[PATCH v4 2/7] vmdk: Add trivial backing_fmt support

[Eric Blake <eblake@redhat.com>]

vmdk already requires that if backing_file is present, that it be
another vmdk image (see vmdk_co_do_create).  Meanwhile, we want to
move towards always being explicit about the backing format for other
drivers where it matters.  So for convenience, make qemu-img create -F
vmdk work, while rejecting all other explicit formats (note that this
is only for QemuOpts usage; there is no change to the QAPI to allow a
format through -blockdev).

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/vmdk.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/block/vmdk.c b/block/vmdk.c
index 20e909d99794..3c7893817afe 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -2611,6 +2611,14 @@ static int coroutine_fn vmdk_co_create_opts(const char *filename, QemuOpts *opts
     bool zeroed_grain;
     bool compat6;
     VMDKCreateOptsData data;
+    char *backing_fmt = NULL;
+
+    backing_fmt = qemu_opt_get_del(opts, BLOCK_OPT_BACKING_FMT);
+    if (backing_fmt && strcmp(backing_fmt, "vmdk") != 0) {
+        error_setg(errp, "backing_file must be a vmdk image");
+        ret = -EINVAL;
+        goto exit;
+    }

     if (filename_decompose(filename, path, prefix, postfix, PATH_MAX, errp)) {
         ret = -EINVAL;
@@ -2669,6 +2677,7 @@ static int coroutine_fn vmdk_co_create_opts(const char *filename, QemuOpts *opts
                             vmdk_co_create_opts_cb, &data, errp);

 exit:
+    g_free(backing_fmt);
     g_free(adapter_type);
     g_free(backing_file);
     g_free(hw_version);
@@ -3005,6 +3014,11 @@ static QemuOptsList vmdk_create_opts = {
             .type = QEMU_OPT_STRING,
             .help = "File name of a base image"
         },
+        {
+            .name = BLOCK_OPT_BACKING_FMT,
+            .type = QEMU_OPT_STRING,
+            .help = "Must be 'vmdk' if present",
+        },
         {
             .name = BLOCK_OPT_COMPAT6,
             .type = QEMU_OPT_BOOL,
-- 
2.25.1