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    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-47639] Fix race with async getHosts request and concurrent topology change</title>
                <link>https://jira.mongodb.org/browse/SERVER-47639</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;See the steps in &quot;Steps To Reproduce&quot; below.&lt;/p&gt;

&lt;p&gt;A fix for this is to invoke server selection at the time of step 4 with a fresh view of the topology while synchronizing with the TopologyManager.&lt;/p&gt;

&lt;p&gt;The topology views (and corresponding topology change events) are serialized, so at step 4 we would either see T0 or T and would block any future topology changes while R is being enqueued.&lt;/p&gt;

&lt;p&gt;In the first case (we see T0), server selection would return no result, the request would be enqueued, and the query will be satisfied after T is applied to the topology manager.&lt;/p&gt;

&lt;p&gt;In the second case (we see T), the query will be satisfied without enqueuing the request.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1319712">SERVER-47639</key>
            <summary>Fix race with async getHosts request and concurrent topology change</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="cheahuychou.mao@mongodb.com">Cheahuychou Mao</assignee>
                                    <reporter username="lamont.nelson@mongodb.com">Lamont Nelson</reporter>
                        <labels>
                    </labels>
                <created>Fri, 17 Apr 2020 18:43:51 +0000</created>
                <updated>Sun, 29 Oct 2023 22:09:18 +0000</updated>
                            <resolved>Tue, 23 Jun 2020 19:46:03 +0000</resolved>
                                                    <fixVersion>4.4.1</fixVersion>
                    <fixVersion>4.7.0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="3319442" author="xgen-internal-githook" created="Tue, 4 Aug 2020 14:23:04 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Cheahuychou Mao&apos;, &apos;email&apos;: &apos;cheahuychou.mao@mongodb.com&apos;, &apos;username&apos;: &apos;cheahuychou&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-47639&quot; title=&quot;Fix race with async getHosts request and concurrent topology change&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-47639&quot;&gt;&lt;del&gt;SERVER-47639&lt;/del&gt;&lt;/a&gt; Fix race with async getHosts request and concurrent topology change&lt;/p&gt;

&lt;p&gt;(cherry picked from commit 36bf915c32d551ad557ec7a1fa41890037e9f54f)&lt;br/&gt;
Branch: v4.4&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/9ffd3632f5fed15e0920fe2fbbff2142676d7537&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/9ffd3632f5fed15e0920fe2fbbff2142676d7537&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3220430" author="xgen-internal-githook" created="Tue, 23 Jun 2020 19:30:57 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Cheahuychou Mao&apos;, &apos;email&apos;: &apos;cheahuychou.mao@mongodb.com&apos;, &apos;username&apos;: &apos;cheahuychou&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-47639&quot; title=&quot;Fix race with async getHosts request and concurrent topology change&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-47639&quot;&gt;&lt;del&gt;SERVER-47639&lt;/del&gt;&lt;/a&gt; Fix race with async getHosts request and concurrent topology change&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/36bf915c32d551ad557ec7a1fa41890037e9f54f&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/36bf915c32d551ad557ec7a1fa41890037e9f54f&lt;/a&gt;&lt;/p&gt;</comment>
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                            <customfieldvalue>&lt;p&gt;Given an initial topology T0, getHosts request R, and topology change event T that can satisfy R.&lt;/p&gt;

&lt;p&gt;0. initial state: T0 cannot satisfy R &lt;br/&gt;
1. The RSM receives R concurrent with T.&lt;br/&gt;
1a. RSM checks topology and sees T0: &lt;a href=&quot;https://github.com/mongodb/mongo/blob/23e6f954d7ef5ab73f5540b46c6b3794b7ecfbdc/src/mongo/client/streamable_replica_set_monitor.cpp#L252&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/23e6f954d7ef5ab73f5540b46c6b3794b7ecfbdc/src/mongo/client/streamable_replica_set_monitor.cpp#L252&lt;/a&gt;&lt;br/&gt;
2. R is scheduled to be fulfilled async: &lt;a href=&quot;https://github.com/mongodb/mongo/blob/23e6f954d7ef5ab73f5540b46c6b3794b7ecfbdc/src/mongo/client/streamable_replica_set_monitor.cpp#L257&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/23e6f954d7ef5ab73f5540b46c6b3794b7ecfbdc/src/mongo/client/streamable_replica_set_monitor.cpp#L257&lt;/a&gt;&lt;br/&gt;
3. T occurs concurrently with step 2 prior to enqueuing R&lt;br/&gt;
4. R is enqueued: &lt;a href=&quot;https://github.com/mongodb/mongo/blob/23e6f954d7ef5ab73f5540b46c6b3794b7ecfbdc/src/mongo/client/streamable_replica_set_monitor.cpp#L279&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/23e6f954d7ef5ab73f5540b46c6b3794b7ecfbdc/src/mongo/client/streamable_replica_set_monitor.cpp#L279&lt;/a&gt;&lt;br/&gt;
5. If no further topology changes occur the query will timeout with FailedToSatisfyReadPreference&lt;/p&gt;</customfieldvalue>

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