feat(pageserver): add force patch index_part API (#11119)

## Problem

As part of the disaster recovery tool. Partly for
https://github.com/neondatabase/neon/issues/9114.

## Summary of changes

* Add a new pageserver API to force patch the fields in index_part and
modify the timeline internal structures.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

test_runner/fixtures/pageserver/http.py

@@ -375,6 +375,19 @@ class PageserverHttpClient(requests.Session, MetricsGetter):
         res = self.post(f"http://localhost:{self.port}/v1/tenant/{tenant_id}/reset", params=params)
         self.verbose_error(res)
 
+    def timeline_patch_index_part(
+        self,
+        tenant_id: TenantId | TenantShardId,
+        timeline_id: TimelineId,
+        data: dict[str, Any],
+    ):
+        res = self.post(
+            f"http://localhost:{self.port}/v1/tenant/{tenant_id}/timeline/{timeline_id}/patch_index_part",
+            json=data,
+        )
+        self.verbose_error(res)
+        return res.json()
+
     def tenant_location_conf(
         self,
         tenant_id: TenantId | TenantShardId,

test_runner/regress/test_pageserver_api.py

@@ -7,7 +7,7 @@ from fixtures.neon_fixtures import (
     NeonEnvBuilder,
 )
 from fixtures.pageserver.http import PageserverHttpClient
-from fixtures.utils import wait_until
+from fixtures.utils import run_only_on_default_postgres, wait_until
 
 
 def check_client(env: NeonEnv, client: PageserverHttpClient):
@@ -138,3 +138,25 @@ def test_pageserver_http_api_client_auth_enabled(neon_env_builder: NeonEnvBuilde
 
     with env.pageserver.http_client(auth_token=pageserver_token) as client:
         check_client(env, client)
+
+
+@run_only_on_default_postgres("it does not use any postgres functionality")
+def test_pageserver_http_index_part_force_patch(neon_env_builder: NeonEnvBuilder):
+    env = neon_env_builder.init_start()
+    tenant_id = env.initial_tenant
+    timeline_id = env.initial_timeline
+    with env.pageserver.http_client() as client:
+        client.timeline_patch_index_part(
+            tenant_id,
+            timeline_id,
+            {"rel_size_migration": "migrating"},
+        )
+        assert client.timeline_detail(tenant_id, timeline_id)["rel_size_migration"] == "migrating"
+        # This is invalid in practice: we should never rollback the migrating state to legacy.
+        # But we do it here to test the API.
+        client.timeline_patch_index_part(
+            tenant_id,
+            timeline_id,
+            {"rel_size_migration": "legacy"},
+        )
+        assert client.timeline_detail(tenant_id, timeline_id)["rel_size_migration"] == "legacy"