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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-33660] Once getMores include lsid, sharded aggregations with $mergeCursors can hang</title>
                <link>https://jira.mongodb.org/browse/SERVER-33660</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;A deadlock is induced on the SessionCatalog:&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;The operation performing the merging half of the pipeline checks out the Session for that lsid.&lt;/li&gt;
	&lt;li&gt;That operation includes a $mergeCursors, which performs getMores on remote hosts, one of which is the same host performing the $mergeCursors.&lt;/li&gt;
	&lt;li&gt;That operation will attempt to check out the same session once the getMore includes the lsid - blocking on a mutex in the SessionCatalog.&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;As a short term fix, we should do the following:&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;Only check out the Session if the operation includes a transaction number.&lt;/li&gt;
	&lt;li&gt;Ban aggregations with a transaction number on mongos.&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;As a long term fix, we will investigate either not using getMores over the network for what are really local reads. If that proves difficult, we will have to re-evaluate.&lt;/p&gt;</description>
                <environment></environment>
        <key id="505762">SERVER-33660</key>
            <summary>Once getMores include lsid, sharded aggregations with $mergeCursors can hang</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="charlie.swanson@mongodb.com">Charlie Swanson</assignee>
                                    <reporter username="charlie.swanson@mongodb.com">Charlie Swanson</reporter>
                        <labels>
                    </labels>
                <created>Mon, 5 Mar 2018 15:41:38 +0000</created>
                <updated>Sun, 29 Oct 2023 22:34:09 +0000</updated>
                            <resolved>Tue, 6 Mar 2018 17:30:20 +0000</resolved>
                                                    <fixVersion>3.7.3</fixVersion>
                                    <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="1824749" author="charlie.swanson" created="Tue, 6 Mar 2018 17:30:20 +0000"  >&lt;p&gt;A workaround has been pushed here to prevent the hang. Future work to improve the system to handle aggregations within a transaction will be tracked by &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-33683&quot; title=&quot;Allow aggregation $mergeCursors stage to run inside a transaction&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-33683&quot;&gt;&lt;del&gt;SERVER-33683&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;Note for drivers: we have banned attaching a &quot;txnNumber&quot; to an aggregation against mongos. We don&apos;t plan for this ban to be permanent, and in fact hope to remove it before 4.0. We want to make sure that drivers are not sending an aggregate with a &quot;txnNumber&quot; just in case we don&apos;t get a chance to do the work necessary to remove the restriction. We believe drivers will only attach this for retryable writes, but let us know if this is not the case.&lt;/p&gt;</comment>
                            <comment id="1824739" author="xgen-internal-githook" created="Tue, 6 Mar 2018 17:23:34 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;email&apos;: &apos;charlie.swanson@mongodb.com&apos;, &apos;name&apos;: &apos;Charlie Swanson&apos;, &apos;username&apos;: &apos;cswanson310&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-33660&quot; title=&quot;Once getMores include lsid, sharded aggregations with $mergeCursors can hang&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-33660&quot;&gt;&lt;del&gt;SERVER-33660&lt;/del&gt;&lt;/a&gt; Only check out session for operations with a txnNumber&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/c3d7752c263d532315671b862695d4316c877ae3&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/c3d7752c263d532315671b862695d4316c877ae3&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1823480" author="james.wahlin@10gen.com" created="Mon, 5 Mar 2018 18:49:23 +0000"  >&lt;p&gt;A session yielding mechanism is definitely worth discussion. I have some concern that it would make session state management more difficult. Also we would need to hold locks during yielding for multi-statement transactions and at present for snapshot reads as well. &lt;/p&gt;</comment>
                            <comment id="1823131" author="kaloian.manassiev" created="Mon, 5 Mar 2018 15:49:20 +0000"  >&lt;p&gt;While the idea of making local {{getMore}}s not go over the network is definitely something, which should be considered, I think there is a more fundamental problem here with session state. Does it make sense to introduce a &quot;session yielding&quot; mechanism, where operations, which have reached a state where they need to perform remote operations can check the session back in (provided perhaps that they aren&apos;t holding any locks), then perform the remote operation assuming it might have to come back?&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=james.wahlin&quot; class=&quot;user-hover&quot; rel=&quot;james.wahlin&quot;&gt;james.wahlin&lt;/a&gt;?&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="299978">SERVER-24978</issuekey>
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