Re: [Libguestfs] [nbdkit PATCH v2 7/7] server: Better newstyle .open failure handling

If a plugin's .open or .get_size or .can_write fails, right now that is fatal to the connection. When nbdkit was first implemented, this made sense (there was no way to report errors to oldstyle or NBD_OPT_EXPORT_NAME). But now that newstyle is around, it's rather abrupt to hang up on the client, and better is to return an error to NBD_OPT_GO, and let the client choose what to do (most clients will probably hang up, whether gracefully with NBD_OPT_ABORT or abruptly, rather than try other NBD_OPT_*, but _we_ shouldn't be the ones forcing their hand). For an example of what this improves, if you run: $ nbdkit -fv sh - <<\EOF case $1 in get_size) echo oops >&2; exit 1;; *) exit 2;; esac EOF Pre-patch, qemu complains about the abrupt server death: $ qemu-nbd --list qemu-nbd: Failed to read option reply: Unexpected end-of-file before all bytes were read Post-patch, qemu gets the desired error message, and exits gracefully: $ qemu-nbd --list qemu-nbd: Requested export not available Note that this does not fix the pre-existing problem that we can end up calling .finalize even when .prepare was skipped or failed (that latent problem was first exposed for the rare client that calls NBD_OPT_INFO before NBD_OPT_GO, see commit a6b88b19); a later patch will have to add better bookkeeping for that, and for better handling of reopen requests from the retry filter. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- server/protocol-handshake-newstyle.c | 32 +++++++++++++++++++++------- server/protocol-handshake-oldstyle.c | 3 +++ 2 files changed, 27 insertions(+), 8 deletions(-) diff --git a/server/protocol-handshake-newstyle.c b/server/protocol-handshake-newstyle.c index 2480d7a3..878fe53f 100644 --- a/server/protocol-handshake-newstyle.c +++ b/server/protocol-handshake-newstyle.c @@ -198,7 +198,7 @@ conn_recv_full (struct connection *conn, void *buf, size_t len, /* Sub-function of negotiate_handshake_newstyle_options below. It * must be called on all non-error paths out of the options for-loop - * in that function. + * in that function, and must not cause any wire traffic. */ static int finish_newstyle_options (struct connection *conn, uint64_t *exportsize) @@ -322,7 +322,9 @@ negotiate_handshake_newstyle_options (struct connection *conn) if (check_export_name (conn, option, data, optlen, optlen, true) == -1) return -1; - /* We have to finish the handshake by sending handshake_finish. */ + /* We have to finish the handshake by sending handshake_finish. + * On failure, we have to disconnect. + */ if (finish_newstyle_options (conn, &exportsize) == -1) return -1; @@ -460,16 +462,30 @@ negotiate_handshake_newstyle_options (struct connection *conn) * or else we drop the support for that context. */ if (check_export_name (conn, option, &data[4], exportnamelen, - optlen - 6, true) == -1) - return -1; + optlen - 6, true) == -1) { + if (send_newstyle_option_reply (conn, option, NBD_REP_ERR_INVALID) + == -1) + return -1; + continue; + } /* The spec is confusing, but it is required that we send back * NBD_INFO_EXPORT, even if the client did not request it! * qemu client in particular does not request this, but will - * fail if we don't send it. + * fail if we don't send it. Note that if .open fails, but we + * succeed at .close, then we merely return an error to the + * client and let them try another NBD_OPT, rather than + * disconnecting. */ - if (finish_newstyle_options (conn, &exportsize) == -1) - return -1; + if (finish_newstyle_options (conn, &exportsize) == -1) { + if (backend->finalize (backend, conn) == -1) + return -1; + backend_close (backend, conn); + if (send_newstyle_option_reply (conn, option, + NBD_REP_ERR_UNKNOWN) == -1) + return -1; + continue; + } if (send_newstyle_option_reply_info_export (conn, option, NBD_REP_INFO, @@ -497,7 +513,7 @@ negotiate_handshake_newstyle_options (struct connection *conn) } /* Unlike NBD_OPT_EXPORT_NAME, NBD_OPT_GO sends back an ACK - * or ERROR packet. + * or ERROR packet. If this was NBD_OPT_LIST, call .close. */ if (send_newstyle_option_reply (conn, option, NBD_REP_ACK) == -1) return -1; diff --git a/server/protocol-handshake-oldstyle.c b/server/protocol-handshake-oldstyle.c index 4efc668b..45a1a486 100644 --- a/server/protocol-handshake-oldstyle.c +++ b/server/protocol-handshake-oldstyle.c @@ -52,6 +52,9 @@ protocol_handshake_oldstyle (struct connection *conn) assert (tls != 2); /* Already filtered in main */ + /* With oldstyle, our only option if .open or friends fail is to + * disconnect, as we cannot report the problem to the client. + */ if (protocol_common_open (conn, &exportsize, &eflags) == -1) return -1;