[Qemu-devel] [PATCH v6 13/15] nbd: Improve server handling of shutdown requests

[Eric Blake <eblake@redhat.com>]

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>

---
v6: rebase
v5: no change
v4: new patch
---
 include/block/nbd.h | 13 +++++++++----
 nbd/nbd-internal.h  |  1 +
 nbd/client.c        | 16 ++++++++++++++++
 nbd/server.c        | 10 ++++++++++
 4 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/include/block/nbd.h b/include/block/nbd.h
index d326308..eea7ef0 100644
--- a/include/block/nbd.h
+++ b/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_INVALID     ((UINT32_C(1) << 31) | 3) /* Invalid length. */
-#define NBD_REP_ERR_TLS_REQD    ((UINT32_C(1) << 31) | 5) /* TLS required */
+
+#define NBD_REP_ERR_UNSUP       NBD_REP_ERR(1)  /* Unknown option */
+#define NBD_REP_ERR_POLICY      NBD_REP_ERR(2)  /* Server denied */
+#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)
diff --git a/nbd/nbd-internal.h b/nbd/nbd-internal.h
index dd57e18..eee20ab 100644
--- a/nbd/nbd-internal.h
+++ b/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)
 {
diff --git a/nbd/client.c b/nbd/client.c
index f5e4c74..b33ae46 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -34,6 +34,8 @@ static int nbd_errno_to_system_errno(int err)
         return ENOMEM;
     case NBD_ENOSPC:
         return ENOSPC;
+    case NBD_ESHUTDOWN:
+        return ESHUTDOWN;
     default:
         TRACE("Squashing unexpected error %d to EINVAL", err);
         /* fallthrough */
@@ -231,11 +233,21 @@ 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);
@@ -784,6 +796,10 @@ ssize_t nbd_receive_reply(QIOChannel *ioc, NBDReply *reply)
         LOG("invalid magic (got 0x%" PRIx32 ")", magic);
         return -EINVAL;
     }
+    if (reply->error == ESHUTDOWN) {
+        LOG("server shutting down");
+        return -EINVAL;
+    }
     return 0;
 }

diff --git a/nbd/server.c b/nbd/server.c
index 20f1086..a7aa2ba 100644
--- a/nbd/server.c
+++ b/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;
@@ -526,6 +528,10 @@ 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) {
@@ -538,6 +544,10 @@ 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:
-- 
2.7.4