libs: add 'generations_api' auth scope (#6783)

## Problem

Even if you're not enforcing auth, the JwtAuth middleware barfs on
scopes it doesn't know about.

Add `generations_api` scope, which was invented in the cloud control
plane for the pageserver's /re-attach and /validate upcalls: this will
be enforced in storage controller's implementation of these in a later
PR.

Unfortunately the scope's naming doesn't match the other scope's naming
styles, so needs a manual serde decorator to give it an underscore.

## Summary of changes

- Add `Scope::GenerationsApi` variant
- Update pageserver + safekeeper auth code to print appropriate message
if they see it.

libs/utils/src/auth.rs

@@ -29,6 +29,9 @@ pub enum Scope {
     // Should only be used e.g. for status check.
     // Currently also used for connection from any pageserver to any safekeeper.
     SafekeeperData,
+    // The scope used by pageservers in upcalls to storage controller and cloud control plane
+    #[serde(rename = "generations_api")]
+    GenerationsApi,
 }
 
 /// JWT payload. See docs/authentication.md for the format

pageserver/src/auth.rs

@@ -14,8 +14,12 @@ pub fn check_permission(claims: &Claims, tenant_id: Option<TenantId>) -> Result<
         }
         (Scope::PageServerApi, None) => Ok(()), // access to management api for PageServerApi scope
         (Scope::PageServerApi, Some(_)) => Ok(()), // access to tenant api using PageServerApi scope
-        (Scope::SafekeeperData, _) => Err(AuthError(
-            "SafekeeperData scope makes no sense for Pageserver".into(),
+        (Scope::SafekeeperData | Scope::GenerationsApi, _) => Err(AuthError(
+            format!(
+                "JWT scope '{:?}' is ineligible for Pageserver auth",
+                claims.scope
+            )
+            .into(),
         )),
     }
 }

safekeeper/src/auth.rs

@@ -12,8 +12,12 @@ pub fn check_permission(claims: &Claims, tenant_id: Option<TenantId>) -> Result<
             }
             Ok(())
         }
-        (Scope::PageServerApi, _) => Err(AuthError(
-            "PageServerApi scope makes no sense for Safekeeper".into(),
+        (Scope::PageServerApi | Scope::GenerationsApi, _) => Err(AuthError(
+            format!(
+                "JWT scope '{:?}' is ineligible for Safekeeper auth",
+                claims.scope
+            )
+            .into(),
         )),
         (Scope::SafekeeperData, _) => Ok(()),
     }

test_runner/regress/test_auth.py

@@ -225,9 +225,7 @@ def test_auth_failures(neon_env_builder: NeonEnvBuilder, auth_enabled: bool):
 
         check_pageserver(True, password=pageserver_token)
 
-        env.pageserver.allowed_errors.append(
-            ".*SafekeeperData scope makes no sense for Pageserver.*"
-        )
+        env.pageserver.allowed_errors.append(".*JWT scope '.+' is ineligible for Pageserver auth.*")
         check_pageserver(False, password=safekeeper_token)
 
     def check_safekeeper(expect_success: bool, **conn_kwargs):