[Libguestfs] [PATCH libnbd 2/3] states: Prevent multi-conn connection unless the server advertizes it.

[Richard W.M. Jones]

It is unsafe to connect using multi-conn to a server unless the server
advertizes it, so fail if the caller tries to do this.
---
 generator/generator | 16 ++++++++++++++--
 lib/flags.c         | 10 ++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/generator/generator b/generator/generator
index a372074..a8e6fb6 100755
--- a/generator/generator
+++ b/generator/generator
@@ -897,7 +897,8 @@ The C<multi_conn> parameter controls whether this feature is
 enabled (if E<gt> 1) or disabled (if C<1>).  The parameter
 passed must not be C<0>.  Usually small powers of 2 (eg. 2, 4, 8)
 will provide increments in performance.  Some servers do not
-support this feature and will return an error on connection.
+support this feature and libnbd will fail on connection to
+these servers if this setting is E<gt> 1.
 
 At present, libnbd assumes that all connections will report the
 same capabilities; a server that behaves otherwise may trigger
@@ -1171,7 +1172,18 @@ connected to and completed the handshake with the server.";
 Returns true if the server supports multi-conn
 (see C<nbd_set_multi_conn>).  Returns false if
 the server does not.  Can return an error if we have not
-connected to and completed the handshake with the server.";
+connected to and completed the handshake with the server.
+
+Note that you I<cannot> set multi-conn on the handle
+to E<gt> 1, connect, and then check this setting, because
+if the server does not support multi-conn the connection
+will fail.  Instead you should connect with multi-conn
+set to C<1> (the default), check this setting, then if
+multi-conn is found to be supported you can call
+C<nbd_set_multi_conn> with a larger value to increase
+the number of connection objects, then call one of the
+C<nbd_connect*> functions again on the handle to connect
+the remaining connections.";
   };
 
   "can_cache", {
diff --git a/lib/flags.c b/lib/flags.c
index 825b326..1956db6 100644
--- a/lib/flags.c
+++ b/lib/flags.c
@@ -42,6 +42,16 @@ nbd_internal_set_size_and_flags (struct nbd_handle *h,
     return -1;
   }
 
+  /* It is unsafe to connect to a server with multi-conn set unless
+   * the server says it is safe to do so.
+   */
+  if (h->multi_conn > 1 && (eflags & NBD_FLAG_CAN_MULTI_CONN) == 0) {
+    set_error (EINVAL, "handshake: multi-conn is set on this handle, "
+               "but the server does not advertize multi-conn support "
+               "so disconnecting because it is not safe to continue");
+    return -1;
+  }
+
   h->exportsize = exportsize;
   h->eflags = eflags;
   return 0;
-- 
2.21.0