Re: [ovs-dev] [PATCH v3] python: Add async DNS support

[Adrian Moreno]

On 6/30/23 13:40, Ilya Maximets wrote:
On 6/30/23 12:39, Adrian Moreno wrote:


On 6/14/23 23:07, Terry Wilson wrote:
This adds a Python version of the async DNS support added in:

771680d96 DNS: Add basic support for asynchronous DNS resolving

The above version uses the unbound C library, and this
implimentation uses the SWIG-wrapped Python version of that.

In the event that the Python unbound library is not available,
a warning will be logged and the resolve() method will just
return None. For the case where inet_parse_active() is passed
an IP address, it will not try to resolve it, so existing
behavior should be preserved in the case that the unbound
library is unavailable.

Intentional differences from the C version are as follows:

    OVS_HOSTS_FILE environment variable can bet set to override
    the system 'hosts' file. This is primarily to allow testing to
    be done without requiring network connectivity.

    Since resolution can still be done via hosts file lookup, DNS
    lookups are not disabled when resolv.conf cannot be loaded.

    The Python socket_util module has fallen behind its C equivalent.
    The bare minimum change was done to inet_parse_active() to support
    sync/async dns, as there is no equivalent to
    parse_sockaddr_components(), inet_parse_passive(), etc. A TODO
    was added to bring socket_util.py up to equivalency to the C
    version.

Signed-off-by: Terry Wilson <twil...@redhat.com>
---
   .github/workflows/build-and-test.yml    |   4 +-
   Documentation/intro/install/general.rst |   4 +-
   Documentation/intro/install/rhel.rst    |   2 +-
   Documentation/intro/install/windows.rst |   2 +-
   NEWS                                    |   4 +-
   debian/control.in                       |   1 +
   m4/openvswitch.m4                       |   8 +-
   python/TODO.rst                         |   7 +
   python/automake.mk                      |   2 +
   python/ovs/dns_resolve.py               | 272 +++++++++++++++++++++++
   python/ovs/socket_util.py               |  21 +-
   python/ovs/stream.py                    |   2 +-
   python/ovs/tests/test_dns_resolve.py    | 280 ++++++++++++++++++++++++
   python/setup.py                         |   6 +-
   rhel/openvswitch-fedora.spec.in         |   2 +-
   tests/vlog.at                           |   2 +
   16 files changed, 601 insertions(+), 18 deletions(-)
   create mode 100644 python/ovs/dns_resolve.py
   create mode 100644 python/ovs/tests/test_dns_resolve.py

diff --git a/.github/workflows/build-and-test.yml 
b/.github/workflows/build-and-test.yml
index f66ab43b0..47d239f10 100644
--- a/.github/workflows/build-and-test.yml
+++ b/.github/workflows/build-and-test.yml
@@ -183,10 +183,10 @@ jobs:
         run:  sudo apt update || true
       - name: install common dependencies
         run:  sudo apt install -y ${{ env.dependencies }}
-    - name: install libunbound libunwind
+    - name: install libunbound libunwind python3-unbound
         # GitHub Actions doesn't have 32-bit versions of these libraries.
         if:   matrix.m32 == ''
-      run:  sudo apt install -y libunbound-dev libunwind-dev
+      run:  sudo apt install -y libunbound-dev libunwind-dev python3-unbound
       - name: install 32-bit libraries
         if:   matrix.m32 != ''
         run:  sudo apt install -y gcc-multilib
diff --git a/Documentation/intro/install/general.rst 
b/Documentation/intro/install/general.rst
index 42b5682fd..19e360d47 100644
--- a/Documentation/intro/install/general.rst
+++ b/Documentation/intro/install/general.rst
@@ -90,7 +90,7 @@ need the following software:
     If libcap-ng is installed, then Open vSwitch will automatically build with
     support for it.
   
-- Python 3.4 or later.
+- Python 3.6 or later.
   
   - Unbound library, from http://www.unbound.net, is optional but recommended if
     you want to enable ovs-vswitchd and other utilities to use DNS names when
@@ -208,7 +208,7 @@ simply install and run Open vSwitch you require the 
following software:
     from iproute2 (part of all major distributions and available at
     https://wiki.linuxfoundation.org/networking/iproute2).
   
-- Python 3.4 or later.
+- Python 3.6 or later.
   
