pageserver: expedite tenant activation on delete (#6190)

## Problem

During startup, a tenant delete request might have to retry for many
minutes waiting for a tenant to enter Active state.

## Summary of changes

- Refactor delete_tenant into TenantManager: this is not a functional
change, but will avoid merge conflicts with
https://github.com/neondatabase/neon/pull/6105 later
- Add 412 responses to the swagger definition of this endpoint.
- Use Tenant::wait_to_become_active in `TenantManager::delete_tenant`

---------

Co-authored-by: Arpad Müller <arpad-m@users.noreply.github.com>

test_runner/regress/test_timeline_size.py

@@ -1,3 +1,4 @@
+import concurrent.futures
 import math
 import queue
 import random
@@ -24,6 +25,7 @@ from fixtures.pageserver.utils import (
     assert_tenant_state,
     timeline_delete_wait_completed,
     wait_for_upload_queue_empty,
+    wait_tenant_status_404,
     wait_until_tenant_active,
 )
 from fixtures.pg_version import PgVersion
@@ -776,6 +778,7 @@ def test_ondemand_activation(neon_env_builder: NeonEnvBuilder):
 
     def get_tenant_states():
         states = {}
+        log.info(f"Tenant ids: {tenant_ids}")
         for tenant_id in tenant_ids:
             tenant = pageserver_http.tenant_status(tenant_id=tenant_id)
             states[tenant_id] = tenant["state"]["slug"]
@@ -872,3 +875,51 @@ def test_ondemand_activation(neon_env_builder: NeonEnvBuilder):
         pageserver_http.get_metric_value("pageserver_tenant_startup_scheduled_total") == n_tenants
     )
     assert pageserver_http.get_metric_value("pageserver_tenant_startup_complete_total") == n_tenants
+
+    # Check that tenant deletion proactively wakes tenants: this is done separately to the main
+    # body of the test because it will disrupt tenant counts
+    env.pageserver.stop()
+    env.pageserver.start(
+        extra_env_vars={"FAILPOINTS": "timeline-calculate-logical-size-pause=pause"}
+    )
+
+    wait_until(10, 1, at_least_one_active)
+    delete_tenant_id = list(
+        [(tid, s) for (tid, s) in get_tenant_states().items() if s == "Attaching"]
+    )[0][0]
+
+    # Deleting a stuck tenant should prompt it to go active
+    with concurrent.futures.ThreadPoolExecutor() as executor:
+        log.info("Starting background delete")
+
+        def delete_tenant():
+            env.pageserver.http_client().tenant_delete(delete_tenant_id)
+
+        background_delete = executor.submit(delete_tenant)
+
+        # Deletion itself won't complete due to our failpoint: Tenant::shutdown can't complete while calculating
+        # logical size is paused in a failpoint.  So instead we will use a log observation to check that
+        # on-demand activation was triggered by the tenant deletion
+        log_match = f".*attach{{tenant_id={delete_tenant_id} shard_id=0000}}: Activating tenant \\(on-demand\\).*"
+
+        def activated_on_demand():
+            assert env.pageserver.log_contains(log_match) is not None
+
+        log.info(f"Waiting for activation message '{log_match}'")
+        try:
+            wait_until(10, 1, activated_on_demand)
+        finally:
+            log.info("Clearing failpoint")
+            pageserver_http.configure_failpoints(("timeline-calculate-logical-size-pause", "off"))
+
+        # Deletion should complete successfully now that failpoint is unblocked
+        log.info("Joining background delete")
+        background_delete.result(timeout=10)
+
+        # Poll for deletion to complete
+        wait_tenant_status_404(pageserver_http, tenant_id=delete_tenant_id, iterations=40)
+        tenant_ids.remove(delete_tenant_id)
+
+    # Check that all the stuck tenants proceed to active (apart from the one that deletes)
+    wait_until(10, 1, all_active)
+    assert len(get_tenant_states()) == n_tenants - 1