Support changing pageserver dynamically (#5542)

## Problem
We currently require full restart of compute if we change the pageserver
url
## Summary of changes
Makes it so that we don't have to do a full restart, but can just send
SIGHUP

test_runner/fixtures/neon_fixtures.py

@@ -1445,6 +1445,20 @@ class NeonCli(AbstractNeonCli):
         res.check_returncode()
         return res
 
+    def endpoint_reconfigure(
+        self,
+        endpoint_id: str,
+        tenant_id: Optional[TenantId] = None,
+        pageserver_id: Optional[int] = None,
+        check_return_code=True,
+    ) -> "subprocess.CompletedProcess[str]":
+        args = ["endpoint", "reconfigure", endpoint_id]
+        if tenant_id is not None:
+            args.extend(["--tenant-id", str(tenant_id)])
+        if pageserver_id is not None:
+            args.extend(["--pageserver-id", str(pageserver_id)])
+        return self.raw_cli(args, check_return_code=check_return_code)
+
     def endpoint_stop(
         self,
         endpoint_id: str,
@@ -2534,6 +2548,10 @@ class Endpoint(PgProtocol):
 
         return self
 
+    def reconfigure(self, pageserver_id: Optional[int] = None):
+        assert self.endpoint_id is not None
+        self.env.neon_cli.endpoint_reconfigure(self.endpoint_id, self.tenant_id, pageserver_id)
+
     def respec(self, **kwargs):
         """Update the endpoint.json file used by control_plane."""
         # Read config

test_runner/regress/test_change_pageserver.py

@@ -0,0 +1,64 @@
+from fixtures.log_helper import log
+from fixtures.neon_fixtures import NeonEnvBuilder
+from fixtures.remote_storage import RemoteStorageKind
+
+
+def test_change_pageserver(neon_env_builder: NeonEnvBuilder):
+    neon_env_builder.num_pageservers = 2
+    neon_env_builder.enable_pageserver_remote_storage(
+        remote_storage_kind=RemoteStorageKind.MOCK_S3,
+    )
+    env = neon_env_builder.init_start()
+
+    env.neon_cli.create_branch("test_change_pageserver")
+    endpoint = env.endpoints.create_start("test_change_pageserver")
+
+    alt_pageserver_id = env.pageservers[1].id
+    env.pageservers[1].tenant_attach(env.initial_tenant)
+
+    pg_conn = endpoint.connect()
+    cur = pg_conn.cursor()
+
+    # Create table, and insert some rows. Make it big enough that it doesn't fit in
+    # shared_buffers, otherwise the SELECT after restart will just return answer
+    # from shared_buffers without hitting the page server, which defeats the point
+    # of this test.
+    cur.execute("CREATE TABLE foo (t text)")
+    cur.execute(
+        """
+        INSERT INTO foo
+            SELECT 'long string to consume some space' || g
+            FROM generate_series(1, 100000) g
+        """
+    )
+
+    # Verify that the table is larger than shared_buffers
+    cur.execute(
+        """
+        select setting::int * pg_size_bytes(unit) as shared_buffers, pg_relation_size('foo') as tbl_size
+        from pg_settings where name = 'shared_buffers'
+        """
+    )
+    row = cur.fetchone()
+    assert row is not None
+    log.info(f"shared_buffers is {row[0]}, table size {row[1]}")
+    assert int(row[0]) < int(row[1])
+
+    cur.execute("SELECT count(*) FROM foo")
+    assert cur.fetchone() == (100000,)
+
+    endpoint.reconfigure(pageserver_id=alt_pageserver_id)
+
+    # Verify that the neon.pageserver_connstring GUC is set to the correct thing
+    cur.execute("SELECT setting FROM pg_settings WHERE name='neon.pageserver_connstring'")
+    connstring = cur.fetchone()
+    assert connstring is not None
+    expected_connstring = f"postgresql://no_user:@localhost:{env.pageservers[1].service_port.pg}"
+    assert expected_connstring == expected_connstring
+
+    env.pageservers[
+        0
+    ].stop()  # Stop the old pageserver just to make sure we're reading from the new one
+
+    cur.execute("SELECT count(*) FROM foo")
+    assert cur.fetchone() == (100000,)