PYTHON-3636 MongoClient should perform SRV resolution lazily

doc/changelog.rst

@@ -9,6 +9,8 @@ PyMongo 4.12 brings a number of changes including:
 - Support for configuring DEK cache lifetime via the ``key_expiration_ms`` argument to
   :class:`~pymongo.encryption_options.AutoEncryptionOpts`.
 - Support for $lookup in CSFLE and QE supported on MongoDB 8.1+.
+- AsyncMongoClient no longer performs dns resolution for "mongodb+srv://" connection string in the constructor.
+  To avoid blocking the asyncio loop, the resolution is now deferred until the client is first connected.
 
 Issues Resolved
 ...............

pymongo/asynchronous/encryption.py

@@ -87,7 +87,7 @@
 from pymongo.results import BulkWriteResult, DeleteResult
 from pymongo.ssl_support import get_ssl_context
 from pymongo.typings import _DocumentType, _DocumentTypeArg
-from pymongo.uri_parser import parse_host
+from pymongo.uri_parser_shared import parse_host
 from pymongo.write_concern import WriteConcern
 
 if TYPE_CHECKING:

pymongo/asynchronous/monitor.py

@@ -25,6 +25,7 @@
 
 from pymongo import common, periodic_executor
 from pymongo._csot import MovingMinimum
+from pymongo.asynchronous.srv_resolver import _SrvResolver
 from pymongo.errors import NetworkTimeout, _OperationCancelled
 from pymongo.hello import Hello
 from pymongo.lock import _async_create_lock
@@ -33,7 +34,6 @@
 from pymongo.pool_options import _is_faas
 from pymongo.read_preferences import MovingAverage
 from pymongo.server_description import ServerDescription
-from pymongo.srv_resolver import _SrvResolver
 
 if TYPE_CHECKING:
     from pymongo.asynchronous.pool import AsyncConnection, Pool, _CancellationContext
@@ -395,7 +395,7 @@ async def _run(self) -> None:
         # Don't poll right after creation, wait 60 seconds first
         if time.monotonic() < self._startup_time + common.MIN_SRV_RESCAN_INTERVAL:
             return
-        seedlist = self._get_seedlist()
+        seedlist = await self._get_seedlist()
         if seedlist:
             self._seedlist = seedlist
             try:
@@ -404,7 +404,7 @@ async def _run(self) -> None:
                 # Topology was garbage-collected.
                 await self.close()
 
-    def _get_seedlist(self) -> Optional[list[tuple[str, Any]]]:
+    async def _get_seedlist(self) -> Optional[list[tuple[str, Any]]]:
         """Poll SRV records for a seedlist.
 
         Returns a list of ServerDescriptions.
@@ -415,7 +415,7 @@ def _get_seedlist(self) -> Optional[list[tuple[str, Any]]]:
                 self._settings.pool_options.connect_timeout,
                 self._settings.srv_service_name,
             )
-            seedlist, ttl = resolver.get_hosts_and_min_ttl()
+            seedlist, ttl = await resolver.get_hosts_and_min_ttl()
             if len(seedlist) == 0:
                 # As per the spec: this should be treated as a failure.
                 raise Exception

