ssh: Do not switch session to non-blocking mode

The libssh does not handle non-blocking mode in SFTP correctly. The
driver code already changes the mode to blocking for the SFTP
initialization, but for some reason changes to non-blocking mode.
This used to work accidentally until libssh in 0.11 branch merged
the patch to avoid infinite looping in case of network errors:

https://gitlab.com/libssh/libssh-mirror/-/merge_requests/498

Since then, the ssh driver in qemu fails to read files over SFTP
as the first SFTP messages exchanged after switching the session
to non-blocking mode return SSH_AGAIN, but that message is lost
int the SFTP internals and interpretted as SSH_ERROR, which is
returned to the caller:

https://gitlab.com/libssh/libssh-mirror/-/issues/280

This is indeed an issue in libssh that we should address in the
long term, but it will require more work on the internals. For
now, the SFTP is not supported in non-blocking mode.

Fixes: https://gitlab.com/libssh/libssh-mirror/-/issues/280
Signed-off-by: Jakub Jelen <[email protected]>
Signed-off-by: Richard W.M. Jones <[email protected]>
Message-ID: <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

block/ssh.c

@@ -866,9 +866,6 @@ static int ssh_open(BlockDriverState *bs, QDict *options, int bdrv_flags,
         goto err;
     }
 
-    /* Go non-blocking. */
-    ssh_set_blocking(s->session, 0);
-
     if (s->attrs->type == SSH_FILEXFER_TYPE_REGULAR) {
         bs->supported_truncate_flags = BDRV_REQ_ZERO_WRITE;
     }