[PATCH v4 1/7] sheepdog: Add trivial backing_fmt support

[Eric Blake <eblake@redhat.com>]

Sheepdog already requires that if backing_file is present, that it be
another sheepdog image (see sd_co_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
sheepdog 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/sheepdog.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/block/sheepdog.c b/block/sheepdog.c
index cfa84338a2d6..376f4ef74638 100644
--- a/block/sheepdog.c
+++ b/block/sheepdog.c
@@ -2166,8 +2166,16 @@ static int coroutine_fn sd_co_create_opts(const char *filename, QemuOpts *opts,
     char *redundancy;
     Error *local_err = NULL;
     int ret;
+    char *backing_fmt = NULL;

     redundancy = qemu_opt_get_del(opts, BLOCK_OPT_REDUNDANCY);
+    backing_fmt = qemu_opt_get_del(opts, BLOCK_OPT_BACKING_FMT);
+
+    if (backing_fmt && strcmp(backing_fmt, "sheepdog") != 0) {
+        error_setg(errp, "backing_file must be a sheepdog image");
+        ret = -EINVAL;
+        goto fail;
+    }

     qdict = qemu_opts_to_qdict(opts, NULL);
     qdict_put_str(qdict, "driver", "sheepdog");
@@ -2232,6 +2240,7 @@ fail:
     qapi_free_BlockdevCreateOptions(create_options);
     qobject_unref(qdict);
     g_free(redundancy);
+    g_free(backing_fmt);
     return ret;
 }

@@ -3189,6 +3198,11 @@ static QemuOptsList sd_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 'sheepdog' if present",
+        },
         {
             .name = BLOCK_OPT_PREALLOC,
             .type = QEMU_OPT_STRING,
-- 
2.25.1