nbd/server: Don't fail NBD_OPT_INFO for byte-aligned sources

In commit 0c1d50b, I added a couple of TODO comments about whether we
consult bl.request_alignment when responding to NBD_OPT_INFO. At the
time, qemu as server was hard-coding an advertised alignment of 512 to
clients that promised to obey constraints, and there was no function
for getting at a device's preferred alignment. But in hindsight,
advertising 512 when the block device prefers 1 caused other
compliance problems, and commit b0245d6 changed one of the two TODO
comments to advertise a more accurate alignment. Time to fix the other
TODO.  Doesn't really impact qemu as client (our normal client doesn't
use NBD_OPT_INFO, and qemu-nbd --list promises to obey block sizes),
but it might prove useful to other clients.

Fixes: b0245d6
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190403030526.12258-4-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

nbd/server.c

@@ -643,11 +643,14 @@ static int nbd_negotiate_handle_info(NBDClient *client, uint16_t myflags,
         return rc;
     }
 
-    /* If the client is just asking for NBD_OPT_INFO, but forgot to
-     * request block sizes, return an error.
-     * TODO: consult blk_bs(blk)->request_align, and only error if it
-     * is not 1? */
-    if (client->opt == NBD_OPT_INFO && !blocksize) {
+    /*
+     * If the client is just asking for NBD_OPT_INFO, but forgot to
+     * request block sizes in a situation that would impact
+     * performance, then return an error. But for NBD_OPT_GO, we
+     * tolerate all clients, regardless of alignments.
+     */
+    if (client->opt == NBD_OPT_INFO && !blocksize &&
+        blk_get_request_alignment(exp->blk) > 1) {
         return nbd_negotiate_send_rep_err(client,
                                           NBD_REP_ERR_BLOCK_SIZE_REQD,
                                           errp,