nbd double disconnect triggered after installation

[ejoerns]

With the current RAUC master, a streaming installation triggers a double disconnect in the kernel.

To reproduce, run

rauc install http://127.0.0.1/example-bundle.raucb

which will trigger the kernel warning:

[24344.459244] block nbd7: NBD_DISCONNECT
[24344.463725] block nbd7: Disconnected due to user request.
[24344.463769] block nbd7: shutting down sockets
[24344.560521] block nbd7: NBD_DISCONNECT
[24344.560541] block nbd7: Send disconnect failed -32

Tested with v5.15.2 kernel.

Either RAUC should only issue a single disconnect, or the kernel should ignore a disconnect on an already-disconnected nbd dev.

[jluebbe]

As far as I can see, the logged warning is the only effect of this issue.

Note that RAUC only sends one NBD_CMD_DISCONNECT, the other is generated by the kernel due to NBD_CFLAG_DISCONNECT_ON_CLOSE. The latter should not be generated by the kernel if the NBD is already closed.