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<body class="article">
<div id="header">
<h1>Git-send-pack internals</h1>
<span id="revdate"></span>
</div>
<div id="content">
<div class="sect1">
<h2 id="_overall_operation">Overall operation</h2>
<div class="sectionbody">
<div class="olist arabic"><ol class="arabic">
<li>
<p>
Connects to the remote side and invokes git-receive-pack.
</p>
</li>
<li>
<p>
Learns what refs the remote has and what commit they point at.
  Matches them to the refspecs we are pushing.
</p>
</li>
<li>
<p>
Checks if there are non-fast-forwards.  Unlike fetch-pack,
  the repository send-pack runs in is supposed to be a superset
  of the recipient in fast-forward cases, so there is no need
  for want/have exchanges, and fast-forward check can be done
  locally.  Tell the result to the other end.
</p>
</li>
<li>
<p>
Calls pack_objects() which generates a packfile and sends it
  over to the other end.
</p>
</li>
<li>
<p>
If the remote side is new enough (v1.1.0 or later), wait for
  the unpack and hook status from the other end.
</p>
</li>
<li>
<p>
Exit with appropriate error codes.
</p>
</li>
</ol></div>
</div>
</div>
<div class="sect1">
<h2 id="_pack_objects_pipeline">Pack_objects pipeline</h2>
<div class="sectionbody">
<div class="paragraph"><p>This function gets one file descriptor (<code>fd</code>) which is either a
socket (over the network) or a pipe (local).  What&#8217;s written to
this fd goes to git-receive-pack to be unpacked.</p></div>
<div class="literalblock">
<div class="content">
<pre><code>send-pack ---&gt; fd ---&gt; receive-pack</code></pre>
</div></div>
<div class="paragraph"><p>The function pack_objects creates a pipe and then forks.  The
forked child execs pack-objects with --revs to receive revision
parameters from its standard input. This process will write the
packfile to the other end.</p></div>
<div class="literalblock">
<div class="content">
<pre><code>send-pack
   |
   pack_objects() ---&gt; fd ---&gt; receive-pack
      | ^ (pipe)
      v |
     (child)</code></pre>
</div></div>
<div class="paragraph"><p>The child dup2&#8217;s to arrange its standard output to go back to
the other end, and read its standard input to come from the
pipe.  After that it exec&#8217;s pack-objects.  On the other hand,
the parent process, before starting to feed the child pipeline,
closes the reading side of the pipe and fd to receive-pack.</p></div>
<div class="literalblock">
<div class="content">
<pre><code>send-pack
   |
   pack_objects(parent)
      |
      v [0]
     pack-objects [0] ---&gt; receive-pack</code></pre>
</div></div>
<div class="paragraph"><p>[jc: the pipeline was much more complex and needed documentation before
 I understood an earlier bug, but now it is trivial and straightforward.]</p></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated
 2025-06-13 22:14:31 UTC
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