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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.
pack_objects() ---> fd ---> receive-pack
| ^ (pipe)
v |
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.
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.]