[Qemu-devel] [PATCH 0/9] QMP/HMP: add error reason to open failures

[Qemu-devel] [PATCH 0/9] QMP/HMP: add error reason to open failures

[Luiz Capitulino]

Today I was surprised to find out that we still have old users of QERR_OPEN_FILE_FAILED that print errors like:

(qemu) dump-guest-memory -p /lkmads/foo
Could not open '/lkmads/foo'
(qemu)

This series converts all those users to a new helper called error_setg_file_open(), which adds error reason to open failures:

(qemu) dump-guest-memory -p /sfmdkjf/foo
Could not open '/sfmdkjf/foo': No such file or directory
(qemu) 

Luiz Capitulino (9):
  error: add error_setg_file_open() helper
  rng-random: use error_setg_file_open()
  block: bdrv_reopen_prepare(): use error_setg_file_open()
  block: mirror_complete(): use error_setg_file_open()
  blockdev: use error_setg_file_open()
  cpus: use error_setg_file_open()
  dump: qmp_dump_guest_memory(): use error_setg_file_open()
  savevm: qmp_xen_save_devices_state(): use error_setg_file_open()
  qerror: drop QERR_OPEN_FILE_FAILED macro

 backends/rng-random.c     | 3 +--
 block.c                   | 3 +--
 block/mirror.c            | 2 +-
 blockdev.c                | 6 +++---
 cpus.c                    | 4 ++--
 dump.c                    | 2 +-
 include/qapi/error.h      | 5 +++++
 include/qapi/qmp/qerror.h | 3 ---
 savevm.c                  | 2 +-
 util/error.c              | 5 +++++
 10 files changed, 20 insertions(+), 15 deletions(-)

-- 
1.8.1.4

[Qemu-devel] [PATCH 1/9] error: add error_setg_file_open() helper

[Luiz Capitulino]

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 include/qapi/error.h | 5 +++++
 util/error.c         | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/include/qapi/error.h b/include/qapi/error.h
index 5cd2f0c..ffd1cea 100644
--- a/include/qapi/error.h
+++ b/include/qapi/error.h
@@ -45,6 +45,11 @@ void error_set_errno(Error **err, int os_error, ErrorClass err_class, const char
     error_set_errno(err, os_error, ERROR_CLASS_GENERIC_ERROR, fmt, ## __VA_ARGS__)
 
 /**
+ * Helper for open() errors
+ */
+void error_setg_file_open(Error **errp, int os_errno, const char *filename);
+
+/**
  * Returns true if an indirect pointer to an error is pointing to a valid
  * error object.
  */
diff --git a/util/error.c b/util/error.c
index 519f6b6..53b0435 100644
--- a/util/error.c
+++ b/util/error.c
@@ -71,6 +71,11 @@ void error_set_errno(Error **errp, int os_errno, ErrorClass err_class,
     *errp = err;
 }
 
+void error_setg_file_open(Error **errp, int os_errno, const char *filename)
+{
+    error_setg_errno(errp, os_errno, "Could not open '%s'", filename);
+}
+
 Error *error_copy(const Error *err)
 {
     Error *err_new;
-- 
1.8.1.4

[Qemu-devel] [PATCH 2/9] rng-random: use error_setg_file_open()

[Luiz Capitulino]

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 backends/rng-random.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/backends/rng-random.c b/backends/rng-random.c
index 830360c..68dfc8a 100644
--- a/backends/rng-random.c
+++ b/backends/rng-random.c
@@ -78,9 +78,8 @@ static void rng_random_opened(RngBackend *b, Error **errp)
                   "filename", "a valid filename");
     } else {
         s->fd = qemu_open(s->filename, O_RDONLY | O_NONBLOCK);
-
         if (s->fd == -1) {
-            error_set(errp, QERR_OPEN_FILE_FAILED, s->filename);
+            error_setg_file_open(errp, errno, s->filename);
         }
     }
 }
-- 
1.8.1.4

[Qemu-devel] [PATCH 3/9] block: bdrv_reopen_prepare(): use error_setg_file_open()

[Luiz Capitulino]

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 block.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/block.c b/block.c
index 79ad33d..c78f152 100644
--- a/block.c
+++ b/block.c
@@ -1291,8 +1291,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
             if (local_err != NULL) {
                 error_propagate(errp, local_err);
             } else {
-                error_set(errp, QERR_OPEN_FILE_FAILED,
-                          reopen_state->bs->filename);
+                error_setg_file_open(errp, errno, reopen_state->bs->filename);
             }
             goto error;
         }
-- 
1.8.1.4

