tests: reproducer for https://github.com/neondatabase/neon/issues/11439

test_runner/regress/test_lsn_mapping.py

@@ -276,3 +276,33 @@ def test_ts_of_lsn_api(neon_env_builder: NeonEnvBuilder):
             if i > 1:
                 before_timestamp = tbl[i - step_size][1]
                 assert timestamp >= before_timestamp, "before_timestamp before timestamp"
+
+
+def test_timestamp_of_lsn_empty_branch(neon_env_builder: NeonEnvBuilder):
+    """
+    Test that getting the timestamp of the head LSN of a newly created branch works.
+    This verifies that we don't get a 404 error when trying to get the timestamp
+    of the head LSN of a branch that was just created.
+
+    Reproducer for https://github.com/neondatabase/neon/issues/11439
+    """
+    env = neon_env_builder.init_start()
+
+    # Create a new branch
+    new_timeline_id = env.create_branch("test_timestamp_of_lsn_empty_branch")
+
+    # Retrieve the commit LSN of the empty branch, which we have never run postgres on
+    detail = env.pageserver.http_client().timeline_detail(
+        tenant_id=env.initial_tenant, timeline_id=new_timeline_id
+    )
+    head_lsn = detail["last_record_lsn"]
+
+    # Verify we can get the timestamp for this LSN
+    with env.pageserver.http_client() as client:
+        result = client.timeline_get_timestamp_of_lsn(
+            env.initial_tenant,
+            new_timeline_id,
+            head_lsn,
+        )
+        # If we get here without a 404 error, the test passes
+        assert result is not None