git allows you to set which paths to use for the git server programs
when connecting over ssh; and we want to provide something similar.
We do this by providing a factory function which can be set as the
remote's transport callback which will set the given paths upon
creation.
As git_clone now has callbacks to configure the details of the
repository and remote, remove the lower-level functions from the public
API, as they lack some of the logic from git_clone proper.
For urls where we do not specify a username, we must handle the case
where the ssh transport asks us for the username.
Test also that switching username fails.
In order to know which authentication methods are supported/allowed by
the ssh server, we need to send a NONE auth request, which needs a
username associated with it.
Most ssh server implementations do not allow switching the username
between authentication attempts, which means we cannot use a dummy
username and then switch. There are two ways around this.
The first is to use a different connection, which an earlier commit
implements, but this increases how long it takes to get set up, and
without knowing the right username, we cannot guarantee that the
list we get in response is the right one.
The second is what's implemented here: if there is no username specified
in the url, ask for it first. We can then ask for the list of auth
methods and use the user's credentials in the same connection.
Before calling the credentials callback, ask the sever which
authentication methods it supports and report that to the user, instead
of simply reporting everything that the transport supports.
In case of an error, we do fall back to listing all of them.
This changes the behavior of callbacks so that the callback error
code is not converted into GIT_EUSER and instead we propagate the
return value through to the caller. Instead of using the
giterr_capture and giterr_restore functions, we now rely on all
functions to pass back the return value from a callback.
To avoid having a return value with no error message, the user
can call the public giterr_set_str or some such function to set
an error message. There is a new helper 'giterr_set_callback'
that functions can invoke after making a callback which ensures
that some error message was set in case the callback did not set
one.
In places where the sign of the callback return value is
meaningful (e.g. positive to skip, negative to abort), only the
negative values are returned back to the caller, obviously, since
the other values allow for continuing the loop.
The hardest parts of this were in the checkout code where positive
return values were overloaded as meaningful values for checkout.
I fixed this by adding an output parameter to many of the internal
checkout functions and removing the overload. This added some
code, but it is probably a better implementation.
There is some funkiness in the network code where user provided
callbacks could be returning a positive or a negative value and
we want to rely on that to cancel the loop. There are still a
couple places where an user error might get turned into GIT_EUSER
there, I think, though none exercised by the tests.
