transitive in the same way, provided you check that “apply to all” box on the user edit screen.