block/curl: Check protocol prefix

If the user has explicitly specified a block driver and thus a protocol,
we have to make sure the URL's protocol prefix matches. Otherwise the
latter will silently override the former which might catch some users by
surprise.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 20170331120431.1767-3-mreitz@redhat.com
Signed-off-by: NJeff Cody <jcody@redhat.com>

block/curl.c

@@ -659,6 +659,7 @@ static int curl_open(BlockDriverState *bs, QDict *options, int flags,
     const char *cookie;
     double d;
     const char *secretid;
+    const char *protocol_delimiter;
 
     static int inited = 0;
 
@@ -700,6 +701,15 @@ static int curl_open(BlockDriverState *bs, QDict *options, int flags,
         goto out_noclean;
     }
 
+    if (!strstart(file, bs->drv->protocol_name, &protocol_delimiter) ||
+        !strstart(protocol_delimiter, "://", NULL))
+    {
+        error_setg(errp, "%s curl driver cannot handle the URL '%s' (does not "
+                   "start with '%s://')", bs->drv->protocol_name, file,
+                   bs->drv->protocol_name);
+        goto out_noclean;
+    }
+
     s->username = g_strdup(qemu_opt_get(opts, CURL_BLOCK_OPT_USERNAME));
     secretid = qemu_opt_get(opts, CURL_BLOCK_OPT_PASSWORD_SECRET);