technical/send-pack-pipeline.html

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 734 </head>
 735 <body class="article">
 736 <div id="header">
 737 <h1>Git-send-pack internals</h1>
 738 <span id="revdate">2023-06-26</span>
 739 </div>
 740 <div id="content">
 741 <div class="sect1">
 742 <h2 id="_overall_operation">Overall operation</h2>
 743 <div class="sectionbody">
 744 <div class="olist arabic"><ol class="arabic">
 745 <li>
 746 <p>
 747 Connects to the remote side and invokes git-receive-pack.
 748 </p>
 749 </li>
 750 <li>
 751 <p>
 752 Learns what refs the remote has and what commit they point at.
 753   Matches them to the refspecs we are pushing.
 754 </p>
 755 </li>
 756 <li>
 757 <p>
 758 Checks if there are non-fast-forwards.  Unlike fetch-pack,
 759   the repository send-pack runs in is supposed to be a superset
 760   of the recipient in fast-forward cases, so there is no need
 761   for want/have exchanges, and fast-forward check can be done
 762   locally.  Tell the result to the other end.
 763 </p>
 764 </li>
 765 <li>
 766 <p>
 767 Calls pack_objects() which generates a packfile and sends it
 768   over to the other end.
 769 </p>
 770 </li>
 771 <li>
 772 <p>
 773 If the remote side is new enough (v1.1.0 or later), wait for
 774   the unpack and hook status from the other end.
 775 </p>
 776 </li>
 777 <li>
 778 <p>
 779 Exit with appropriate error codes.
 780 </p>
 781 </li>
 782 </ol></div>
 783 </div>
 784 </div>
 785 <div class="sect1">
 786 <h2 id="_pack_objects_pipeline">Pack_objects pipeline</h2>
 787 <div class="sectionbody">
 788 <div class="paragraph"><p>This function gets one file descriptor (<code>fd</code>) which is either a
 789 socket (over the network) or a pipe (local).  What&#8217;s written to
 790 this fd goes to git-receive-pack to be unpacked.</p></div>
 791 <div class="literalblock">
 792 <div class="content">
 793 <pre><code>send-pack ---&gt; fd ---&gt; receive-pack</code></pre>
 794 </div></div>
 795 <div class="paragraph"><p>The function pack_objects creates a pipe and then forks.  The
 796 forked child execs pack-objects with --revs to receive revision
 797 parameters from its standard input. This process will write the
 798 packfile to the other end.</p></div>
 799 <div class="literalblock">
 800 <div class="content">
 801 <pre><code>send-pack
 802    |
 803    pack_objects() ---&gt; fd ---&gt; receive-pack
 804       | ^ (pipe)
 805       v |
 806      (child)</code></pre>
 807 </div></div>
 808 <div class="paragraph"><p>The child dup2&#8217;s to arrange its standard output to go back to
 809 the other end, and read its standard input to come from the
 810 pipe.  After that it exec&#8217;s pack-objects.  On the other hand,
 811 the parent process, before starting to feed the child pipeline,
 812 closes the reading side of the pipe and fd to receive-pack.</p></div>
 813 <div class="literalblock">
 814 <div class="content">
 815 <pre><code>send-pack
 816    |
 817    pack_objects(parent)
 818       |
 819       v [0]
 820      pack-objects [0] ---&gt; receive-pack</code></pre>
 821 </div></div>
 822 <div class="paragraph"><p>[jc: the pipeline was much more complex and needed documentation before
 823  I understood an earlier bug, but now it is trivial and straightforward.]</p></div>
 824 </div>
 825 </div>
 826 </div>
 827 <div id="footnotes"><hr /></div>
 828 <div id="footer">
 829 <div id="footer-text">
 830 Last updated
 831  2020-03-10 15:02:33 PDT
 832 </div>
 833 </div>
 834 </body>
 835 </html>