[PATCH 3/2] qemu-img: Improve error for rebase without backing format

[Eric Blake <eblake@redhat.com>]

When removeing support for qemu-img being able to create backing
chains without embedded backing formats, we caused a poor error
message as caught by iotest 114.  Improve the situation to inform the
user what went wrong.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 qemu-img.c                 | 3 +++
 tests/qemu-iotests/114.out | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/qemu-img.c b/qemu-img.c
index 7c0e73882dd4..b017734c255a 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3786,6 +3786,9 @@ static int img_rebase(int argc, char **argv)
     if (ret == -ENOSPC) {
         error_report("Could not change the backing file to '%s': No "
                      "space left in the file header", out_baseimg);
+    } else if (ret == -EINVAL && out_baseimg && !out_basefmt) {
+        error_report("Could not change the backing file to '%s': backing "
+                     "format must be specified", out_baseimg);
     } else if (ret < 0) {
         error_report("Could not change the backing file to '%s': %s",
             out_baseimg, strerror(-ret));
diff --git a/tests/qemu-iotests/114.out b/tests/qemu-iotests/114.out
index 172454401257..016e9ce3ecfb 100644
--- a/tests/qemu-iotests/114.out
+++ b/tests/qemu-iotests/114.out
@@ -14,7 +14,7 @@ qemu-io: can't open device TEST_DIR/t.qcow2: Could not open backing file: Unknow
 no file open, try 'help open'
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-qemu-img: Could not change the backing file to '/home/eblake/qemu/build/tests/qemu-iotests/scratch/t.qcow2.base': Invalid argument
+qemu-img: Could not change the backing file to '/home/eblake/qemu/build/tests/qemu-iotests/scratch/t.qcow2.base': backing format must be specified
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 *** done
-- 
2.31.1