On Wed, 28 Feb 2018 15:22:37 -0800
Brandon Williams [off-list ref] wrote:

+    output = *section
+    section = (acknowledgments | packfile)
+	      (flush-pkt | delim-pkt)
+
+    acknowledgments = PKT-LINE("acknowledgments" LF)
+		      (nak | *ack)
+		      (ready)
+    ready = PKT-LINE("ready" LF)
+    nak = PKT-LINE("NAK" LF)
+    ack = PKT-LINE("ACK" SP obj-id LF)
+
+    packfile = PKT-LINE("packfile" LF)
+	       [PACKFILE]

I should have noticed this earlier, but "PACKFILE" is not defined anywhere -
it's probably better written as:

    *PKT-LINE(%x01-03 *%x00-ff)"

or something like that.

+    acknowledgments section
+	* Always begins with the section header "acknowledgments"
+
+	* The server will respond with "NAK" if none of the object ids sent
+	  as have lines were common.
+
+	* The server will respond with "ACK obj-id" for all of the
+	  object ids sent as have lines which are common.
+
+	* A response cannot have both "ACK" lines as well as a "NAK"
+	  line.
+
+	* The server will respond with a "ready" line indicating that
+	  the server has found an acceptable common base and is ready to
+	  make and send a packfile (which will be found in the packfile
+	  section of the same response)
+
+	* If the client determines that it is finished with negotiations
+	  by sending a "done" line, the acknowledgments sections MUST be
+	  omitted from the server's response.
+
+	* If the server has found a suitable cut point and has decided
+	  to send a "ready" line, then the server can decide to (as an
+	  optimization) omit any "ACK" lines it would have sent during
+	  its response.  This is because the server will have already
+	  determined the objects it plans to send to the client and no
+	  further negotiation is needed.
+
+----
+    packfile section
+	* Always begins with the section header "packfile"
+
+	* The transmission of the packfile begins immediately after the
+	  section header
+
+	* The data transfer of the packfile is always multiplexed, using
+	  the same semantics of the 'side-band-64k' capability from
+	  protocol version 1.  This means that each packet, during the
+	  packfile data stream, is made up of a leading 4-byte pkt-line
+	  length (typical of the pkt-line format), followed by a 1-byte
+	  stream code, followed by the actual data.
+
+	  The stream code can be one of:
+		1 - pack data
+		2 - progress messages
+		3 - fatal error message just before stream aborts
+
+	* This section is only included if the client has sent 'want'
+	  lines in its request and either requested that no more
+	  negotiation be done by sending 'done' or if the server has
+	  decided it has found a sufficient cut point to produce a
+	  packfile.

For both the sections, I think that the conditions for
inclusion/non-inclusion ("This section is only included if...") should
be the first point.

+static void upload_pack_data_init(struct upload_pack_data *data)
+{
+	struct object_array wants = OBJECT_ARRAY_INIT;
+	struct oid_array haves = OID_ARRAY_INIT;
+
+	memset(data, 0, sizeof(*data));
+	data->wants = wants;
+	data->haves = haves;
+}

Any reason to use a initializer function instead of a static literal?