nbd: Let client skip portions of server reply

The server has a nice helper function nbd_negotiate_drop_sync()
which lets it easily ignore fluff from the client (such as the
payload to an unknown option request).  We can't quite make it
common, since it depends on nbd_negotiate_read() which handles
coroutine magic, but we can copy the idea into the client where
we have places where we want to ignore data (such as the
description tacked on the end of NBD_REP_SERVER).
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <1476469998-28592-11-git-send-email-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

nbd/client.c

@@ -75,6 +75,32 @@ static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
 
 */
 
+/* Discard length bytes from channel.  Return -errno on failure, or
+ * the amount of bytes consumed. */
+static ssize_t drop_sync(QIOChannel *ioc, size_t size)
+{
+    ssize_t ret, dropped = size;
+    char small[1024];
+    char *buffer;
+
+    buffer = sizeof(small) < size ? small : g_malloc(MIN(65536, size));
+    while (size > 0) {
+        ret = read_sync(ioc, buffer, MIN(65536, size));
+        if (ret < 0) {
+            goto cleanup;
+        }
+        assert(ret <= size);
+        size -= ret;
+    }
+    ret = dropped;
+
+ cleanup:
+    if (buffer != small) {
+        g_free(buffer);
+    }
+    return ret;
+}
+
 /* Send an option request.
  *
  * The request is for option @opt, with @data containing @len bytes of
@@ -285,19 +311,12 @@ static int nbd_receive_list(QIOChannel *ioc, char **name, Error **errp)
         }
         (*name)[namelen] = '\0';
         len -= namelen;
-        if (len) {
-            char *buf = g_malloc(len + 1);
-            if (read_sync(ioc, buf, len) != len) {
-                error_setg(errp, "failed to read export description");
-                g_free(*name);
-                g_free(buf);
-                *name = NULL;
-                nbd_send_opt_abort(ioc);
-                return -1;
-            }
-            buf[len] = '\0';
-            TRACE("Ignoring export description: %s", buf);
-            g_free(buf);
+        if (drop_sync(ioc, len) != len) {
+            error_setg(errp, "failed to read export description");
+            g_free(*name);
+            *name = NULL;
+            nbd_send_opt_abort(ioc);
+            return -1;
         }
     } else {
         error_setg(errp, "Unexpected reply type %" PRIx32 " expected %x",
@@ -577,7 +596,7 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint16_t *flags,
     }
 
     TRACE("Size is %" PRIu64 ", export flags %" PRIx16, *size, *flags);
-    if (read_sync(ioc, &buf, 124) != 124) {
+    if (drop_sync(ioc, 124) != 124) {
         error_setg(errp, "Failed to read reserved block");
         goto fail;
     }