[Test] Extract common PD server setup into base fixture (#22080)

python/sglang/test/server_fixtures/disaggregation_fixture.py

@@ -12,6 +12,7 @@ from sglang.test.test_utils import (
     DEFAULT_URL_FOR_TEST,
     CustomTestCase,
     is_in_ci,
+    popen_launch_pd_server,
     popen_with_error_check,
 )
 from sglang.utils import wait_for_http_ready
@@ -56,6 +57,59 @@ class PDDisaggregationServerBase(CustomTestCase):
                 msg = "No RDMA devices specified for disaggregation test, using default settings."
                 warnings.warn(msg)
 
+    # Subclasses can set these to customize server args
+    extra_prefill_args = []
+    extra_decode_args = []
+
+    @classmethod
+    def start_prefill(cls):
+        prefill_args = [
+            "--trust-remote-code",
+            "--disaggregation-mode",
+            "prefill",
+            "--disaggregation-bootstrap-port",
+            cls.bootstrap_port,
+            "--tp",
+            "1",
+        ] + list(cls.extra_prefill_args)
+        prefill_args += cls.transfer_backend + cls.rdma_devices
+        cls.process_prefill = popen_launch_pd_server(
+            cls.model,
+            cls.prefill_url,
+            timeout=DEFAULT_TIMEOUT_FOR_SERVER_LAUNCH,
+            other_args=prefill_args,
+        )
+
+    @classmethod
+    def start_decode(cls):
+        decode_args = [
+            "--trust-remote-code",
+            "--disaggregation-mode",
+            "decode",
+            "--disaggregation-bootstrap-port",
+            cls.bootstrap_port,
+            "--tp",
+            "1",
+            "--base-gpu-id",
+            "1",
+        ] + list(cls.extra_decode_args)
+        decode_args += cls.transfer_backend + cls.rdma_devices
+        cls.process_decode = popen_launch_pd_server(
+            cls.model,
+            cls.decode_url,
+            timeout=DEFAULT_TIMEOUT_FOR_SERVER_LAUNCH,
+            other_args=decode_args,
+        )
+
+    @classmethod
+    def launch_all(cls):
+        """Start prefill, decode, wait for health, and launch LB."""
+        cls.start_prefill()
+        cls.start_decode()
+        cls.wait_server_ready(cls.prefill_url + "/health", process=cls.process_prefill)
+        cls.wait_server_ready(cls.decode_url + "/health", process=cls.process_decode)
+        cls.launch_lb()
+
     @classmethod
     def launch_lb(cls):
         lb_command = [