Refactor to use race().

src/allmydata/storage_client.py

@@ -82,7 +82,7 @@ from allmydata.util.observer import ObserverList
 from allmydata.util.rrefutil import add_version_to_remote_reference
 from allmydata.util.hashutil import permute_server_hash
 from allmydata.util.dictutil import BytesKeyDict, UnicodeKeyDict
-from allmydata.util.deferredutil import async_to_deferred
+from allmydata.util.deferredutil import async_to_deferred, race
 from allmydata.storage.http_client import (
     StorageClient, StorageClientImmutables, StorageClientGeneral,
     ClientException as HTTPClientException, StorageClientMutables,
@@ -1017,42 +1017,23 @@ def _pick_a_http_server(
     Fires with ``None`` if no server was found, or with the ``DecodedURL`` of
     the first successfully-connected server.
     """
+    queries = race([
+        request(reactor, nurl).addCallback(lambda _, nurl=nurl: nurl)
+        for nurl in nurls
+    ])
 
-    to_cancel : list[defer.Deferred] = []
-
-    def cancel(result: Optional[defer.Deferred]):
-        for d in to_cancel:
-            if not d.called:
-                d.cancel()
-        if result is not None:
-            result.errback(defer.CancelledError())
-
-    result : defer.Deferred[Optional[DecodedURL]] = defer.Deferred(canceller=cancel)
-
-    def succeeded(nurl: DecodedURL, result=result):
-        # Only need the first successful NURL:
-        if result.called:
-            return
-        result.callback(nurl)
-        # No point in continuing other requests if we're connected:
-        cancel(None)
-
-    def failed(failure, failures=[], result=result):
+    def failed(failure: Failure):
         # Logging errors breaks a bunch of tests, and it's not a _bug_ to
         # have a failed connection, it's often expected and transient. More
         # of a warning, really?
         log.msg("Failed to connect to NURL: {}".format(failure))
-        failures.append(None)
-        if len(failures) == len(nurls):
-            # All our potential NURLs failed...
-            result.callback(None)
+        return None
 
-    for index, nurl in enumerate(nurls):
-        d = request(reactor, nurl)
-        to_cancel.append(d)
-        d.addCallback(lambda _, nurl=nurl: nurl).addCallbacks(succeeded, failed)
+    def succeeded(result: tuple[int, DecodedURL]):
+        _, nurl = result
+        return nurl
 
-    return result
+    return queries.addCallbacks(succeeded, failed)
 
 
 @implementer(IServer)