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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>
    </build-info>


<item>
            <title>[SERVER-70974] Fix early-exits triggered when user specifies TCP Fast Open server parameters</title>
                <link>https://jira.mongodb.org/browse/SERVER-70974</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;h3&gt;&lt;a name=&quot;Background%2FProblem&quot;&gt;&lt;/a&gt;Background/Problem&lt;/h3&gt;

&lt;p&gt;As part of implementing TCP Fast Open support, we &lt;a href=&quot;https://github.com/mongodb/mongo/blob/b220a5176b0b70a158d483d8cbf8ab7d7744c5a1/src/mongo/transport/transport_layer_asio.cpp#L1019-L1023&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;calculate a variable&lt;/a&gt; that indicates whether the user specified any TCP Fast Open server parameters.&lt;/p&gt;

&lt;p&gt;We use this variable in some places to decide whether to emit logs about &lt;a href=&quot;https://github.com/mongodb/mongo/blob/b220a5176b0b70a158d483d8cbf8ab7d7744c5a1/src/mongo/transport/transport_layer_asio.cpp#L1112-L1114&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;success/failure enabling TCP Fast Open&lt;/a&gt;. We also &lt;em&gt;intend&lt;/em&gt; to use this to &lt;a href=&quot;https://github.com/mongodb/mongo/blob/b220a5176b0b70a158d483d8cbf8ab7d7744c5a1/src/mongo/transport/transport_layer_asio.cpp#L1247-L1249&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;decide whether to fail from connection establishment&lt;/a&gt;. &lt;/p&gt;

&lt;p&gt;However, we forget to check whether the given error code indicates success or failure &amp;#8211; the consequence is that whenever a user specifies &lt;em&gt;any&lt;/em&gt; TCP Fast Open parameter, whether to turn it on, turn it off, or change the queue size, we will &lt;b&gt;always&lt;/b&gt; fail connection establishment.&lt;/p&gt;

&lt;p&gt;In some cases, this can trigger a crash, as seen in the linked HELP ticket. In other cases, this just permanently stalls the server, because we never establish the listener thread that would accept connections.&lt;/p&gt;

&lt;p&gt;Relatedly, we always enable TCP Fast Open, even if the user attempts to disable it via server parameter.&lt;/p&gt;

&lt;h3&gt;&lt;a name=&quot;Solution&quot;&gt;&lt;/a&gt;Solution&lt;/h3&gt;
&lt;ul&gt;
	&lt;li&gt;Only return early from connection establishment if the error code is a failure.&lt;/li&gt;
	&lt;li&gt;Add logic only to enable TCP Fast Open if either the user doesn&apos;t specify (implicit enablement) or if the user specifies to enable TCP Fast Open. Do not enable it if the user indicates not to enable TCP Fast Open.&lt;/li&gt;
	&lt;li&gt;Add tests to ensure that the behavior has been fixed.&lt;/li&gt;
&lt;/ul&gt;
</description>
                <environment></environment>
        <key id="2170702">SERVER-70974</key>
            <summary>Fix early-exits triggered when user specifies TCP Fast Open server parameters</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="billy.donahue@mongodb.com">Billy Donahue</assignee>
                                    <reporter username="blake.oler@mongodb.com">Blake Oler</reporter>
                        <labels>
                    </labels>
                <created>Mon, 31 Oct 2022 16:34:16 +0000</created>
                <updated>Thu, 30 Nov 2023 18:51:39 +0000</updated>
                            <resolved>Tue, 1 Aug 2023 13:34:55 +0000</resolved>
                                                    <fixVersion>7.1.0-rc0</fixVersion>
                    <fixVersion>7.0.5</fixVersion>
                    <fixVersion>6.0.13</fixVersion>
                                                        <votes>0</votes>
                                    <watches>8</watches>
                                                                                                                <comments>
                            <comment id="5913087" author="xgen-internal-githook" created="Wed, 29 Nov 2023 17:24:52 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Billy Donahue&apos;, &apos;email&apos;: &apos;billy.donahue@mongodb.com&apos;, &apos;username&apos;: &apos;BillyDonahue&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-70974&quot; title=&quot;Fix early-exits triggered when user specifies TCP Fast Open server parameters&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-70974&quot;&gt;&lt;del&gt;SERVER-70974&lt;/del&gt;&lt;/a&gt; Fix early-exits triggered when user specifies TFO&lt;br/&gt;
Branch: v7.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/4cfb9179e4e7241852c7c2d6dffffaa7d271ab24&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/4cfb9179e4e7241852c7c2d6dffffaa7d271ab24&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5913085" author="xgen-internal-githook" created="Wed, 29 Nov 2023 17:24:13 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Billy Donahue&apos;, &apos;email&apos;: &apos;billy.donahue@mongodb.com&apos;, &apos;username&apos;: &apos;BillyDonahue&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-70974&quot; title=&quot;Fix early-exits triggered when user specifies TCP Fast Open server parameters&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-70974&quot;&gt;&lt;del&gt;SERVER-70974&lt;/del&gt;&lt;/a&gt; Fix early-exits triggered when user specifies TCP Fast Open server parameters&lt;br/&gt;
Branch: v6.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/ac2ac3fef3d9d0d14b996180b19ec70f986d6115&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/ac2ac3fef3d9d0d14b996180b19ec70f986d6115&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5617191" author="billy.donahue" created="Mon, 7 Aug 2023 14:54:52 +0000"  >&lt;p&gt;Going to backport only the minimal changes to the transport layer.&lt;/p&gt;