   On Linux you should ensure that ``/dev/urandom`` exists. To support TAP
   devices, you must also ensure that ``/dev/net/tun`` exists.
diff --git a/Documentation/intro/install/rhel.rst 
b/Documentation/intro/install/rhel.rst
index d1fc42021..f2151d890 100644
--- a/Documentation/intro/install/rhel.rst
+++ b/Documentation/intro/install/rhel.rst
@@ -92,7 +92,7 @@ Once that is completed, remove the file ``/tmp/ovs.spec``.
   If python3-sphinx package is not available in your version of RHEL, you can
   install it via pip with 'pip install sphinx'.
   
-Open vSwitch requires python 3.4 or newer which is not available in older
+Open vSwitch requires python 3.6 or newer which is not available in older
   distributions. In the case of RHEL 6.x and its derivatives, one option is
   to install python34 from `EPEL`_.
   
diff --git a/Documentation/intro/install/windows.rst b/Documentation/intro/install/windows.rst
index 78f60f35a..fce099d5d 100644
--- a/Documentation/intro/install/windows.rst
+++ b/Documentation/intro/install/windows.rst
@@ -56,7 +56,7 @@ The following explains the steps in some detail.
   
         'C:/MinGW /mingw'.
   
-- Python 3.4 or later.
+- Python 3.6 or later.
   
     Install the latest Python 3.x from python.org and verify that its path is
     part of Windows' PATH environment variable.
diff --git a/NEWS b/NEWS
index cfd466663..24c694b8f 100644
--- a/NEWS
+++ b/NEWS
@@ -36,7 +36,9 @@ Post-v3.1.0
          process extra privileges when mapping physical interconnect memory.
      - SRv6 Tunnel Protocol
        * Added support for userspace datapath (only).
-
+   - Python
+     * Added async DNS support
+     * Dropped support for Python < 3.6
   
   v3.1.0 - 16 Feb 2023
   --------------------
diff --git a/debian/control.in b/debian/control.in
index 19f590d06..64b0a4ce0 100644
--- a/debian/control.in
+++ b/debian/control.in
@@ -287,6 +287,7 @@ Depends:
   Suggests:
    python3-netaddr,
    python3-pyparsing,
+ python3-unbound,
   Description: Python 3 bindings for Open vSwitch
    Open vSwitch is a production quality, multilayer, software-based,
    Ethernet virtual switch. It is designed to enable massive network
diff --git a/m4/openvswitch.m4 b/m4/openvswitch.m4
index 14d9249b8..373a7e413 100644
--- a/m4/openvswitch.m4
+++ b/m4/openvswitch.m4
@@ -371,16 +371,16 @@ dnl Checks for valgrind/valgrind.h.
   AC_DEFUN([OVS_CHECK_VALGRIND],
     [AC_CHECK_HEADERS([valgrind/valgrind.h])])
   
-dnl Checks for Python 3.4 or later.
+dnl Checks for Python 3.6 or later.
   AC_DEFUN([OVS_CHECK_PYTHON3],
     [AC_CACHE_CHECK(
-     [for Python 3 (version 3.4 or later)],
+     [for Python 3 (version 3.6 or later)],
        [ovs_cv_python3],
        [if test -n "$PYTHON3"; then
           ovs_cv_python3=$PYTHON3
         else
           ovs_cv_python3=no
-        for binary in python3 python3.4 python3.5 python3.6 python3.7; do
+        for binary in python3{,.{6..12}}; do
             ovs_save_IFS=$IFS; IFS=$PATH_SEPARATOR
             for dir in $PATH; do
               IFS=$ovs_save_IFS
@@ -397,7 +397,7 @@ else:
           done
         fi])
      if test "$ovs_cv_python3" = no; then
-     AC_MSG_ERROR([Python 3.4 or later is required but not found in $PATH, 
please install it or set $PYTHON3 to point to it])
+     AC_MSG_ERROR([Python 3.6 or later is required but not found in $PATH, 
please install it or set $PYTHON3 to point to it])
      fi
      AC_ARG_VAR([PYTHON3])
      PYTHON3=$ovs_cv_python3])
diff --git a/python/TODO.rst b/python/TODO.rst
index 3a53489f1..acc5461e2 100644
--- a/python/TODO.rst
+++ b/python/TODO.rst
@@ -32,3 +32,10 @@ Python Bindings To-do List
   
