PSCP and PSFTP can only work over a protocol enough like SSH to be
able to run subsystems (or at the very least a remote command, for
old-style PSCP). Historically we've implemented this restriction by
having them not support any protocol-selection command-line options at
all, and hardwiring them to instantiating ssh_backend.
This commit regularises them to be more like the rest of the tools.
You can select a protocol using the appropriate command-line option,
provided it's a protocol in those tools' backends[] array. And the
setup code will find the BackendVtable to instantiate by the usual
method of calling backend_vt_from_proto.
Currently, this makes essentially no difference: those tools link in
be_ssh.c, which means the only supported backend is SSH. So the effect
is that now -ssh is an accepted option with no effect, instead of
being rejected. But it opens the way to add other protocols that are
SSH-like enough to run file transfer over.
Nearly every part of the code that ever handles a full backend
structure has historically done it using a pair of pointer variables,
one pointing at a constant struct full of function pointers, and the
other pointing to a 'void *' state object that's passed to each of
those.
While I'm modernising the rest of the code, this seems like a good
time to turn that into the same more or less type-safe and less
cumbersome system as I'm using for other parts of the code, such as
Socket, Plug, BinaryPacketProtocol and so forth: the Backend structure
contains a vtable pointer, and a system of macro wrappers handles
dispatching through that vtable.
nonstandard port number when loading a saved session.
Occurs because those tools include be_none.c which defines no entries
in backends[] at all, as a result of which settings.c doesn't
recognise the word 'ssh' in the saved session's protocol field and
instead sets the protocol to something idiotic - which _then_ means
that when pscp.c forces the protocol to PROT_SSH, it also resets the
port number as it would when overriding a saved session specifying a
protocol other than SSH.
The immediate solution is to define a new be_ssh.c citing only
ssh_backend, and include that in the SSH-only tools. However, I wonder
if a better approach (perhaps when I redesign session loading and
saving) would be not to be so clever, and just have all the tools
contain a complete list of known protocol names for purposes of
understanding what's in the saved session data, and complain if you
try to use one they don't know how to actually speak.
[originally from svn r9254]
