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    <title>MongoDB Jira</title>
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    <description>This file is an XML representation of an issue</description>
    <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-26116] CSRS leaves the balancer lock unprotected briefly between leaving drain mode and the balancer acquiring it</title>
                <link>https://jira.mongodb.org/browse/SERVER-26116</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Draining mode unlocks all of the config server held distributed locks. The Balancer reacquires the {_id: &quot;balancer&quot;} distlock after drain mode has been left.&lt;/p&gt;

&lt;p&gt;Theoretically, a 3.2 Mongos could take the {_id: &quot;balancer&quot;} distlock in this window. It could briefly hold the distlock for a manual moveChunk, or indefinitely if auto-balancing is enabled. It could be held indefinitely if the config server is holding collection distlocks for migrations it is trying to recover after failover.&lt;/p&gt;</description>
                <environment></environment>
        <key id="316279">SERVER-26116</key>
            <summary>CSRS leaves the balancer lock unprotected briefly between leaving drain mode and the balancer acquiring it</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="9">Done</resolution>
                                        <assignee username="nathan.myers">Nathan Myers</assignee>
                                    <reporter username="dianna.hohensee@mongodb.com">Dianna Hohensee</reporter>
                        <labels>
                    </labels>
                <created>Wed, 14 Sep 2016 21:05:48 +0000</created>
                <updated>Sat, 19 Nov 2016 00:05:10 +0000</updated>
                            <resolved>Tue, 15 Nov 2016 20:41:43 +0000</resolved>
                                                    <fixVersion>3.4.0-rc4</fixVersion>
                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>8</watches>
                                                                                                                <comments>
                            <comment id="1434935" author="xgen-internal-githook" created="Tue, 15 Nov 2016 20:40:13 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;DiannaHohensee&apos;, u&apos;name&apos;: u&apos;Dianna Hohensee&apos;, u&apos;email&apos;: u&apos;dianna.hohensee@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-26116&quot; title=&quot;CSRS leaves the balancer lock unprotected briefly between leaving drain mode and the balancer acquiring it&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-26116&quot;&gt;&lt;del&gt;SERVER-26116&lt;/del&gt;&lt;/a&gt; reacquire the balancer distlock in drain mode during config primary step-up&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/eacdb58313a1b464e89c44868527fcadc22a67a6&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/eacdb58313a1b464e89c44868527fcadc22a67a6&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1423995" author="kaloian.manassiev" created="Wed, 2 Nov 2016 15:12:40 +0000"  >&lt;p&gt;The proposed way to fix this is to try to acquire the &lt;tt&gt;&quot;balancer&quot;&lt;/tt&gt; distributed lock first thing after the migration manager &lt;a href=&quot;https://github.com/mongodb/mongo/blob/64a44616e62e9e35a81bad24889a4e2a8c816980/src/mongo/db/s/balancer/migration_manager.cpp#L292&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;enters the recovering state&lt;/a&gt;. This acquisition must be done with a local write concern since at that point the server is still in draining mode.&lt;/p&gt;

&lt;p&gt;If this aquisition fails for any reason, there are a couple of cases to be considered:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;There are &lt;b&gt;no&lt;/b&gt; migration documents - in this case it is harmless since there are no active migrations to be recovered and the failure most likely happened because a 3.2 mongos after an upgrade had a lease on the lock, which will expire after 15 minutes and the lock will be acquired by the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/64a44616e62e9e35a81bad24889a4e2a8c816980/src/mongo/db/s/balancer/balancer.cpp#L308&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;balancer thread&lt;/a&gt;, which guarantees that no new migrations will occur until then.&lt;/li&gt;
	&lt;li&gt;There are some migration documents - this is the unexpected case and can only happen due to local write problem, which is a pretty severe condition in itself. In this case, just log a warning and continue recovery, since there is not much that can be done.&lt;/li&gt;
&lt;/ul&gt;

</comment>
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