Make proxy shutdown when all connections are closed (#3764)

## Describe your changes
Makes Proxy start draining connections on SIGTERM.
## Issue ticket number and link
#3333

test_runner/fixtures/neon_fixtures.py

@@ -2041,6 +2041,17 @@ class NeonProxy(PgProtocol):
         self._wait_until_ready()
         return self
 
+    # Sends SIGTERM to the proxy if it has been started
+    def terminate(self):
+        if self._popen:
+            self._popen.terminate()
+
+    # Waits for proxy to exit if it has been opened with a default timeout of
+    # two seconds. Raises subprocess.TimeoutExpired if the proxy does not exit in time.
+    def wait_for_exit(self, timeout=2):
+        if self._popen:
+            self._popen.wait(timeout=2)
+
     @backoff.on_exception(backoff.expo, requests.exceptions.RequestException, max_time=10)
     def _wait_until_ready(self):
         requests.get(f"http://{self.host}:{self.http_port}/v1/status")

test_runner/regress/test_proxy.py

@@ -1,3 +1,5 @@
+import subprocess
+
 import psycopg2
 import pytest
 from fixtures.neon_fixtures import PSQL, NeonProxy, VanillaPostgres
@@ -134,3 +136,19 @@ def test_forward_params_to_client(static_proxy: NeonProxy):
             for name, value in cur.fetchall():
                 # Check that proxy has forwarded this parameter.
                 assert conn.get_parameter_status(name) == value
+
+
+@pytest.mark.timeout(5)
+def test_close_on_connections_exit(static_proxy: NeonProxy):
+    # Open two connections, send SIGTERM, then ensure that proxy doesn't exit
+    # until after connections close.
+    with static_proxy.connect(options="project=irrelevant"), static_proxy.connect(
+        options="project=irrelevant"
+    ):
+        static_proxy.terminate()
+        with pytest.raises(subprocess.TimeoutExpired):
+            static_proxy.wait_for_exit(timeout=2)
+        # Ensure we don't accept any more connections
+        with pytest.raises(psycopg2.OperationalError):
+            static_proxy.connect(options="project=irrelevant")
+    static_proxy.wait_for_exit()