&lt;p&gt;The main PR for this ticket was a VERY hard-won testability refactor and lots of platform juggling to get the tests to pass reliably, so I don&apos;t think it&apos;s good to backport all that extra stuff. The basic fix is just a few tweaks to a few &lt;tt&gt;if&lt;/tt&gt; conditions.&lt;/p&gt;</comment>
                            <comment id="5606576" author="xgen-internal-githook" created="Wed, 2 Aug 2023 04:13:44 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Billy Donahue&apos;, &apos;email&apos;: &apos;billy.donahue@mongodb.com&apos;, &apos;username&apos;: &apos;BillyDonahue&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-70974&quot; title=&quot;Fix early-exits triggered when user specifies TCP Fast Open server parameters&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-70974&quot;&gt;&lt;del&gt;SERVER-70974&lt;/del&gt;&lt;/a&gt;  Fix early-exits triggered when user specifies TCP Fast Open server parameters&lt;br/&gt;
Branch: minh.luu-no_compile_sys-perf&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/16cabd2354977e9e1d2cf4a6a89416cad240ee76&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/16cabd2354977e9e1d2cf4a6a89416cad240ee76&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5604055" author="xgen-internal-githook" created="Tue, 1 Aug 2023 07:59:43 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Billy Donahue&apos;, &apos;email&apos;: &apos;billy.donahue@mongodb.com&apos;, &apos;username&apos;: &apos;BillyDonahue&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-70974&quot; title=&quot;Fix early-exits triggered when user specifies TCP Fast Open server parameters&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-70974&quot;&gt;&lt;del&gt;SERVER-70974&lt;/del&gt;&lt;/a&gt;  Fix early-exits triggered when user specifies TCP Fast Open server parameters&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/16cabd2354977e9e1d2cf4a6a89416cad240ee76&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/16cabd2354977e9e1d2cf4a6a89416cad240ee76&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5603609" author="billy.donahue" created="Mon, 31 Jul 2023 23:37:13 +0000"  >&lt;p&gt;merging origin/master in revealed some issues, since new tests disabled TFO the old-fashioned way (which doesn&apos;t work anymore). Took care of that.&lt;/p&gt;

