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    <title>MongoDB Jira</title>
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    <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-31992] getMore on tailable awaitData cursor doesn&apos;t attempt to fill batch on mongos</title>
                <link>https://jira.mongodb.org/browse/SERVER-31992</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Once the cluster cursor returns any result, RouterStageMerge &lt;a href=&quot;https://github.com/mongodb/mongo/blob/r3.6.0-rc4/src/mongo/s/query/router_stage_merge.cpp#L73&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;doesn&apos;t issue any new getMore&lt;/a&gt;. However the batch may haven&apos;t been filled up. If the result set isn&apos;t empty, RouterStageMerge should &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e7f03f979c98ea7cc90151839d6d91276f5d32ed/src/mongo/s/query/async_results_merger.cpp#L254-L257&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;wait for ARM to return an EOF&lt;/a&gt; when ARM gets an empty batch rather than checking &lt;tt&gt;_arm.ready()&lt;/tt&gt;.&lt;/p&gt;

&lt;p&gt;The following patch fixes it. However, if the batch size is enforced for getMore&apos;s, getMore&apos;s will always wait for a full awaitDataTimeout (1 second by default) if there isn&apos;t enough data to fill a batch. The original implementation can return some results earlier without waiting. The difference is interesting when resuming change streams where returning full batches is more efficient.&lt;/p&gt;

&lt;p/&gt;
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					&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;diff --git a/src/mongo/s/query/router_stage_merge.cpp b/src/mongo/s/query/router_stage_merge.cpp&lt;/span&gt;&lt;/pre&gt;
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					&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;index e571ed4ed2..fcf6f560fa 100644&lt;/span&gt;&lt;/pre&gt;
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						&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: #2B91AF; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;--- a/src/mongo/s/query/router_stage_merge.cpp&lt;/span&gt;&lt;/pre&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;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: #2B91AF; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;+++ b/src/mongo/s/query/router_stage_merge.cpp&lt;/span&gt;&lt;/pre&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;   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;@@ -70,7 +70,7 @@ StatusWith&amp;lt;ClusterQueryResult&amp;gt; RouterStageMerge::awaitNextWithTimeout(ExecContex&lt;/span&gt;&lt;/pre&gt;
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						&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: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;     invariant(_params-&amp;gt;tailableMode == TailableMode::kTailableAndAwaitData);&lt;/span&gt;&lt;/pre&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;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;     // If we are in kInitialFind or kGetMoreWithAtLeastOneResultInBatch context and the ARM is not&lt;/span&gt;&lt;/pre&gt;
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						&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: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;     // ready, we don&apos;t block. Fall straight through to the return statement.&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: red; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;-    while (!_arm.ready() &amp;amp;&amp;amp; execCtx == ExecContext::kGetMoreNoResultsYet) {&lt;/span&gt;&lt;/pre&gt;
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						&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: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;+    while (!_arm.ready() &amp;amp;&amp;amp; execCtx != ExecContext::kInitialFind) {&lt;/span&gt;&lt;/pre&gt;
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						&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: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;         auto nextEventStatus = getNextEvent();&lt;/span&gt;&lt;/pre&gt;
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						&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: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;         if (!nextEventStatus.isOK()) {&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;</description>
                <environment></environment>
        <key id="459521">SERVER-31992</key>
            <summary>getMore on tailable awaitData cursor doesn&apos;t attempt to fill batch on mongos</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="5" iconUrl="https://jira.mongodb.org/images/icons/priorities/trivial.svg">Trivial - P5</priority>
                        <status id="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-execution">Backlog - Query Execution</assignee>
                                    <reporter username="siyuan.zhou@mongodb.com">Siyuan Zhou</reporter>
                        <labels>
                            <label>change-streams-improvements</label>
                            <label>query-44-grooming</label>
                    </labels>
                <created>Wed, 15 Nov 2017 23:56:10 +0000</created>
                <updated>Tue, 6 Dec 2022 03:46:29 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>8</watches>
                                                                                                                <comments>
                            <comment id="1816081" author="david.storch" created="Mon, 26 Feb 2018 16:13:11 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=asya&quot; class=&quot;user-hover&quot; rel=&quot;asya&quot;&gt;asya&lt;/a&gt;, I don&apos;t think so. It doesn&apos;t appear on our prioritized list of change streams improvements.&lt;/p&gt;</comment>
                            <comment id="1813295" author="asya" created="Thu, 22 Feb 2018 19:19:34 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.storch&quot; class=&quot;user-hover&quot; rel=&quot;david.storch&quot;&gt;david.storch&lt;/a&gt; should this be part of ongoing change streams epic?&lt;/p&gt;</comment>
                            <comment id="1727155" author="siyuan.zhou@10gen.com" created="Thu, 16 Nov 2017 00:00:49 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=charlie.swanson&quot; class=&quot;user-hover&quot; rel=&quot;charlie.swanson&quot;&gt;charlie.swanson&lt;/a&gt;, here&apos;s the issue found by testing shell helper. The behavior is tested on an unshared collection. Not sure if this issue applies to sharded collections.&lt;/p&gt;</comment>
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