[Qemu-devel] [PATCH 4/9] block: mirror_complete(): use error_setg_file_open()

[Luiz Capitulino]

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 block/mirror.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/mirror.c b/block/mirror.c
index 8b07dec..89d531d 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -512,7 +512,7 @@ static void mirror_complete(BlockJob *job, Error **errp)
         char backing_filename[PATH_MAX];
         bdrv_get_full_backing_filename(s->target, backing_filename,
                                        sizeof(backing_filename));
-        error_set(errp, QERR_OPEN_FILE_FAILED, backing_filename);
+        error_setg_file_open(errp, errno, backing_filename);
         return;
     }
     if (!s->synced) {
-- 
1.8.1.4

[Qemu-devel] [PATCH 5/9] blockdev: use error_setg_file_open()

[Luiz Capitulino]

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 blockdev.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/blockdev.c b/blockdev.c
index 9937311..c09adba 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -899,7 +899,7 @@ static void external_snapshot_prepare(BlkTransactionStates *common,
     ret = bdrv_open(states->new_bs, new_image_file, NULL,
                     flags | BDRV_O_NO_BACKING, drv);
     if (ret != 0) {
-        error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
+        error_setg_file_open(errp, errno, new_image_file);
     }
 }
 
@@ -1063,7 +1063,7 @@ static void qmp_bdrv_open_encrypted(BlockDriverState *bs, const char *filename,
                                     const char *password, Error **errp)
 {
     if (bdrv_open(bs, filename, NULL, bdrv_flags, drv) < 0) {
-        error_set(errp, QERR_OPEN_FILE_FAILED, filename);
+        error_setg_file_open(errp, errno, filename);
         return;
     }
 
@@ -1483,7 +1483,7 @@ void qmp_drive_mirror(const char *device, const char *target,
 
     if (ret < 0) {
         bdrv_delete(target_bs);
-        error_set(errp, QERR_OPEN_FILE_FAILED, target);
+        error_setg_file_open(errp, errno, target);
         return;
     }
 
-- 
1.8.1.4

[Qemu-devel] [PATCH 6/9] cpus: use error_setg_file_open()

[Luiz Capitulino]

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 cpus.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/cpus.c b/cpus.c
index c232265..c8bc8ad 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1278,7 +1278,7 @@ void qmp_memsave(int64_t addr, int64_t size, const char *filename,
 
     f = fopen(filename, "wb");
     if (!f) {
-        error_set(errp, QERR_OPEN_FILE_FAILED, filename);
+        error_setg_file_open(errp, errno, filename);
         return;
     }
 
@@ -1308,7 +1308,7 @@ void qmp_pmemsave(int64_t addr, int64_t size, const char *filename,
 
     f = fopen(filename, "wb");
     if (!f) {
-        error_set(errp, QERR_OPEN_FILE_FAILED, filename);
+        error_setg_file_open(errp, errno, filename);
         return;
     }
 
-- 
1.8.1.4

[Qemu-devel] [PATCH 7/9] dump: qmp_dump_guest_memory(): use error_setg_file_open()

[Luiz Capitulino]

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 dump.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/dump.c b/dump.c
index c0d3da5..28fd296 100644
--- a/dump.c
+++ b/dump.c
@@ -847,7 +847,7 @@ void qmp_dump_guest_memory(bool paging, const char *file, bool has_begin,
     if  (strstart(file, "file:", &p)) {
         fd = qemu_open(p, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, S_IRUSR);
         if (fd < 0) {
-            error_set(errp, QERR_OPEN_FILE_FAILED, p);
+            error_setg_file_open(errp, errno, p);
             return;
         }
     }
-- 
1.8.1.4

[Qemu-devel] [PATCH 8/9] savevm: qmp_xen_save_devices_state(): use error_setg_file_open()

[Luiz Capitulino]

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 savevm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/savevm.c b/savevm.c
index 2ce439f..ff5ece6 100644
--- a/savevm.c
+++ b/savevm.c
@@ -2410,7 +2410,7 @@ void qmp_xen_save_devices_state(const char *filename, Error **errp)
 
     f = qemu_fopen(filename, "wb");
     if (!f) {
-        error_set(errp, QERR_OPEN_FILE_FAILED, filename);
+        error_setg_file_open(errp, errno, filename);
         goto the_end;
     }
     ret = qemu_save_device_state(f);
-- 
1.8.1.4

[Qemu-devel] [PATCH 9/9] qerror: drop QERR_OPEN_FILE_FAILED macro

[Luiz Capitulino]

Not used since the last commit.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 include/qapi/qmp/qerror.h | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/include/qapi/qmp/qerror.h b/include/qapi/qmp/qerror.h
index 6c0a18d..c30c2f6 100644
--- a/include/qapi/qmp/qerror.h
+++ b/include/qapi/qmp/qerror.h
@@ -177,9 +177,6 @@ void assert_no_error(Error *err);
 #define QERR_NOT_SUPPORTED \
     ERROR_CLASS_GENERIC_ERROR, "Not supported"
 
-#define QERR_OPEN_FILE_FAILED \
-    ERROR_CLASS_GENERIC_ERROR, "Could not open '%s'"
-
 #define QERR_PERMISSION_DENIED \
     ERROR_CLASS_GENERIC_ERROR, "Insufficient permission to perform this operation"
 
-- 
1.8.1.4

Re: [Qemu-devel] [PATCH 3/9] block: bdrv_reopen_prepare(): use error_setg_file_open()

[Stefan Hajnoczi]

On Fri, Jun 07, 2013 at 03:52:29PM -0400, Luiz Capitulino wrote:
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> ---
>  block.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 79ad33d..c78f152 100644
> --- a/block.c
> +++ b/block.c
> @@ -1291,8 +1291,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
>              if (local_err != NULL) {
>                  error_propagate(errp, local_err);
>              } else {
> -                error_set(errp, QERR_OPEN_FILE_FAILED,
> -                          reopen_state->bs->filename);
> +                error_setg_file_open(errp, errno, reopen_state->bs->filename);

s/errno/-ret/

Re: [Qemu-devel] [PATCH 3/9] block: bdrv_reopen_prepare(): use error_setg_file_open()

[Stefan Hajnoczi]

On Fri, Jun 07, 2013 at 03:52:29PM -0400, Luiz Capitulino wrote:
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> ---
>  block.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 79ad33d..c78f152 100644
> --- a/block.c
> +++ b/block.c
> @@ -1291,8 +1291,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
>              if (local_err != NULL) {
>                  error_propagate(errp, local_err);
>              } else {
> -                error_set(errp, QERR_OPEN_FILE_FAILED,
> -                          reopen_state->bs->filename);
> +                error_setg_file_open(errp, errno, reopen_state->bs->filename);

Looking closer, my suggestion was wrong too.

I think QERR_OPEN_FILE_FAILED is simply the wrong error here.  We don't
know that the error occurred when trying to open a file.

errno does not necessarily contain the error value!

Stefan

Re: [Qemu-devel] [PATCH 4/9] block: mirror_complete(): use error_setg_file_open()

[Stefan Hajnoczi]

On Fri, Jun 07, 2013 at 03:52:30PM -0400, Luiz Capitulino wrote:
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> ---
>  block/mirror.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 8b07dec..89d531d 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -512,7 +512,7 @@ static void mirror_complete(BlockJob *job, Error **errp)
>          char backing_filename[PATH_MAX];
>          bdrv_get_full_backing_filename(s->target, backing_filename,
>                                         sizeof(backing_filename));
> -        error_set(errp, QERR_OPEN_FILE_FAILED, backing_filename);
> +        error_setg_file_open(errp, errno, backing_filename);

s/errno/-ret/

Re: [Qemu-devel] [PATCH 5/9] blockdev: use error_setg_file_open()

[Stefan Hajnoczi]

On Fri, Jun 07, 2013 at 03:52:31PM -0400, Luiz Capitulino wrote:
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> ---
>  blockdev.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)

s/errno/-ret/g

Re: [Qemu-devel] [PATCH 3/9] block: bdrv_reopen_prepare(): use error_setg_file_open()

[Luiz Capitulino]

On Mon, 10 Jun 2013 10:43:47 +0200
Stefan Hajnoczi <stefanha@gmail.com> wrote:

> On Fri, Jun 07, 2013 at 03:52:29PM -0400, Luiz Capitulino wrote:
> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > ---
> >  block.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/block.c b/block.c
> > index 79ad33d..c78f152 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -1291,8 +1291,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
> >              if (local_err != NULL) {
> >                  error_propagate(errp, local_err);
> >              } else {
> > -                error_set(errp, QERR_OPEN_FILE_FAILED,
> > -                          reopen_state->bs->filename);
> > +                error_setg_file_open(errp, errno, reopen_state->bs->filename);
> 
> Looking closer, my suggestion was wrong too.
> 
> I think QERR_OPEN_FILE_FAILED is simply the wrong error here.  We don't
> know that the error occurred when trying to open a file.

