feat: Support dynamic query parameters on reconnect

The `ConnectStrategy` can be created with a `query_params` callable.
This callable will return a set of parameters that should be used to
update any static query parameters initially configured.

This functionality enables FDv2 selector behavior where we want to
resume from our last known checkpoint.

Author: keelerm84

ld_eventsource/config/connect_strategy.py

@@ -1,11 +1,13 @@
 from __future__ import annotations
 
+from dataclasses import dataclass
 from logging import Logger
 from typing import Callable, Iterator, Optional, Union
 
 from urllib3 import PoolManager
 
-from ld_eventsource.http import _HttpClientImpl, _HttpConnectParams
+from ld_eventsource.http import (DynamicQueryParams, _HttpClientImpl,
+                                 _HttpConnectParams)
 
 
 class ConnectStrategy:
@@ -38,6 +40,7 @@ def http(
         headers: Optional[dict] = None,
         pool: Optional[PoolManager] = None,
         urllib3_request_options: Optional[dict] = None,
+        query_params: Optional[DynamicQueryParams] = None
     ) -> ConnectStrategy:
         """
         Creates the default HTTP implementation, specifying request parameters.
@@ -47,9 +50,11 @@ def http(
         :param pool: optional urllib3 ``PoolManager`` to provide an HTTP client
         :param urllib3_request_options: optional ``kwargs`` to add to the ``request`` call; these
             can include any parameters supported by ``urllib3``, such as ``timeout``
+        :param query_params: optional callable that can be used to affect query parameters
+            dynamically for each connection attempt
         """
         return _HttpConnectStrategy(
-            _HttpConnectParams(url, headers, pool, urllib3_request_options)
+            _HttpConnectParams(url, headers, pool, urllib3_request_options, query_params)
         )
 
 

ld_eventsource/http.py

@@ -1,5 +1,6 @@
 from logging import Logger
 from typing import Callable, Iterator, Optional, Tuple
+from urllib.parse import parse_qsl, urlencode, urlsplit, urlunsplit
 
 from urllib3 import PoolManager
 from urllib3.exceptions import MaxRetryError
@@ -9,6 +10,12 @@
 
 _CHUNK_SIZE = 10000
 
+DynamicQueryParams = Callable[[], dict[str, str]]
+"""
+A callable that returns a dictionary of query parameters to add to the URL.
+This can be used to modify query parameters dynamically for each connection attempt.
+"""
+
 
 class _HttpConnectParams:
     def __init__(
@@ -17,16 +24,22 @@ def __init__(
         headers: Optional[dict] = None,
         pool: Optional[PoolManager] = None,
         urllib3_request_options: Optional[dict] = None,
+        query_params: Optional[DynamicQueryParams] = None
     ):
         self.__url = url
         self.__headers = headers
         self.__pool = pool
         self.__urllib3_request_options = urllib3_request_options
+        self.__query_params = query_params
 
     @property
     def url(self) -> str:
         return self.__url
 
+    @property
+    def query_params(self) -> Optional[DynamicQueryParams]:
+        return self.__query_params
+
     @property
     def headers(self) -> Optional[dict]:
         return self.__headers
@@ -48,7 +61,16 @@ def __init__(self, params: _HttpConnectParams, logger: Logger):
         self.__logger = logger
 
     def connect(self, last_event_id: Optional[str]) -> Tuple[Iterator[bytes], Callable]:
-        self.__logger.info("Connecting to stream at %s" % self.__params.url)
+        url = self.__params.url
+        if self.__params.query_params is not None:
+            qp = self.__params.query_params()
+            if qp:
+                url_parts = list(urlsplit(url))
+                query = dict(parse_qsl(url_parts[3]))
+                query.update(qp)
+                url_parts[3] = urlencode(query)
+                url = urlunsplit(url_parts)
+        self.__logger.info("Connecting to stream at %s" % url)
 
         headers = self.__params.headers.copy() if self.__params.headers else {}
         headers['Cache-Control'] = 'no-cache'
@@ -67,7 +89,7 @@ def connect(self, last_event_id: Optional[str]) -> Tuple[Iterator[bytes], Callab
         try:
             resp = self.__pool.request(
                 'GET',
-                self.__params.url,
+                url,
                 preload_content=False,
                 retries=Retry(
                     total=None, read=0, connect=0, status=0, other=0, redirect=3

ld_eventsource/testing/http_util.py

@@ -113,7 +113,7 @@ def do_POST(self):
     def _do_request(self):
         server_wrapper = self.server.server_wrapper
         server_wrapper.requests.put(MockServerRequest(self))
-        handler = server_wrapper.matchers.get(self.path)
+        handler = server_wrapper.matchers.get(self.path.split("?")[0], None)
         if handler:
             handler.write(self)
         else:

ld_eventsource/testing/test_http_connect_strategy_with_sse_client.py

@@ -1,3 +1,5 @@
+from urllib.parse import parse_qsl
+
 from ld_eventsource import *
 from ld_eventsource.config import *
 from ld_eventsource.testing.helpers import *
@@ -56,6 +58,48 @@ def test_sse_client_reconnects_after_socket_closed():
                     assert event2.data == 'data2'
 
 
+def test_sse_client_allows_modifying_query_params_dynamically():
+    count = 0
+
+    def dynamic_query_params() -> dict[str, str]:
+        nonlocal count
+        count += 1
+        params = {'count': str(count)}
+        if count > 1:
+            params['option'] = 'updated'
+
+        return params
+
+    with start_server() as server:
+        with make_stream() as stream1:
+            with make_stream() as stream2:
+                server.for_path('/', SequentialHandler(stream1, stream2))
+                stream1.push("event: a\ndata: data1\nid: id123\n\n")
+                stream2.push("event: b\ndata: data2\n\n")
+                with SSEClient(
+                    connect=ConnectStrategy.http(f"{server.uri}?basis=unchanging&option=initial", query_params=dynamic_query_params),
+                    error_strategy=ErrorStrategy.always_continue(),
+                    initial_retry_delay=0,
+                ) as client:
+                    client.start()
+                    next(client.events)
+                    stream1.close()
+                    next(client.events)
+                    r1 = server.await_request()
+                    r1_query_params = dict(parse_qsl(r1.path.split('?', 1)[1]))
+
+                    # Ensure we can add, retain, and modify query parameters
+                    assert r1_query_params.get('count') == '1'
+                    assert r1_query_params.get('basis') == 'unchanging'
+                    assert r1_query_params.get('option') == 'initial'
+
+                    r2 = server.await_request()
+                    r2_query_params = dict(parse_qsl(r2.path.split('?', 1)[1]))
+                    assert r2_query_params.get('count') == '2'
+                    assert r2_query_params.get('basis') == 'unchanging'
+                    assert r2_query_params.get('option') == 'updated'
+
+
 def test_sse_client_sends_last_event_id_on_reconnect():
     with start_server() as server:
         with make_stream() as stream1: