tests: reinstate some syntethic size tests (#7294)

## Problem

`test_empty_tenant_size` was marked `xfail` and a few other tests were
skipped.

## Summary of changes

Stabilise `test_empty_tenant_size`. This test attempted to disable
checkpointing for the postgres instance
and expected that the synthetic size remains stable for an empty tenant.
When debugging I noticed that
postgres *was* issuing a checkpoint after the transaction in the test
(perhaps something changed since the
test was introduced). Hence, I relaxed the size check to allow for the
checkpoint key written on the pageserver.

Also removed the checks for synthetic size inputs since the expected
values differ between postgres versions.

Closes https://github.com/neondatabase/neon/issues/7138

test_runner/regress/test_tenant_size.py

@@ -20,9 +20,10 @@ from fixtures.pg_version import PgVersion
 from fixtures.types import Lsn, TenantId, TimelineId
 
 
-@pytest.mark.xfail
-def test_empty_tenant_size(neon_simple_env: NeonEnv, test_output_dir: Path):
-    env = neon_simple_env
+def test_empty_tenant_size(neon_env_builder: NeonEnvBuilder):
+    env = neon_env_builder.init_configs()
+    env.start()
+
     (tenant_id, _) = env.neon_cli.create_tenant()
     http_client = env.pageserver.http_client()
     initial_size = http_client.tenant_size(tenant_id)
@@ -35,66 +36,25 @@ def test_empty_tenant_size(neon_simple_env: NeonEnv, test_output_dir: Path):
     branch_name, main_timeline_id = env.neon_cli.list_timelines(tenant_id)[0]
     assert branch_name == main_branch_name
 
-    with env.endpoints.create_start(
+    endpoint = env.endpoints.create_start(
         main_branch_name,
         tenant_id=tenant_id,
         config_lines=["autovacuum=off", "checkpoint_timeout=10min"],
-    ) as endpoint:
-        with endpoint.cursor() as cur:
-            cur.execute("SELECT 1")
-            row = cur.fetchone()
-            assert row is not None
-            assert row[0] == 1
-        size = http_client.tenant_size(tenant_id)
-        # we've disabled the autovacuum and checkpoint
-        # so background processes should not change the size.
-        # If this test will flake we should probably loosen the check
-        assert (
-            size == initial_size
-        ), f"starting idle compute should not change the tenant size (Currently {size}, expected {initial_size})"
+    )
 
-    # the size should be the same, until we increase the size over the
-    # gc_horizon
-    size, inputs = http_client.tenant_size_and_modelinputs(tenant_id)
-    assert (
-        size == initial_size
-    ), f"tenant_size should not be affected by shutdown of compute (Currently {size}, expected {initial_size})"
+    with endpoint.cursor() as cur:
+        cur.execute("SELECT 1")
+        row = cur.fetchone()
+        assert row is not None
+        assert row[0] == 1
 
-    expected_inputs = {
-        "segments": [
-            {
-                "segment": {"parent": None, "lsn": 23694408, "size": 25362432, "needed": True},
-                "timeline_id": f"{main_timeline_id}",
-                "kind": "BranchStart",
-            },
-            {
-                "segment": {"parent": 0, "lsn": 23694528, "size": None, "needed": True},
-                "timeline_id": f"{main_timeline_id}",
-                "kind": "BranchEnd",
-            },
-        ],
-        "timeline_inputs": [
-            {
-                "timeline_id": f"{main_timeline_id}",
-                "ancestor_id": None,
-                "ancestor_lsn": "0/0",
-                "last_record": "0/1698CC0",
-                "latest_gc_cutoff": "0/1698C48",
-                "horizon_cutoff": "0/0",
-                "pitr_cutoff": "0/0",
-                "next_gc_cutoff": "0/0",
-                "retention_param_cutoff": None,
-            }
-        ],
-    }
-    expected_inputs = mask_model_inputs(expected_inputs)
-    actual_inputs = mask_model_inputs(inputs)
+    # The transaction above will make the compute generate a checkpoint.
+    # In turn, the pageserver persists the checkpoint. This should only be
+    # one key with a size of a couple hundred bytes.
+    wait_for_last_flush_lsn(env, endpoint, tenant_id, main_timeline_id)
+    size = http_client.tenant_size(tenant_id)
 
-    assert expected_inputs == actual_inputs
-
-    size_debug_file = open(test_output_dir / "size_debug.html", "w")
-    size_debug = http_client.tenant_size_debug(tenant_id)
-    size_debug_file.write(size_debug)
+    assert size >= initial_size and size - initial_size < 1024
 
 
 def test_branched_empty_timeline_size(neon_simple_env: NeonEnv, test_output_dir: Path):
@@ -190,7 +150,6 @@ def test_branched_from_many_empty_parents_size(neon_simple_env: NeonEnv, test_ou
     size_debug_file.write(size_debug)
 
 
-@pytest.mark.skip("This should work, but is left out because assumed covered by other tests")
 def test_branch_point_within_horizon(neon_simple_env: NeonEnv, test_output_dir: Path):
     """
     gc_horizon = 15
@@ -233,7 +192,6 @@ def test_branch_point_within_horizon(neon_simple_env: NeonEnv, test_output_dir:
     size_debug_file.write(size_debug)
 
 
-@pytest.mark.skip("This should work, but is left out because assumed covered by other tests")
 def test_parent_within_horizon(neon_simple_env: NeonEnv, test_output_dir: Path):
     """
     gc_horizon = 5
@@ -282,7 +240,6 @@ def test_parent_within_horizon(neon_simple_env: NeonEnv, test_output_dir: Path):
     size_debug_file.write(size_debug)
 
 
-@pytest.mark.skip("This should work, but is left out because assumed covered by other tests")
 def test_only_heads_within_horizon(neon_simple_env: NeonEnv, test_output_dir: Path):
     """
     gc_horizon = small