Right.

> errno does not necessarily contain the error value!

There are two ways to fix it (and they're not mutually exclusive):

 1. We could review all bdrv_reopen_prepare() methods and make sure they
    set errno on failure

 2. We set errno=0 before calling the bdrv_reopen_prepare() method and if
    there's an error and if errno != 0 we use it, otherwise we set a generic
    "failed to prepare to reopen image" error

Option 1 goes a bit beyond the time I'd like to spent on this series.
Option 2 is quite reasonable.

What do you think?

Re: [Qemu-devel] [PATCH 5/9] blockdev: use error_setg_file_open()

[Luiz Capitulino]

On Mon, 10 Jun 2013 10:45:23 +0200
Stefan Hajnoczi <stefanha@gmail.com> wrote:

> On Fri, Jun 07, 2013 at 03:52:31PM -0400, Luiz Capitulino wrote:
> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > ---
> >  blockdev.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> s/errno/-ret/g

I'll fix this and double check the other patches.

Thanks for reviewing Stefan!

Re: [Qemu-devel] [PATCH 3/9] block: bdrv_reopen_prepare(): use error_setg_file_open()

[Kevin Wolf]

Am 10.06.2013 um 15:21 hat Luiz Capitulino geschrieben:
> On Mon, 10 Jun 2013 10:43:47 +0200
> Stefan Hajnoczi <stefanha@gmail.com> wrote:
> 
> > On Fri, Jun 07, 2013 at 03:52:29PM -0400, Luiz Capitulino wrote:
> > > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > > ---
> > >  block.c | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > 
> > > diff --git a/block.c b/block.c
> > > index 79ad33d..c78f152 100644
> > > --- a/block.c
> > > +++ b/block.c
> > > @@ -1291,8 +1291,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
> > >              if (local_err != NULL) {
> > >                  error_propagate(errp, local_err);
> > >              } else {
> > > -                error_set(errp, QERR_OPEN_FILE_FAILED,
> > > -                          reopen_state->bs->filename);
> > > +                error_setg_file_open(errp, errno, reopen_state->bs->filename);
> > 
> > Looking closer, my suggestion was wrong too.
> > 
> > I think QERR_OPEN_FILE_FAILED is simply the wrong error here.  We don't
> > know that the error occurred when trying to open a file.
> 
> Right.
> 
> > errno does not necessarily contain the error value!
> 
> There are two ways to fix it (and they're not mutually exclusive):
> 
>  1. We could review all bdrv_reopen_prepare() methods and make sure they
>     set errno on failure
> 
>  2. We set errno=0 before calling the bdrv_reopen_prepare() method and if
>     there's an error and if errno != 0 we use it, otherwise we set a generic
>     "failed to prepare to reopen image" error
> 
> Option 1 goes a bit beyond the time I'd like to spent on this series.
> Option 2 is quite reasonable.
> 
> What do you think?

errno is definitely not reliable here and won't be. You must use -ret if
you want a meaningful error message.

I think Stefan's point was more that "Could not open" might not be the
right message for a reopen, so we'd have to use error_setg_errno()
directly with a different message here, like "Could not prepare reopen
for '%s'".

The _real_ solution, of course, is to make bdrv_prepare_reopen() a void
function and always set errp when something goes wrong.

Kevin

Re: [Qemu-devel] [PATCH 3/9] block: bdrv_reopen_prepare(): use error_setg_file_open()

[Luiz Capitulino]

On Mon, 10 Jun 2013 15:54:10 +0200
Kevin Wolf <kwolf@redhat.com> wrote:

