add dedicated doc on details of allowing access

docs/source/index.md

@@ -47,6 +47,7 @@ Topic guides go more in-depth on a particular topic.
 :maxdepth: 2
 :caption: Topic guides
 
+topic/allowing
 topic/extending
 ```
 

docs/source/topic/allowing.md

@@ -0,0 +1,142 @@
+(allowing)=
+
+# Allowing access to your JupyterHub
+
+OAuthenticator is about deferring **authentication** to an external source,
+assuming your users all have accounts _somewhere_.
+But many of these sources (e.g. Google, GitHub) have _lots_ of users, and you don't want _all_ of them to be able to use your hub.
+This is where **authorization** comes in.
+
+In OAuthenticator, authorization is represented via configuration options that start with `allow` or `block`.
+
+There are also lots of OAuth providers, and as a result, lots of ways to tell OAuthenticator who should be allowed to access your hub.
+
+## Default behavior: nobody is allowed!
+
+The default behavior of OAuthenticator (starting with version 16) is to block all users unless explicitly authorized via _some_ `allow` configuration.
+If you want anyone to be able to use your hub, you must specify at least one `allow` configuration.
+
+```{versionchanged} 16
+Prior to OAuthenticator 16, `allow_all` was _implied_ if no other `allow` configuration was specified.
+Starting from 16, `allow_all` can only be enabled explicitly.
+```
+
+## Allowing access
+
+There are several `allow_` configuration options, to grant access to users according to different rules.
+
+When you have only one `allow` configuration, the behavior is generally unambiguous: anyone allowed by the rule can login to the Hub, while anyone not explicitly allowed cannot login.
+However, once you start adding additional `allow` configuration, there is some ambiguity in how multiple rules are combined.
+
+```{important}
+Additional allow rules **can only grant access**, meaning they only _expand_ who has access to your hub.
+Adding an `allow` rule cannot prevent access granted by another `allow` rule.
+To block access, use `block` configuration.
+```
+
+That is, if a user is granted access by _any_ `allow` configuration, they are allowed.
+An allow rule cannot _exclude_ access granted by another `allow` rule.
+
+An example:
+
+```python
+c.GitHubOAuthenticator.allowed_users = {"mensah", "art"}
+c.GitHubOAuthenticator.allowed_organizations = {"preservation"}
+```
+
+means that the users `mensah` and `art` are allowed, _and_ any member of the `preservation` organization are allowed.
+Any user that doesn't meet any of the allow rules will not be allowed.
+
+| user  | allowed | reason                                                  |
+| ----- | ------- | ------------------------------------------------------- |
+| art   | True    | in `allowed_users`                                      |
+| amena | True    | member of `preservation`                                |
+| tlacy | False   | not in `allowed_users` and not member of `preservation` |
+
+### `allow_all`
+
+The first and simplest way to allow access is to any user who can successfully authenticate:
+
+```python
+c.OAuthenticator.allow_all = True
+```
+
+This is appropriate when you use an authentication provider (e.g. an institutional single-sign-on provider), where everyone who has an account in the provider should have access to your Hub.
+It may also be appropriate for unadvertised short-lived hubs, e.g. dedicated hubs for workshops that will be shutdown after a day, where you may decide it is acceptable to allow anyone who finds your hub to login.
+
+If `allow_all` is enabled, no other `allow` configuration will have any effect.
+
+```{seealso}
+Configuration documentation for {attr}`.OAuthenticator.allow_all`
+```
+
+### `allowed_users`
+
+This is top-level JupyterHub configuration, shared by all Authenticators.
+This specifies a list of users that are allowed by name.
+This is the simplest authorization mechanism when you have a small group of users whose usernames you know:
+
+```python
+c.OAuthenticator.allowed_users = {"mensah", "ratthi"}
+```
+
+If this is your only configuration, only these users will be allowed, no others.
+
+Note that any additional usernames in the deprecated `admin_users` configuration will also be allowed to login.
+
+```{seealso}
+Configuration documentation for {attr}`.OAuthenticator.allowed_users`
+```
+
+### `allow_existing_users`
+
+JupyterHub can allow you to add and remove users while the Hub is running via the admin page.
+If you add or remove users this way, they will be added to the JupyterHub database, but they will not be able to login unless they are also granted access via an `allow` rule.
+
+To enable managing users via the admin panel, set
+
+```python
+c.OAuthenticator.allow_existing_users = True
+```
+
+```{warning}
+Enabling `allow_existing_users` means that _removing_ users from any explicit allow mechanisms will no longer revoke their access.
+Once the user has been added to the database, the only way to revoke their access to the hub is to remove the user from JupyterHub entirely, via the admin page.
+```
+
+```{seealso}
