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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-23807] Updates should always throw WriteConflictException on unindexing</title>
                <link>https://jira.mongodb.org/browse/SERVER-23807</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;During an index build, the collection is locked in mode IX. Consider a background index build occurring with concurrent updates. Then, the following race scenario could occur:&lt;/p&gt;

&lt;p&gt;1. Both the updater and the index builder read the same document.&lt;br/&gt;
2. To perform the update, the updater unindexes the document&apos;s old value.&lt;br/&gt;
3. Because the document has yet to be indexed, the WiredTiger returns WT_NOTFOUND. &lt;b&gt;No&lt;/b&gt; WCE is generated.&lt;br/&gt;
4. The update proceeds to index the new value.&lt;br/&gt;
5. The indexer in the background, unaware of any WCEs, indexes the value it has read. The result is that the index has one too many keys.&lt;/p&gt;

&lt;p&gt;We can fix this by having the update path trigger a write conflict exception even in the &quot;no-op&quot; case when unindexing returns WT_NOTFOUND.&lt;/p&gt;

&lt;p&gt;(An alternative is to simply lock the collection in X mode during an index build, but this will probably have a large impact on performance.)&lt;/p&gt;</description>
                <environment></environment>
        <key id="280825">SERVER-23807</key>
            <summary>Updates should always throw WriteConflictException on unindexing</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</type>
                                            <priority id="2" iconUrl="https://jira.mongodb.org/images/icons/priorities/critical.svg">Critical - P2</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="kyle.suarez@mongodb.com">Kyle Suarez</assignee>
                                    <reporter username="kyle.suarez@mongodb.com">Kyle Suarez</reporter>
                        <labels>
                            <label>code-only</label>
                    </labels>
                <created>Tue, 19 Apr 2016 17:36:37 +0000</created>
                <updated>Thu, 2 Aug 2018 21:22:53 +0000</updated>
                            <resolved>Thu, 21 Apr 2016 19:04:34 +0000</resolved>
                                    <version>3.2.5</version>
                    <version>3.3.4</version>
                                    <fixVersion>3.0.12</fixVersion>
                    <fixVersion>3.2.6</fixVersion>
                    <fixVersion>3.3.5</fixVersion>
                                    <component>Index Maintenance</component>
                    <component>WiredTiger</component>
                                        <votes>1</votes>
                                    <watches>10</watches>
                                                                                                                <comments>
                            <comment id="1250826" author="xgen-internal-githook" created="Fri, 29 Apr 2016 19:48:24 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;ksuarz&apos;, u&apos;name&apos;: u&apos;Kyle Suarez&apos;, u&apos;email&apos;: u&apos;ksuarz@gmail.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-23807&quot; title=&quot;Updates should always throw WriteConflictException on unindexing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-23807&quot;&gt;&lt;del&gt;SERVER-23807&lt;/del&gt;&lt;/a&gt; index build must X-lock collection&lt;/p&gt;

&lt;p&gt;Index builds must lock the collection with an X lock, rather than an IX lock, to&lt;br/&gt;
prevent a race with concurrent updates.&lt;br/&gt;
Branch: v3.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/dc729bf1e79318b17bcb02550ce7a0a51b814d72&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/dc729bf1e79318b17bcb02550ce7a0a51b814d72&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1242933" author="xgen-internal-githook" created="Thu, 21 Apr 2016 19:12:58 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;ksuarz&apos;, u&apos;name&apos;: u&apos;Kyle Suarez&apos;, u&apos;email&apos;: u&apos;ksuarz@gmail.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-22970&quot; title=&quot;Background index build produces an index with mismatched index keys and documents&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-22970&quot;&gt;&lt;del&gt;SERVER-22970&lt;/del&gt;&lt;/a&gt; fix update race with background index build&lt;/p&gt;

&lt;p&gt;There are two distinct fixes that must be done together:&lt;/p&gt;

&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-23807&quot; title=&quot;Updates should always throw WriteConflictException on unindexing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-23807&quot;&gt;&lt;del&gt;SERVER-23807&lt;/del&gt;&lt;/a&gt;: update should throw write conflict on unindex&lt;/li&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-18844&quot; title=&quot;Reacquire the snapshot after commit/abort&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-18844&quot;&gt;&lt;del&gt;SERVER-18844&lt;/del&gt;&lt;/a&gt;: background index builds should reacquire the snapshot to&lt;br/&gt;
                properly detect write conflicts with concurrent updates&lt;br/&gt;
Branch: v3.2&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/be81cc9e83d09d4dc206c59656fc7e51c3e4fc12&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/be81cc9e83d09d4dc206c59656fc7e51c3e4fc12&lt;/a&gt;&lt;/li&gt;
&lt;/ul&gt;
</comment>
                            <comment id="1242908" author="xgen-internal-githook" created="Thu, 21 Apr 2016 19:01:24 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;ksuarz&apos;, u&apos;name&apos;: u&apos;Kyle Suarez&apos;, u&apos;email&apos;: u&apos;ksuarz@gmail.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-22970&quot; title=&quot;Background index build produces an index with mismatched index keys and documents&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-22970&quot;&gt;&lt;del&gt;SERVER-22970&lt;/del&gt;&lt;/a&gt; fix update race with background index build&lt;/p&gt;

&lt;p&gt;There are two distinct fixes that must be done together:&lt;/p&gt;

&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-23807&quot; title=&quot;Updates should always throw WriteConflictException on unindexing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-23807&quot;&gt;&lt;del&gt;SERVER-23807&lt;/del&gt;&lt;/a&gt;: update should throw write conflict on unindex&lt;/li&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-18844&quot; title=&quot;Reacquire the snapshot after commit/abort&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-18844&quot;&gt;&lt;del&gt;SERVER-18844&lt;/del&gt;&lt;/a&gt;: background index builds should reacquire the snapshot to&lt;br/&gt;
                properly detect write conflicts with concurrent updates&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/f27b0cac4869fa506c6ed6f0dfc885b9edcd765a&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/f27b0cac4869fa506c6ed6f0dfc885b9edcd765a&lt;/a&gt;&lt;/li&gt;
&lt;/ul&gt;
</comment>
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