pymongo/asynchronous/srv_resolver.py

@@ -0,0 +1,158 @@
+# Copyright 2019-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you
+# may not use this file except in compliance with the License.  You
+# may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.  See the License for the specific language governing
+# permissions and limitations under the License.
+
+"""Support for resolving hosts and options from mongodb+srv:// URIs."""
+from __future__ import annotations
+
+import ipaddress
+import random
+from typing import TYPE_CHECKING, Any, Optional, Union
+
+from pymongo.common import CONNECT_TIMEOUT
+from pymongo.errors import ConfigurationError
+
+if TYPE_CHECKING:
+    from dns import resolver
+
+_IS_SYNC = False
+
+
+def _have_dnspython() -> bool:
+    try:
+        import dns  # noqa: F401
+
+        return True
+    except ImportError:
+        return False
+
+
+# dnspython can return bytes or str from various parts
+# of its API depending on version. We always want str.
+def maybe_decode(text: Union[str, bytes]) -> str:
+    if isinstance(text, bytes):
+        return text.decode()
+    return text
+
+
+# PYTHON-2667 Lazily call dns.resolver methods for compatibility with eventlet.
+async def _resolve(*args: Any, **kwargs: Any) -> resolver.Answer:
+    if _IS_SYNC:
+        from dns import resolver
+
+        if hasattr(resolver, "resolve"):
+            # dnspython >= 2
+            return resolver.resolve(*args, **kwargs)
+        # dnspython 1.X
+        return resolver.query(*args, **kwargs)
+    else:
+        from dns import asyncresolver
+
+        if hasattr(asyncresolver, "resolve"):
+            # dnspython >= 2
+            return await asyncresolver.resolve(*args, **kwargs)  # type:ignore[return-value]
+        raise ConfigurationError("Upgrade to dnspython version >= 2.0")
+
+
+_INVALID_HOST_MSG = (
+    "Invalid URI host: %s is not a valid hostname for 'mongodb+srv://'. "
+    "Did you mean to use 'mongodb://'?"
+)
+
+
+class _SrvResolver:
+    def __init__(
+        self,
+        fqdn: str,
+        connect_timeout: Optional[float],
+        srv_service_name: str,
+        srv_max_hosts: int = 0,
+    ):
+        self.__fqdn = fqdn
+        self.__srv = srv_service_name
+        self.__connect_timeout = connect_timeout or CONNECT_TIMEOUT
+        self.__srv_max_hosts = srv_max_hosts or 0
+        # Validate the fully qualified domain name.
+        try:
+            ipaddress.ip_address(fqdn)
+            raise ConfigurationError(_INVALID_HOST_MSG % ("an IP address",))
+        except ValueError:
+            pass
+
+        try:
+            self.__plist = self.__fqdn.split(".")[1:]
+        except Exception:
+            raise ConfigurationError(_INVALID_HOST_MSG % (fqdn,)) from None
+        self.__slen = len(self.__plist)
+        if self.__slen < 2:
+            raise ConfigurationError(_INVALID_HOST_MSG % (fqdn,))
+
+    async def get_options(self) -> Optional[str]:
+        from dns import resolver
+
+        try:
+            results = await _resolve(self.__fqdn, "TXT", lifetime=self.__connect_timeout)
+        except (resolver.NoAnswer, resolver.NXDOMAIN):
+            # No TXT records
+            return None
+        except Exception as exc:
+            raise ConfigurationError(str(exc)) from None
+        if len(results) > 1:
+            raise ConfigurationError("Only one TXT record is supported")
+        return (b"&".join([b"".join(res.strings) for res in results])).decode("utf-8")  # type: ignore[attr-defined]
+
+    async def _resolve_uri(self, encapsulate_errors: bool) -> resolver.Answer:
+        try:
+            results = await _resolve(
+                "_" + self.__srv + "._tcp." + self.__fqdn, "SRV", lifetime=self.__connect_timeout
+            )
+        except Exception as exc:
+            if not encapsulate_errors:
+                # Raise the original error.
+                raise
+            # Else, raise all errors as ConfigurationError.
+            raise ConfigurationError(str(exc)) from None
+        return results
+
+    async def _get_srv_response_and_hosts(
+        self, encapsulate_errors: bool
+    ) -> tuple[resolver.Answer, list[tuple[str, Any]]]:
+        results = await self._resolve_uri(encapsulate_errors)
+
+        # Construct address tuples
+        nodes = [
+            (maybe_decode(res.target.to_text(omit_final_dot=True)), res.port)  # type: ignore[attr-defined]
+            for res in results
+        ]
+
+        # Validate hosts
+        for node in nodes:
+            try:
+                nlist = node[0].lower().split(".")[1:][-self.__slen :]
+            except Exception:
+                raise ConfigurationError(f"Invalid SRV host: {node[0]}") from None
+            if self.__plist != nlist:
+                raise ConfigurationError(f"Invalid SRV host: {node[0]}")
+        if self.__srv_max_hosts:
+            nodes = random.sample(nodes, min(self.__srv_max_hosts, len(nodes)))
+        return results, nodes
+
+    async def get_hosts(self) -> list[tuple[str, Any]]:
+        _, nodes = await self._get_srv_response_and_hosts(True)
+        return nodes
+
+    async def get_hosts_and_min_ttl(self) -> tuple[list[tuple[str, Any]], int]:
+        results, nodes = await self._get_srv_response_and_hosts(False)
+        rrset = results.rrset
+        ttl = rrset.ttl if rrset else 0
+        return nodes, ttl