# Git-send-pack internals

2023-11-02

## Overall operation

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

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

3. 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.

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

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

6. 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.]