+Configuration documentation for {attr}`.OAuthenticator.allow_existing_users`
+```
+
+### Provider-specific rules
+
+Each OAuthenticator provider may have its own provider-specific rules to allow groups of users access, such as:
+
+- {attr}`.GitHubOAuthenticator.allowed_organizations`
+- {attr}`.GitLabOAuthenticator.allowed_gitlab_groups`
+- {attr}`.GlobusOAuthenticator.allowed_globus_groups`
+- {attr}`.GoogleOAuthenticator.allowed_google_groups`
+
+## Blocking Access
+
+It's possible that you want to limit who has access to your Hub to less than all of the users granted access by your `allow` configuration.
+`block` configuration always has higher priority than `allow` configuration, so if a user is both allowed _and_ blocked, they will not be able to login.
+
+The only `block` configuration is the base Authenticators `block_users`,
+a set of usernames that will not be allowed to login.
+
+### Revoking previously-allowed access
+
+Any users who have logged in previously will be present in the JupyterHub database.
+Removing a user's login permissions (e.g. removing them from a GitLab project when using {attr}`.GitLabOAuthenticator.project_ids`) only prevents future logins;
+it does not remove the user from the JupyterHub database.
+This means that:
+
+1. any API tokens that the user still has access to will continue to be valid, and can continue to be used
+2. any still-valid browser sessions will continue to be logged in.
+
+```{important}
+To fully remove a user's access to JupyterHub,
+their login permission must be revoked _and_ their user fully deleted from the Hub,
+e.g. via the admin page.
+```

oauthenticator/oauth2.py

@@ -269,6 +269,8 @@ class OAuthenticator(Authenticator):
         help="""
         Allow all authenticated users to login.
 
+        Overrides all other `allow` configuration.
+
         .. versionadded:: 16.0
         """,
     )
@@ -279,37 +281,29 @@ class OAuthenticator(Authenticator):
         help="""
         Allow existing users to login.
 
-        An existing user is a user in JupyterHub's database of users, and it
-        includes all users that has previously logged in.
+        Enable this if you want to manage user access via the JupyterHub admin page (/hub/admin).
+
+        With this enabled, all users present in the JupyterHub database are allowed to login.
+        This has the effect of any user who has _previously_ been allowed to login
+        via any means will continue to be allowed until the user is deleted via the /hub/admin page
+        or REST API.
 
         .. warning::
 
            Before enabling this you should review the existing users in the
            JupyterHub admin panel at `/hub/admin`. You may find users existing
-           there because they have once been declared in config such as
-           `allowed_users` or once been allowed to sign in.
+           there because they have previously been declared in config such as
+           `allowed_users` or allowed to sign in.
 
         .. warning::
 
-           When this is enabled and you are to remove access for one or more
-           users allowed via other config options, you must make sure that they
-           are not part of the database of users still. This can be tricky to do
+           When this is enabled and you wish to remove access for one or more
+           users previously allowed, you must make sure that they
+           are removed from the jupyterhub database. This can be tricky to do
            if you stop allowing a group of externally managed users for example.
 
         With this enabled, JupyterHub admin users can visit `/hub/admin` or use
-        JupyterHub's REST API to add and remove users as a way to allow them
-        access.
-
-        The username for existing users must match the normalized username
-        returned by the authenticator. When creating users, only lowercase
-        letters should be used unless `MWOAuthenticator` is used.
-
-        .. note::
-
-           Allowing existing users is done by adding existing users on startup
-           and newly created users to the `allowed_users` set. Due to that, you
-           can't rely on this config to independently allow existing users if
-           you for example would reset `allowed_users` after startup.
+        JupyterHub's REST API to add and remove users to manage who can login.
 
         .. versionadded:: 16.0
 
@@ -1086,3 +1080,18 @@ def __init__(self, **kwargs):
                 self._deprecated_oauth_trait, names=list(self._deprecated_oauth_aliases)
             )
         super().__init__(**kwargs)
+
+
+# patch allowed_users help string to match our definition
+# base Authenticator class help string gives the wrong impression
+# when combined with other allow options
+OAuthenticator.class_traits()[
+    "allowed_users"
+].help = """
+Set of usernames that should be allowed to login.
+
+If unspecified, grants no access. You must set at least one other `allow` configuration
+if any users are to have permission to access the Hub.
+
+Any usernames in `admin_users` will also be allowed to login.
+"""