nbd: Improve server handling of shutdown requests

NBD commit 6d34500b clarified how clients and servers are supposed
to behave before closing a connection. It added NBD_REP_ERR_SHUTDOWN
(for the server to announce it is about to go away during option
haggling, so the client should quit sending NBD_OPT_* other than
NBD_OPT_ABORT) and ESHUTDOWN (for the server to announce it is about
to go away during transmission, so the client should quit sending
NBD_CMD_* other than NBD_CMD_DISC).  It also clarified that
NBD_OPT_ABORT gets a reply, while NBD_CMD_DISC does not.

This patch merely adds the missing reply to NBD_OPT_ABORT and teaches
the client to recognize server errors.  Actually teaching the server
to send NBD_REP_ERR_SHUTDOWN or ESHUTDOWN would require knowing that
the server has been requested to shut down soon (maybe we could do
that by installing a SIGINT handler in qemu-nbd, which transitions
from RUNNING to a new state that waits for the client to react,
rather than just out-right quitting - but that's a bigger task for
another day).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1476469998-28592-15-git-send-email-eblake@redhat.com>
[Move dummy ESHUTDOWN to include/qemu/osdep.h. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/block/nbd.h

@@ -83,12 +83,17 @@ typedef struct NBDReply NBDReply;
 #define NBD_FLAG_C_NO_ZEROES      (1 << 1) /* End handshake without zeroes. */
 
 /* Reply types. */
+#define NBD_REP_ERR(value) ((UINT32_C(1) << 31) | (value))
+
 #define NBD_REP_ACK             (1)             /* Data sending finished. */
 #define NBD_REP_SERVER          (2)             /* Export description. */
-#define NBD_REP_ERR_UNSUP       ((UINT32_C(1) << 31) | 1) /* Unknown option. */
+
-#define NBD_REP_ERR_POLICY      ((UINT32_C(1) << 31) | 2) /* Server denied */
+#define NBD_REP_ERR_UNSUP       NBD_REP_ERR(1)  /* Unknown option */
-#define NBD_REP_ERR_INVALID     ((UINT32_C(1) << 31) | 3) /* Invalid length. */
+#define NBD_REP_ERR_POLICY      NBD_REP_ERR(2)  /* Server denied */
-#define NBD_REP_ERR_TLS_REQD    ((UINT32_C(1) << 31) | 5) /* TLS required */
+#define NBD_REP_ERR_INVALID     NBD_REP_ERR(3)  /* Invalid length */
+#define NBD_REP_ERR_PLATFORM    NBD_REP_ERR(4)  /* Not compiled in */
+#define NBD_REP_ERR_TLS_REQD    NBD_REP_ERR(5)  /* TLS required */
+#define NBD_REP_ERR_SHUTDOWN    NBD_REP_ERR(7)  /* Server shutting down */
 
 /* Request flags, sent from client to server during transmission phase */
 #define NBD_CMD_FLAG_FUA        (1 << 0)

include/qemu/osdep.h

@@ -128,6 +128,9 @@ extern int daemon(int, int);
 #if !defined(EMEDIUMTYPE)
 #define EMEDIUMTYPE 4098
 #endif
+#if !defined(ESHUTDOWN)
+#define ESHUTDOWN 4099
+#endif
 #ifndef TIME_MAX
 #define TIME_MAX LONG_MAX
 #endif

nbd/client.c

@@ -40,6 +40,9 @@ static int nbd_errno_to_system_errno(int err)
     case NBD_ENOSPC:
         ret = ENOSPC;
         break;
+    case NBD_ESHUTDOWN:
+        ret = ESHUTDOWN;
+        break;
     default:
         TRACE("Squashing unexpected error %d to EINVAL", err);
         /* fallthrough */
@@ -239,11 +242,21 @@ static int nbd_handle_reply_err(QIOChannel *ioc, nbd_opt_reply *reply,
                    reply->option);
         break;
 
+    case NBD_REP_ERR_PLATFORM:
+        error_setg(errp, "Server lacks support for option %" PRIx32,
+                   reply->option);
+        break;
+
     case NBD_REP_ERR_TLS_REQD:
         error_setg(errp, "TLS negotiation required before option %" PRIx32,
                    reply->option);
         break;
 
+    case NBD_REP_ERR_SHUTDOWN:
+        error_setg(errp, "Server shutting down before option %" PRIx32,
+                   reply->option);
+        break;
+
     default:
         error_setg(errp, "Unknown error code when asking for option %" PRIx32,
                    reply->option);
@@ -785,6 +798,11 @@ ssize_t nbd_receive_reply(QIOChannel *ioc, NBDReply *reply)
 
     reply->error = nbd_errno_to_system_errno(reply->error);
 
+    if (reply->error == ESHUTDOWN) {
+        /* This works even on mingw which lacks a native ESHUTDOWN */
+        LOG("server shutting down");
+        return -EINVAL;
+    }
     TRACE("Got reply: { magic = 0x%" PRIx32 ", .error = % " PRId32
           ", handle = %" PRIu64" }",
           magic, reply->error, reply->handle);

nbd/nbd-internal.h

@@ -92,6 +92,7 @@
 #define NBD_ENOMEM     12
 #define NBD_EINVAL     22
 #define NBD_ENOSPC     28
+#define NBD_ESHUTDOWN  108
 
 static inline ssize_t read_sync(QIOChannel *ioc, void *buffer, size_t size)
 {

nbd/server.c

@@ -39,6 +39,8 @@ static int system_errno_to_nbd_errno(int err)
     case EFBIG:
     case ENOSPC:
         return NBD_ENOSPC;
+    case ESHUTDOWN:
+        return NBD_ESHUTDOWN;
     case EINVAL:
     default:
         return NBD_EINVAL;
@@ -527,6 +529,10 @@ static int nbd_negotiate_options(NBDClient *client)
                 if (ret < 0) {
                     return ret;
                 }
+                /* Let the client keep trying, unless they asked to quit */
+                if (clientflags == NBD_OPT_ABORT) {
+                    return -EINVAL;
+                }
                 break;
             }
         } else if (fixedNewstyle) {
@@ -539,6 +545,10 @@ static int nbd_negotiate_options(NBDClient *client)
                 break;
 
             case NBD_OPT_ABORT:
+                /* NBD spec says we must try to reply before
+                 * disconnecting, but that we must also tolerate
+                 * guests that don't wait for our reply. */
+                nbd_negotiate_send_rep(client->ioc, NBD_REP_ACK, clientflags);
                 return -EINVAL;
 
             case NBD_OPT_EXPORT_NAME: