Git-send-pack internals ======================= 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.]