> Am 10.06.2013 um 15:21 hat Luiz Capitulino geschrieben:
> > On Mon, 10 Jun 2013 10:43:47 +0200
> > Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > 
> > > On Fri, Jun 07, 2013 at 03:52:29PM -0400, Luiz Capitulino wrote:
> > > > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > > > ---
> > > >  block.c | 3 +--
> > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > > 
> > > > diff --git a/block.c b/block.c
> > > > index 79ad33d..c78f152 100644
> > > > --- a/block.c
> > > > +++ b/block.c
> > > > @@ -1291,8 +1291,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
> > > >              if (local_err != NULL) {
> > > >                  error_propagate(errp, local_err);
> > > >              } else {
> > > > -                error_set(errp, QERR_OPEN_FILE_FAILED,
> > > > -                          reopen_state->bs->filename);
> > > > +                error_setg_file_open(errp, errno, reopen_state->bs->filename);
> > > 
> > > Looking closer, my suggestion was wrong too.
> > > 
> > > I think QERR_OPEN_FILE_FAILED is simply the wrong error here.  We don't
> > > know that the error occurred when trying to open a file.
> > 
> > Right.
> > 
> > > errno does not necessarily contain the error value!
> > 
> > There are two ways to fix it (and they're not mutually exclusive):
> > 
> >  1. We could review all bdrv_reopen_prepare() methods and make sure they
> >     set errno on failure
> > 
> >  2. We set errno=0 before calling the bdrv_reopen_prepare() method and if
> >     there's an error and if errno != 0 we use it, otherwise we set a generic
> >     "failed to prepare to reopen image" error
> > 
> > Option 1 goes a bit beyond the time I'd like to spent on this series.
> > Option 2 is quite reasonable.
> > 
> > What do you think?
> 
> errno is definitely not reliable here and won't be. You must use -ret if
> you want a meaningful error message.
> 
> I think Stefan's point was more that "Could not open" might not be the
> right message for a reopen, so we'd have to use error_setg_errno()
> directly with a different message here, like "Could not prepare reopen
> for '%s'".

Ok, but Stefan also said that -ret is not reliable. And after quickly
checking the code I see he's right, as there are methods that return -1.

I think the safest thing to do is to have a generic error message for
for this now and in the future we should propagate errp or return -errno.

Re: [Qemu-devel] [PATCH 3/9] block: bdrv_reopen_prepare(): use error_setg_file_open()

[Kevin Wolf]

Am 10.06.2013 um 16:02 hat Luiz Capitulino geschrieben:
> On Mon, 10 Jun 2013 15:54:10 +0200
> Kevin Wolf <kwolf@redhat.com> wrote:
> 
> > Am 10.06.2013 um 15:21 hat Luiz Capitulino geschrieben:
> > > On Mon, 10 Jun 2013 10:43:47 +0200
> > > Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > > 
> > > > On Fri, Jun 07, 2013 at 03:52:29PM -0400, Luiz Capitulino wrote:
> > > > > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > > > > ---
> > > > >  block.c | 3 +--
> > > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/block.c b/block.c
> > > > > index 79ad33d..c78f152 100644
> > > > > --- a/block.c
> > > > > +++ b/block.c
> > > > > @@ -1291,8 +1291,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
> > > > >              if (local_err != NULL) {
> > > > >                  error_propagate(errp, local_err);
> > > > >              } else {
> > > > > -                error_set(errp, QERR_OPEN_FILE_FAILED,
> > > > > -                          reopen_state->bs->filename);
> > > > > +                error_setg_file_open(errp, errno, reopen_state->bs->filename);
> > > > 
> > > > Looking closer, my suggestion was wrong too.
> > > > 
> > > > I think QERR_OPEN_FILE_FAILED is simply the wrong error here.  We don't
> > > > know that the error occurred when trying to open a file.
> > > 
> > > Right.
> > > 
> > > > errno does not necessarily contain the error value!
> > > 
> > > There are two ways to fix it (and they're not mutually exclusive):
> > > 
> > >  1. We could review all bdrv_reopen_prepare() methods and make sure they
> > >     set errno on failure
> > > 
> > >  2. We set errno=0 before calling the bdrv_reopen_prepare() method and if
> > >     there's an error and if errno != 0 we use it, otherwise we set a generic
> > >     "failed to prepare to reopen image" error
> > > 
> > > Option 1 goes a bit beyond the time I'd like to spent on this series.
> > > Option 2 is quite reasonable.
> > > 
> > > What do you think?
> > 
> > errno is definitely not reliable here and won't be. You must use -ret if
> > you want a meaningful error message.
> > 
> > I think Stefan's point was more that "Could not open" might not be the
> > right message for a reopen, so we'd have to use error_setg_errno()
> > directly with a different message here, like "Could not prepare reopen
> > for '%s'".
> 
> Ok, but Stefan also said that -ret is not reliable. And after quickly
> checking the code I see he's right, as there are methods that return -1.
> 
> I think the safest thing to do is to have a generic error message for
> for this now and in the future we should propagate errp or return -errno.

Ah, yes. I agree, let's make it error_setg() without errno for now.

Kevin