From 791a16b049ea1ce2c450acd367fce774d9aab5b1 Mon Sep 17 00:00:00 2001
From: "Richard W.M. Jones" <rjones@redhat.com>
Date: Tue, 31 Aug 2021 08:27:15 +0100
Subject: [PATCH] lib: Autodetect backing format for qemu-img create -b

qemu 6.1 has decided to change qemu-img create so that a backing
format (-F) is required if a backing file (-b) is specified.  Since we
don't want to change the libguestfs API to force callers to specify
this because that would be an API break, autodetect it.

This is similar to commit c8c181e8d9 ("launch: libvirt: Autodetect
backing format for readonly drive overlays").

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1998820
(cherry picked from commit 45de287447bb18d59749fbfc1ec5072413090109)
---
 lib/create.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/lib/create.c b/lib/create.c
index 44a7df25f..75a4d3a28 100644
--- a/lib/create.c
+++ b/lib/create.c
@@ -255,6 +255,7 @@ disk_create_qcow2 (guestfs_h *g, const char *filename, int64_t size,
                    const struct guestfs_disk_create_argv *optargs)
 {
   const char *backingformat = NULL;
+  CLEANUP_FREE char *backingformat_free = NULL;
   const char *preallocation = NULL;
   const char *compat = NULL;
   int clustersize = -1;
@@ -270,6 +271,14 @@ disk_create_qcow2 (guestfs_h *g, const char *filename, int64_t size,
       return -1;
     }
   }
+  else if (backingfile) {
+    /* Since qemu 6.1, qemu-img create has requires a backing format (-F)
+     * parameter if backing file (-b) is used (RHBZ#1998820).
+     */
+    backingformat = backingformat_free = guestfs_disk_format (g, backingfile);
+    if (!backingformat)
+      return -1;
+  }
   if (optargs->bitmask & GUESTFS_DISK_CREATE_PREALLOCATION_BITMASK) {
     if (STREQ (optargs->preallocation, "off") ||
         STREQ (optargs->preallocation, "sparse"))
-- 
2.31.1