Server: explain why it's OK to disconnect in case of any read error

Signed-off-by: Gabor Igloi <gabor.igloi@citrix.com>

lib/server.ml

@@ -238,6 +238,15 @@ let serve t (type t) ?(read_only=false) block (b:t) =
       end
       >>= loop
     | { ty = Command.Read; from; len; handle } ->
+      (* It is okay to disconnect here in case of errors. The NBD protocol
+         documentation says about NBD_CMD_READ:
+         "If an error occurs, the server SHOULD set the appropriate error code
+         in the error field. The server MAY then initiate a hard disconnect.
+         If it chooses not to, it MUST NOT send any payload for this request.
+         If an error occurs while reading after the server has already sent out
+         the reply header with an error field set to zero (i.e., signalling no
+         error), the server MUST immediately initiate a hard disconnect; it
+         MUST NOT send any further data to the client." *)
       if Int64.(rem from (of_int info.Block.sector_size)) <> 0L || Int64.(rem (of_int32 len) (of_int info.Block.sector_size) <> 0L)
       then error t handle `EINVAL
       else begin
@@ -249,12 +258,6 @@ let serve t (type t) ?(read_only=false) block (b:t) =
           Block.read b Int64.(div offset (of_int info.Block.sector_size)) [ subblock ]
           >>= function
           | `Error e ->
-            (* The NBD protocol documentation says about NBD_CMD_READ:
-               "If an error occurs while reading after the server has already
-               sent out the reply header with an error field set to zero (i.e.,
-               signalling no error), the server MUST immediately initiate a
-               hard disconnect; it MUST NOT send any further data to the
-               client." *)
             Lwt.fail_with (Printf.sprintf "Partial failure during a Block.read: %s; terminating the session" (Block_error_printer.to_string e))
           | `Ok () ->
             t.channel.write subblock