[Qemu-trivial] [PATCH 2/2] qemu-img: better error message when opening a backing file fails

[Maxim Levitsky <mlevitsk@redhat.com>]

Currently we print message like that:

"
new_file.qcow2 : error message
"

However the error could have come from opening the backing file (e.g when it missing encryption keys),
thus try to clarify this by using this format:

"
qemu-img: error creating new_file.qcow2: base_file.qcow2: error message
Could not open backing image to determine size.
"


Test used:

qemu-img create -f qcow2 \
        --object secret,id=sec0,data=hunter9 \
        --object secret,id=sec1,data=my_new_secret_password \
        -b 'json:{ "encrypt.key-secret": "sec1", "driver": "qcow2", "file": { "driver": "file", "filename": "base.qcow2" }}' \
        -o encrypt.format=luks,encrypt.key-secret=sec1 \
        sn.qcow2


Error message before:

qemu-img: sn.qcow2: Invalid password, cannot unlock any keyslot
Could not open backing image to determine size.


Error message after:

qemu-img: error creating sn.qcow2: \
	json:{ "encrypt.key-secret": "sec1", "driver": "qcow2", "file": { "driver": "file", "filename": "base.qcow2" }}: \
	Invalid password, cannot unlock any keyslot
Could not open backing image to determine size.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 block.c    | 1 +
 qemu-img.c | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 29e931e217..5eb47b2199 100644
--- a/block.c
+++ b/block.c
@@ -5790,6 +5790,7 @@ void bdrv_img_create(const char *filename, const char *fmt,
                             "This may become an error in future versions.\n");
             local_err = NULL;
         } else if (!bs) {
+            error_prepend(&local_err, "%s: ", backing_file);
             /* Couldn't open bs, do not have size */
             error_append_hint(&local_err,
                               "Could not open backing image to determine size.\n");
diff --git a/qemu-img.c b/qemu-img.c
index 79983772de..134bf2fbe0 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -545,7 +545,7 @@ static int img_create(int argc, char **argv)
     bdrv_img_create(filename, fmt, base_filename, base_fmt,
                     options, img_size, flags, quiet, &local_err);
     if (local_err) {
-        error_reportf_err(local_err, "%s: ", filename);
+        error_reportf_err(local_err, "error creating %s: ", filename);
         goto fail;
     }
 
-- 
2.17.2