&lt;p&gt;Windows hits a segfault tearing down one of the new tests. Not sure what that&apos;s about.&lt;br/&gt;
Trying to revert some small details to fix this problem. Not sure what it is yet.&lt;br/&gt;
&lt;a href=&quot;https://spruce.mongodb.com/task/mongodb_mongo_master_windows_compile_required_run_unittests_patch_71a4c41f6e33cabc882b3442df027490eda7a900_64c81a48d6d80a5ccb0288db_23_07_31_20_32_45/tests?execution=0&amp;amp;sortBy=STATUS&amp;amp;sortDir=ASC&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://spruce.mongodb.com/task/mongodb_mongo_master_windows_compile_required_run_unittests_patch_71a4c41f6e33cabc882b3442df027490eda7a900_64c81a48d6d80a5ccb0288db_23_07_31_20_32_45/tests?execution=0&amp;amp;sortBy=STATUS&amp;amp;sortDir=ASC&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5575564" author="billy.donahue" created="Tue, 18 Jul 2023 22:12:07 +0000"  >&lt;p&gt;Still some evergreen troubles. Working on it.&lt;/p&gt;</comment>
                            <comment id="5567653" author="billy.donahue" created="Fri, 14 Jul 2023 18:27:10 +0000"  >&lt;p&gt;As long as that journey was, it was not the end.&lt;br/&gt;
Changing to nonblocking IO client connect calls didn&apos;t fix the problem.&lt;/p&gt;

&lt;p&gt;The problem was ALSO a server-side problem.&lt;/p&gt;

&lt;p&gt;The listen call always specifies a backlog depth. In the jstest (mongod) this is 128.&lt;br/&gt;
In the C++ unit test it&apos;s 0. Termporary fix:&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;
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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-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;@ src/mongo/transport/asio/asio_transport_layer.cpp:1163 @ void AsioTransportLayer::_runListener() noexcept {&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: gray; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    for (auto&amp;amp; acceptorRecord : _acceptorRecords) {&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;        asio::error_code ec;&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: red; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;-       acceptorRecord-&amp;gt;acceptor.listen(serverGlobalParams.listenBacklog, ec);&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: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;+       int backlog = serverGlobalParams.listenBacklog;&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&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;+       if (backlog == 0)&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;+           backlog = SOMAXCONN;&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: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;+       acceptorRecord-&amp;gt;acceptor.listen(backlog, ec);&lt;/span&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;
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&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;I&apos;m partially blaming the obscurity of IDL and server parameter initialization for this.&lt;br/&gt;
The default value of this listenBacklog is not defined in C&amp;#43;&amp;#43;. It&apos;s a C&amp;#43;&amp;#43; expression hiding in yaml.&lt;/p&gt;
&lt;p/&gt;
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&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;
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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;   &apos;net.listenBacklog&apos;:&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;        description: &apos;Set socket listen backlog size&apos;&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;        short_name: listenBacklog&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;        arg_vartype: Int&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;   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;        default: { expr: &apos;SOMAXCONN&apos; }&lt;/span&gt;&lt;/pre&gt;
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&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;But it has a &quot;fake&quot; initial value of 0 at its C++ definition, for a reason I don&apos;t understand:&lt;/p&gt;
&lt;p/&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-top: 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;span style=&quot;color: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;int&lt;/span&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; listenBacklog = 0;  &lt;/span&gt;&lt;span style=&quot;color: #008200; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;// --listenBacklog, real default is SOMAXCONN&lt;/span&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;(what does &quot;real default&quot; mean?)&lt;/p&gt;

&lt;p&gt;So somehow the mocked environment of the transport_test doesn&apos;t go through all the same initialization steps of fetching the REAL default from IDL, and sticks with the C++ struct default or whatever, so we end up with a listen backlog of 0. This is not an error but it disables TFO. This doesn&apos;t actually break anything UNTIL you try to use TFO. TFO requires backlog capacity because the connection isn&apos;t fully TCP OPEN until it&apos;s been replied to, and it isn&apos;t replied to until you sendmsg a response, so a TFO connection is using a listen backlog slot until we read from it and send a reply.&lt;/p&gt;

