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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <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-34494] Background index builds on secondaries should set the collection minimumVisibleSnapshot when they start</title>
                <link>https://jira.mongodb.org/browse/SERVER-34494</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;During batch application, starting a background index build on a secondary does not set the minimumVisibleSnapshot on the collection until the index build finishes. If a secondary reader without the PBWM lock reads at a timestamp A before an index build timestamped for B, a reader may iterate through the indexes for a collection and reach &lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/storage/bson_collection_catalog_entry.cpp#L185&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;this invariant&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;The problem is that &lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/catalog/index_catalog_impl.cpp#L429&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;_setupInMemoryStructures&lt;/a&gt; is called before the actual catalog entry is updated on disk, so a reader may see an in-memory state inconsistent with the state on disk. This can be prevented by updating the minimumVisibleSnapshot on the collection to force readers to wait for a later timestamp.&lt;/p&gt;</description>
                <environment></environment>
        <key id="528251">SERVER-34494</key>
            <summary>Background index builds on secondaries should set the collection minimumVisibleSnapshot when they start</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="louis.williams@mongodb.com">Louis Williams</assignee>
                                    <reporter username="louis.williams@mongodb.com">Louis Williams</reporter>
                        <labels>
                    </labels>
                <created>Mon, 16 Apr 2018 16:12:42 +0000</created>
                <updated>Sun, 29 Oct 2023 22:32:40 +0000</updated>
                            <resolved>Tue, 17 Apr 2018 17:07:36 +0000</resolved>
                                    <version>3.7.4</version>
                                    <fixVersion>3.7.5</fixVersion>
                                    <component>Storage</component>
                                        <votes>0</votes>
                                    <watches>5</watches>
                                                                                                                <comments>
                            <comment id="1866589" author="xgen-internal-githook" created="Tue, 17 Apr 2018 17:04:33 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Louis Williams&apos;, &apos;email&apos;: &apos;louis.williams@mongodb.com&apos;, &apos;username&apos;: &apos;louiswilliams&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-34494&quot; title=&quot;Background index builds on secondaries should set the collection minimumVisibleSnapshot when they start&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-34494&quot;&gt;&lt;del&gt;SERVER-34494&lt;/del&gt;&lt;/a&gt; Background index builds on secondaries should set the collection minimumVisibleSnapshot&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/228106a7416288ad6263ab67878477cc23e9c23f&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/228106a7416288ad6263ab67878477cc23e9c23f&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1865667" author="daniel.gottlieb@10gen.com" created="Mon, 16 Apr 2018 20:31:50 +0000"  >&lt;blockquote&gt;
&lt;p&gt;The problem is that _setupInMemoryStructures is called before the actual catalog entry is updated on disk, so a reader may see an in-memory state inconsistent with the state on disk. This can be prevented by updating the minimumVisibleSnapshot on the collection to force readers to wait for a later timestamp.&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;That&apos;s not how I had it. That paragraph suggests there&apos;s a lack of locking where a reader can come in and view an inconsistent state.&lt;/p&gt;

&lt;p&gt;I had thought that a writer could create the index entry in its &quot;ready: false&quot; state at a time T, but not set the &lt;tt&gt;minimumVisibleSnapshot&lt;/tt&gt; field. This allows a reader to come in on a collection with a &lt;tt&gt;readTimestamp&lt;/tt&gt; of &lt;tt&gt;T-1&lt;/tt&gt;, causing the crash (because of the in-memory/&quot;on disk&quot; state). The crash still happens because of a split state, but as far as I know, there&apos;s sufficient locking to avoid the low-level race, if only a minimum visible snapshot were set (which is what&apos;s being suggested).&lt;/p&gt;</comment>
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