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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-50349] Requests for writable Collection should be under WUOW</title>
                <link>https://jira.mongodb.org/browse/SERVER-50349</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;We need to be under a WUOW when requesting a writable Collection for metadata updates under MODE_X lock.&lt;/p&gt;

&lt;p&gt;Note that simple insert, update, delete of documents under MODE_IX should not require this.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1445179">SERVER-50349</key>
            <summary>Requests for writable Collection should be under WUOW</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</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="henrik.edin@mongodb.com">Henrik Edin</assignee>
                                    <reporter username="henrik.edin@mongodb.com">Henrik Edin</reporter>
                        <labels>
                    </labels>
                <created>Tue, 18 Aug 2020 12:23:24 +0000</created>
                <updated>Sun, 29 Oct 2023 22:04:22 +0000</updated>
                            <resolved>Tue, 15 Sep 2020 17:51:29 +0000</resolved>
                                                    <fixVersion>4.8.0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>1</watches>
                                                                                                                <comments>
                            <comment id="3394488" author="xgen-internal-githook" created="Tue, 15 Sep 2020 14:28:47 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Henrik Edin&apos;, &apos;email&apos;: &apos;henrik.edin@mongodb.com&apos;, &apos;username&apos;: &apos;henrikedin&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-50349&quot; title=&quot;Requests for writable Collection should be under WUOW&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-50349&quot;&gt;&lt;del&gt;SERVER-50349&lt;/del&gt;&lt;/a&gt; Getting a writable collection now requires the caller to be inside a WUOW by default&lt;/p&gt;

&lt;p&gt;There are three modes when accessing a writable Collection:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Managed in WUOW (default)&lt;/li&gt;
	&lt;li&gt;Unmanaged (users need to commit/rollback)&lt;/li&gt;
	&lt;li&gt;Inplace that provides direct access to the Collection in the catalog. (Users need to ensure there&apos;s no concurrent operations going on)&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Added a helper RAII type CollectionWriter that abstract the three modes above and also provides abstraction on different methods of accessing Collections (AutoGetCollection or manual lookups).&lt;/p&gt;

&lt;p&gt;Writable Collection is aquired lazily when needed (usually inside a WUOW).&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/accc7e7cd7e7a984347361d03ee76514c4a54163&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/accc7e7cd7e7a984347361d03ee76514c4a54163&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3368455" author="henrik.edin" created="Mon, 31 Aug 2020 19:00:14 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=daniel.gottlieb&quot; class=&quot;user-hover&quot; rel=&quot;daniel.gottlieb&quot;&gt;daniel.gottlieb&lt;/a&gt;&#160;Yes of course!&lt;/p&gt;

&lt;p&gt;This is just referring to&#160;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-50317&quot; title=&quot;Make uses of Collection const-correct&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-50317&quot;&gt;&lt;del&gt;SERVER-50317&lt;/del&gt;&lt;/a&gt; where insert, update and delete can now be performed on a const-collection (but still under a WUOW of course)&lt;/p&gt;

&lt;p&gt;You can do&#160;lookupCollectionByUUID (that will return a const-collection soon) without a WUOW, but any subsequent write op still needs a WUOW.&lt;/p&gt;

&lt;p&gt;I do not anticipate any risk with code like you show, as an MODE_IX lock is still being held so there can&apos;t be another holder of a MODE_X lock that can make the Collection pointer stale.&lt;/p&gt;

&lt;p&gt;This is just prerequisite work for&#160;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-50145&quot; title=&quot;Have DDL operations and MODE_X writers install a new Collection object&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-50145&quot;&gt;&lt;del&gt;SERVER-50145&lt;/del&gt;&lt;/a&gt; where a new Collection instance will be created for MODE_X writers that do metadata changes to the collection. We need to be in a WUOW in that case so any changes can be rolled back.&lt;/p&gt;

&lt;p&gt;Hope that makes it clearer.&lt;/p&gt;</comment>
                            <comment id="3347071" author="daniel.gottlieb@10gen.com" created="Wed, 19 Aug 2020 14:15:59 +0000"  >&lt;blockquote&gt;
&lt;p&gt;Note that simple insert, update, delete of documents under MODE_IX should not require this.&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;Certainly one still needs a WUOW for doing the insert/update/delete? Are there any anticipated risks with code that does:&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;AutoGetCollection coll(opCtx, &quot;foo.bar&quot;, MODE_IX);&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;writeConflictRetry {&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;  WUOW wuow(opCtx);&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;  coll-&amp;gt;insert(doc);&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;  wuow.commit();&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;}&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;Specifically, if the collection pointer is stale[1] can that prevent a write from succeeding? Will those failures only ever be uasserts? Or is it plausible those could be WCE landing the code in an infinite loop because the collection is never refreshed?&lt;/p&gt;

&lt;p&gt;[1] In most cases, the locks/collection objects are acquired prior to a storage engine snapshot being allocated (which I think proves the collection object cannot be stale). But that&apos;s not necessarily true with multi-statement transactions (and maybe some other kinds of writes that read at a no-holes point?)&lt;/p&gt;</comment>
                    </comments>
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                            <issuelinktype id="10012">
                    <name>Related</name>
                                            <outwardlinks description="related to">
                                        <issuelink>
            <issuekey id="1764965">SERVER-57324</issuekey>
        </issuelink>
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