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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-43412] Create a wrapper around getCursor calls for the oplog collection that ensures oplog visibility rules are enforced</title>
                <link>https://jira.mongodb.org/browse/SERVER-43412</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Motivation: enforce oplog visibility rules. An oplog collection read should in most cases be the first operation in a WT transaction, so as to select a snapshot using the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/409713e3b6585327e5d39eca31705c7ae484d0ed/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp#L483-L485&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;_oplogVisibleTs&lt;/a&gt; and not see any oplog holes. Currently, &lt;a href=&quot;https://github.com/mongodb/mongo/blob/409713e3b6585327e5d39eca31705c7ae484d0ed/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp#L2031&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;getCursor() will call setIsOplogReader() for oplog reads&lt;/a&gt;, so the the _oplogVisibleTs is used when opening a WT txn.&lt;br/&gt;
-----------------------------------&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-30771&quot; title=&quot;investigate oplog stones initialization&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-30771&quot;&gt;&lt;del&gt;SERVER-30771&lt;/del&gt;&lt;/a&gt; discovered two difficult issues with adding an invariant (&lt;tt&gt;!inActiveTxn() || _isOplogReader&lt;/tt&gt;) to the &lt;tt&gt;setIsOplogReader()&lt;/tt&gt; function. The first is that &lt;a href=&quot;https://github.com/mongodb/mongo/blob/409713e3b6585327e5d39eca31705c7ae484d0ed/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp#L2029&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;getCursor()&lt;/tt&gt; has a MODE_X lock bypass&lt;/a&gt; where we throw the &lt;tt&gt;(!inActiveTxn || _isOplogReader)&lt;/tt&gt; rules out the window. Secondly, that, particularly around oplog truncation, there are multiple use cases where we should see the oplog in its entirety without visibility rules: for example, &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e91ba00e77b5c330d37474740b425f2c0d681e4f/src/mongo/db/repl/replication_recovery.cpp#L437-L457&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;_truncateOplogTo&lt;/tt&gt;&lt;/a&gt; fetches a reverse oplog cursor without visibility restrictions, starting a txn, then calls &lt;tt&gt;capperTruncateAfter&lt;/tt&gt;, which opens a forward oplog cursor with visibility restrictions.&lt;/p&gt;

&lt;p&gt;Therefore, we&apos;re proposing trying to create a wrapper function around &lt;tt&gt;getCursor()&lt;/tt&gt; calls against the oplog collection, say &lt;tt&gt;getOplogCursor()&lt;/tt&gt;. This new function could take a boolean for visibility rules: the default is to obey the visibility rule; but it can be explicitly overridden by callers we do not wish to follow the restrictions.&lt;/p&gt;

&lt;p&gt;Given how many callers there are of getCursor(), it may be simpler to do something like this&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;getCursor() {&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;    if (_nss == oplog) {&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;        getOplogCursor();&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;    }&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;    .....&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&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;And swap oplog cursor only codepaths using &lt;tt&gt;getCursor()&lt;/tt&gt; with &lt;tt&gt;getOplogCursor(/&amp;#42;ignoreOplogVisibilityRule&amp;#42;/ true)&lt;/tt&gt; (if they ignore the visibility rules). However, there are also callers like in the createCollection code path, where we would need to add special &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;{    &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;    if (nss != oplog) {&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;        getCursor();&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;    } else { &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;        getOplogCursor(/\*ignoreOplogVisibilityRule\*/ true);&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;    }&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;So maybe &lt;tt&gt;getCursor()&lt;/tt&gt; should have a visibility boolean, too.&lt;/p&gt;

&lt;p&gt;May need to elevate the &lt;tt&gt;setIsOplogReader()&lt;/tt&gt; and &lt;tt&gt;getIsOplogReader()&lt;/tt&gt; functions from the WiredTigerRecoveryUnit to the RecoveryUnit interface, so higher level callers external to WT can call them &amp;#8211; would need a new &lt;tt&gt;unsetIsOplogReader()&lt;/tt&gt; function, too, if this implementation is desired. &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-30771&quot; title=&quot;investigate oplog stones initialization&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-30771&quot;&gt;&lt;del&gt;SERVER-30771&lt;/del&gt;&lt;/a&gt; has a WIP that reflects this: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-30771&quot; title=&quot;investigate oplog stones initialization&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-30771&quot;&gt;&lt;del&gt;SERVER-30771&lt;/del&gt;&lt;/a&gt;&apos;s solution, to invariant (&lt;tt&gt;!inActiveTxn() || _isOplogReader&lt;/tt&gt;) was abandoned because there appear to be too many special cases where we do not obey the visibility rules and would have to litter special calls to &lt;tt&gt;setIsOplogReader()&lt;/tt&gt; in many code paths prior to any WT txn starting on a different collection.&lt;/p&gt;</description>
                <environment></environment>
        <key id="935236">SERVER-43412</key>
            <summary>Create a wrapper around getCursor calls for the oplog collection that ensures oplog visibility rules are enforced</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</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="2">Won&apos;t Fix</resolution>
                                        <assignee username="backlog-server-execution">Backlog - Storage Execution Team</assignee>
                                    <reporter username="dianna.hohensee@mongodb.com">Dianna Hohensee</reporter>
                        <labels>
                    </labels>
                <created>Mon, 23 Sep 2019 14:10:10 +0000</created>
                <updated>Tue, 6 Dec 2022 02:47:41 +0000</updated>
                            <resolved>Thu, 2 Apr 2020 21:14:15 +0000</resolved>
                                                                    <component>Storage</component>
                                        <votes>0</votes>
                                    <watches>0</watches>
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                            <issuelinktype id="10520">
                    <name>Problem/Incident</name>
                                                                <inwardlinks description="is caused by">
                                        <issuelink>
            <issuekey id="418792">SERVER-30771</issuekey>
        </issuelink>
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