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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-21434] FindAndModify query unexpectedly returns null when using multiple readers</title>
                <link>https://jira.mongodb.org/browse/SERVER-21434</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;If a findAndModify request specifies a sort, we use a limit of 1 to reduce the memory requirements and use a top-k sort. With such a request, if a &lt;tt&gt;WriteConflictException&lt;/tt&gt; is encountered while trying to update or delete a matching document, the operation will return no matches. This happens even if there are other documents that match the query. We should retry the entire operation in this scenario to avoid not returning a document when there were results that matched the query.&lt;/p&gt;

&lt;h5&gt;&lt;a name=&quot;OriginalDescription&quot;&gt;&lt;/a&gt;Original Description&lt;/h5&gt;
&lt;p&gt;Using MongoDb with WT to create a simple message queue, I used one writer to add some documents with a state of &apos;Q&apos;. I then use 50 readers (threads) to consume these documents, with each thread using a FindAndModify query to atomically update the state of a document to &apos;in process&apos; and return it for processing. At the end of processing the document state is updated to &apos;processed&apos;.&lt;/p&gt;

&lt;p&gt;The query (using the c# driver) looks as follows:&lt;/p&gt;

&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;var message = await m_QueueMessages.FindOneAndUpdateAsync(Builders&amp;lt;MongoMessage&amp;gt;.Filter.Where(m =&amp;gt; m.QueueType == QueueType.Queued),&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                                                Builders&amp;lt;MongoMessage&amp;gt;.Update.Set(m =&amp;gt; m.QueueType, QueueType.InProcess),&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                                                new FindOneAndUpdateOptions&amp;lt;MongoMessage&amp;gt; { Sort = Builders&amp;lt;MongoMessage&amp;gt;.Sort.Ascending(m =&amp;gt; m.ObjectId), IsUpsert = false, MaxTime = TimeSpan.FromSeconds(5) },&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                                                CancellationToken.None);&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;This query often returns null, even though there are documents matching the filter. Removal of the sort apparently fixes this. I have confirmed with Craig Wilson (MongoDb c# driver) that this is not a driver issue.&lt;/p&gt;</description>
                <environment></environment>
        <key id="239085">SERVER-21434</key>
            <summary>FindAndModify query unexpectedly returns null when using multiple readers</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="charlie.swanson@mongodb.com">Charlie Swanson</assignee>
                                    <reporter username="nick@innsenroute.com">Nick Judson</reporter>
                        <labels>
                    </labels>
                <created>Thu, 12 Nov 2015 17:05:07 +0000</created>
                <updated>Tue, 19 Jan 2016 19:51:22 +0000</updated>
                            <resolved>Fri, 13 Nov 2015 17:02:35 +0000</resolved>
                                    <version>3.2.0-rc2</version>
                                    <fixVersion>3.2.0-rc3</fixVersion>
                                    <component>Querying</component>
                    <component>Write Ops</component>
                                        <votes>0</votes>
                                    <watches>10</watches>
                                                                                                                <comments>
                            <comment id="1087993" author="xgen-internal-githook" created="Fri, 13 Nov 2015 17:01:20 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;cswanson310&apos;, u&apos;name&apos;: u&apos;Charlie Swanson&apos;, u&apos;email&apos;: u&apos;charlie.swanson@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-21434&quot; title=&quot;FindAndModify query unexpectedly returns null when using multiple readers&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-21434&quot;&gt;&lt;del&gt;SERVER-21434&lt;/del&gt;&lt;/a&gt; Retry a findAndModify if it conflicts on an update or delete&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/cd766108b26e7e272f3c367f9de8767399feeadd&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/cd766108b26e7e272f3c367f9de8767399feeadd&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1087054" author="charlie.swanson" created="Thu, 12 Nov 2015 19:38:39 +0000"  >&lt;p&gt;Updated the description to match our understanding of the problem.&lt;/p&gt;</comment>
                            <comment id="1086998" author="nick@innsenroute.com" created="Thu, 12 Nov 2015 18:55:03 +0000"  >&lt;p&gt;Great - thanks.&lt;/p&gt;</comment>
                            <comment id="1086966" author="david.storch" created="Thu, 12 Nov 2015 18:35:12 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=nick%40innsenroute.com&quot; class=&quot;user-hover&quot; rel=&quot;nick@innsenroute.com&quot;&gt;nick@innsenroute.com&lt;/a&gt;, thanks for reporting this. I think we understand the issue from the details you already provided. We are working on a repro script and a fix, so please continue to watch this ticket for updates.&lt;/p&gt;</comment>
                            <comment id="1086850" author="nick@innsenroute.com" created="Thu, 12 Nov 2015 17:05:45 +0000"  >&lt;p&gt;This may be a duplicate of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-21227&quot; title=&quot;MultiPlanStage::invalidate() should not flag and drop invalidated WorkingSetMembers&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-21227&quot;&gt;&lt;del&gt;SERVER-21227&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;</comment>
                    </comments>
                <issuelinks>
                            <issuelinktype id="10010">
                    <name>Duplicate</name>
                                                                <inwardlinks description="is duplicated by">
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                            <issuelinktype id="10012">
                    <name>Related</name>
                                            <outwardlinks description="related to">
                                        <issuelink>
            <issuekey id="244970">SERVER-22002</issuekey>
        </issuelink>
                            </outwardlinks>
                                                        </issuelinktype>
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                <attachments>
                            <attachment id="98102" name="Threading_Test.7z" size="483393" author="nick@innsenroute.com" created="Thu, 12 Nov 2015 17:05:07 +0000"/>
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