pageserver: add a /reset API for tenants (#6014)

## Problem

Traditionally we would detach/attach directly with curl if we wanted to
"reboot" a single tenant. That's kind of inconvenient these days,
because one needs to know a generation number to issue an attach
request.

Closes: https://github.com/neondatabase/neon/issues/6011

## Summary of changes

- Introduce a new `/reset` API, which remembers the LocationConf from
the current attachment so that callers do not have to work out the
correct configuration/generation to use.
- As an additional support tool, allow an optional `drop_cache` query
parameter, for situations where we are concerned that some on-disk state
might be bad and want to clear that as well as the in-memory state.

One might wonder why I didn't call this "reattach" -- it's because
there's already a PS->CP API of that name and it could get confusing.

test_runner/fixtures/pageserver/http.py

@@ -260,6 +260,14 @@ class PageserverHttpClient(requests.Session):
         res = self.post(f"http://localhost:{self.port}/v1/tenant/{tenant_id}/detach", params=params)
         self.verbose_error(res)
 
+    def tenant_reset(self, tenant_id: TenantId, drop_cache: bool):
+        params = {}
+        if drop_cache:
+            params["drop_cache"] = "true"
+
+        res = self.post(f"http://localhost:{self.port}/v1/tenant/{tenant_id}/reset", params=params)
+        self.verbose_error(res)
+
     def tenant_delete(self, tenant_id: TenantId):
         res = self.delete(f"http://localhost:{self.port}/v1/tenant/{tenant_id}")
         self.verbose_error(res)

test_runner/regress/test_tenant_detach.py

@@ -1,4 +1,5 @@
 import asyncio
+import enum
 import random
 import time
 from threading import Thread
@@ -51,11 +52,20 @@ def do_gc_target(
         log.info("gc http thread returning")
 
 
+class ReattachMode(str, enum.Enum):
+    REATTACH_EXPLICIT = "explicit"
+    REATTACH_RESET = "reset"
+    REATTACH_RESET_DROP = "reset"
+
+
 # Basic detach and re-attach test
 @pytest.mark.parametrize("remote_storage_kind", available_remote_storages())
+@pytest.mark.parametrize(
+    "mode",
+    [ReattachMode.REATTACH_EXPLICIT, ReattachMode.REATTACH_RESET, ReattachMode.REATTACH_RESET_DROP],
+)
 def test_tenant_reattach(
-    neon_env_builder: NeonEnvBuilder,
-    remote_storage_kind: RemoteStorageKind,
+    neon_env_builder: NeonEnvBuilder, remote_storage_kind: RemoteStorageKind, mode: str
 ):
     neon_env_builder.enable_pageserver_remote_storage(remote_storage_kind)
 
@@ -100,8 +110,15 @@ def test_tenant_reattach(
         ps_metrics.query_one("pageserver_last_record_lsn", filter=tenant_metric_filter).value
     )
 
-    pageserver_http.tenant_detach(tenant_id)
-    pageserver_http.tenant_attach(tenant_id)
+    if mode == ReattachMode.REATTACH_EXPLICIT:
+        # Explicitly detach then attach the tenant as two separate API calls
+        pageserver_http.tenant_detach(tenant_id)
+        pageserver_http.tenant_attach(tenant_id)
+    elif mode in (ReattachMode.REATTACH_RESET, ReattachMode.REATTACH_RESET_DROP):
+        # Use the reset API to detach/attach in one shot
+        pageserver_http.tenant_reset(tenant_id, mode == ReattachMode.REATTACH_RESET_DROP)
+    else:
+        raise NotImplementedError(mode)
 
     time.sleep(1)  # for metrics propagation