&lt;p&gt;If there&apos;s no backlog capacity, TFO can&apos;t be engaged and the server will simply accept the connection earlier (before recvmsg or sendmsg happens) and reply to the SYN with a SYNACK acking none of the data in the TFO SYN packet.&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;This is a failed TFO attempt because listen backlog was full (0 actually).&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;02:53:36.888552 IP localhost.54482 &amp;gt; localhost.37097: Flags [S], seq 423420387:423420427, win 65495, options [mss 65495,sackOK,TS val 4149143320 ecr 0,nop,wscale 7,tfo  cookie ed87c9091b811d74,nop,nop], length 40&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;&amp;nbsp;&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;// Only acks 423420388, which is the SYN sequence number +1, a normal TCP open.&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;02:53:36.888562 IP localhost.37097 &amp;gt; localhost.54482: Flags [S.], seq 1804626240, ack 423420388, win 65483, options [mss 65495,sackOK,TS val 4149143312 ecr 4149143320,nop,wscale 7], length 0&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;&amp;nbsp;&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;// The 40 payload bytes are not acked and must be retransmitted here:&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;02:53:36.888568 IP localhost.54482 &amp;gt; localhost.37097: Flags [P.], seq 1:41, ack 1, win 512, options [nop,nop,TS val 4149143320 ecr 4149143312], length 40&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;02:53:36.888575 IP localhost.37097 &amp;gt; localhost.54482: Flags [.], ack 41, win 502, options [nop,nop,TS val 4149143320 ecr 4149143320], length 0&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;PHWEW!&lt;/p&gt;</comment>
                            <comment id="5565394" author="billy.donahue" created="Thu, 13 Jul 2023 21:48:20 +0000"  >&lt;p&gt;Was having a great deal of trouble testing TFO usage. Jstest did it, but C++ test didn&apos;t.&lt;/p&gt;

&lt;p&gt;Strace logs:&lt;/p&gt;

&lt;p&gt;tcp_fastopen_test.js performs a successful TFO connection.&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;13310 16:29:53 socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 14&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;13310 16:29:53 setsockopt(14, SOL_TCP, TCP_FASTOPEN_CONNECT, [1], 4) = 0&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;13310 16:29:53 ioctl(14, FIONBIO, [1])  = 0&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;13310 16:29:53 connect(14, {sa_family=AF_INET, sin_port=htons(20040), sin_addr=inet_addr(&quot;127.0.0.1&quot;)}, 16) = 0&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;13310 16:29:53 ioctl(14, FIONBIO, [0])  = 0&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;13310 16:29:53 getsockname(14, {sa_family=AF_INET, sin_port=htons(50038), sin_addr=inet_addr(&quot;127.0.0.1&quot;)}, [128-&amp;gt;16]) = 0&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;13310 16:29:53 setsockopt(14, SOL_TCP, TCP_NODELAY, [1], 4) = 0&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;13310 16:29:53 setsockopt(14, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0&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;13310 16:29:53 getsockopt(14, SOL_TCP, TCP_KEEPIDLE, [7200], [4]) = 0&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;13310 16:29:53 setsockopt(14, SOL_TCP, TCP_KEEPIDLE, [300], 4) = 0&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;13310 16:29:53 getsockopt(14, SOL_TCP, TCP_KEEPINTVL, [75], [4]) = 0&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;13310 16:29:53 setsockopt(14, SOL_TCP, TCP_KEEPINTVL, [1], 4) = 0&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;13310 16:29:53 getsockname(14, {sa_family=AF_INET, sin_port=htons(50038), sin_addr=inet_addr(&quot;127.0.0.1&quot;)}, [128-&amp;gt;16]) = 0&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;13310 16:29:53 ioctl(14, FIONBIO, [0])  = 0&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;13310 16:29:53 sendmsg(14, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base=&quot;\r\1\0\0\21\0\0\0\0\0\0\0\335\7\0\0\1\0\0\0\0\364\0\0\0\20hello\0&quot;..., iov_len=269}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL &amp;lt;unfinished ...&amp;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;asio_transport_layer_test performs a TCP connection, failing TFO:&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;4028  20:14:51 socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 16&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;4028  20:14:51 setsockopt(16, SOL_TCP, TCP_FASTOPEN_CONNECT, [1], 4) = 0&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;4028  20:14:51 connect(16, {sa_family=AF_INET, sin_port=htons(43019), sin_addr=inet_addr(&quot;127.0.0.1&quot;)}, 16) = 0&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;4028  20:14:51 sendmsg(16, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base=&quot;(\0\0\0\0\0\0\0\0\0\0\0\335\7\0\0\1\0\0\0\0\17\0\0\0\20ping\0\1&quot;..., iov_len=40}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 40&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;
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			&lt;/td&gt;
		&lt;/tr&gt;
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&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;Finally seeing these side by side, the ioctl call to select nonblocking IO before the connect (but then deselecting nonblocking IO immediately after the connect) is a smoking gun.&lt;/p&gt;</comment>
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