     * Support write-only-changed monitor mode (equivalent of
       OVSDB_IDL_WRITE_CHANGED_ONLY).
+
+* socket_util:
+
+  * Add equivalent fuctions to inet_parse_passive, parse_sockaddr_components,
+    et al. to better support using async dns. The reconnect code will
+    currently log a warning when inet_parse_active() returns w/o yet having
+    resolved an address, but will continue to connect and eventually succeed.
diff --git a/python/automake.mk b/python/automake.mk
index d00911828..82a508787 100644
--- a/python/automake.mk
+++ b/python/automake.mk
@@ -16,6 +16,7 @@ ovs_pyfiles = \
        python/ovs/compat/sortedcontainers/sorteddict.py \
        python/ovs/compat/sortedcontainers/sortedset.py \
        python/ovs/daemon.py \
+       python/ovs/dns_resolve.py \
        python/ovs/db/__init__.py \
        python/ovs/db/custom_index.py \
        python/ovs/db/data.py \
@@ -55,6 +56,7 @@ ovs_pyfiles = \
   
   ovs_pytests = \
        python/ovs/tests/test_decoders.py \
+       python/ovs/tests/test_dns_resolve.py \
        python/ovs/tests/test_filter.py \
        python/ovs/tests/test_kv.py \
        python/ovs/tests/test_list.py \
diff --git a/python/ovs/dns_resolve.py b/python/ovs/dns_resolve.py
new file mode 100644
index 000000000..7b0f6c266
--- /dev/null
+++ b/python/ovs/dns_resolve.py
@@ -0,0 +1,272 @@
+# Copyright (c) 2023 Red Hat, 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.
+
+import collections
+import enum
+import functools
+import ipaddress
+import os
+import time
+import typing
+
+try:
+    import unbound  # type: ignore
+except ImportError:
+    pass
+
+import ovs.vlog
+
+vlog = ovs.vlog.Vlog("dns_resolve")
+
+
+class ReqState(enum.Enum):
+    INVALID = 0
+    PENDING = 1
+    GOOD = 2
+    ERROR = 3
+
+
+class DNSRequest:
+

nit: extra space

+    def __init__(self, name: str):
+        self.name: str = name
+        self.state: ReqState = ReqState.INVALID
+        self.time: typing.Optional[float] = None
+        # set by DNSResolver._callback
+        self.result: typing.Optional[str] = None
+        self.ttl: typing.Optional[float] = None
+
+    @property
+    def expired(self):
+        return time.time() > self.time + self.ttl
+
+    @property
+    def is_valid(self):
+        return self.state == ReqState.GOOD and not self.expired
+
+    def __str__(self):
+        return (f"DNSRequest(name={self.name}, state={self.state}, "
+                f"time={self.time}, result={self.result})")
+
+
+class DefaultReqDict(collections.defaultdict):
+    def __init__(self):
+        super().__init__(DNSRequest)
+
+    def __missing__(self, key):
+        ret = self.default_factory(key)
+        self[key] = ret
+        return ret
+
+
+class UnboundException(Exception):
+    def __init__(self, message, errno):
+        try:
+            msg = f"{message}: {unbound.ub_strerror(errno)}"
+        except NameError:
+            msg = message
+        super().__init__(msg)
+
+
+def dns_enabled(func):
+    @functools.wraps(func)
+    def wrapper(self, *args, **kwargs):
+        if self.dns_enabled:
+            return func(self, *args, **kwargs)
+        vlog.err("DNS support requires the python unbound library")
+    return wrapper
+


+
+class singleton:
+    def __init__(self, klass):
+        self._klass = klass
+        self._instance = None
+
+    def __call__(self, *args, **kwargs):
+        if self._instance is None:
+            self._instance = self._klass(*args, **kwargs)
+        return self._instance
+

Having a singleton class that accepts a parameter in the constructor could be a
bit confusing. For instance: "d = DNSResolver(is_daemon=True)"
might yield a resolver where "is_daemon" is actually "False" if some other part
of the code created it before.

Woudn't it be cleaner if it just exposed "resove_sync" and "resolve_async" and
let users choose how to resolve?

I suppose, the problem is that most of the resolving is happening
from the inside of OVS libraries.  So, it will not be a choice of
an application.


Not sure I understand. Currently if a user does:
'DNSResolver(is_daemon=False).resolve("openvswitch.org")', it'll resolve 
synchronously and so will all the rest of the resolutions of that program.

The internal variable "_is_daemon" is only used to select the way to resolve. 
The same behavior would be achieved if, for instance, "is_daemon" is a default 
argument to "resolve()" with the extra benefit of the first resolution not 
influencing all the rest.

Maybe a warning that the parameter is different will be enough?
> Best regards, Ilya Maximets.


--
Adrián Moreno

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