technical/send-pack-pipeline.txt

git-send-pack
=============

Overall operation
-----------------

. Connects to the remote side and invokes git-receive-pack.

. Learns what refs the remote has and what commit they point at.
 Matches them to the refspecs we are pushing.

. 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.

. Calls pack_objects() which generates a packfile and sends it
 over to the other end.

. If the remote side is new enough (v1.1.0 or later), wait for
 the unpack and hook status from the other end.

. Exit with appropriate error codes.


Pack_objects pipeline
---------------------

This function gets one file descriptor (`fd`) which is either a
socket (over the network) or a pipe (local). What's written to
this fd goes to git-receive-pack to be unpacked.

 send-pack ---> fd ---> receive-pack

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.

 send-pack
 |
 pack_objects() ---> fd ---> receive-pack
 | ^ (pipe)
 v |
 (child)

The child dup2'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'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.

 send-pack
 |
 pack_objects(parent)
 |
 v [0]
 pack-objects [0] ---> receive-pack


[jc: the pipeline was much more complex and needed documentation before
 I understood an earlier bug, but now it is trivial and straightforward.]