[Qemu-devel] [PATCH] qemu-img: fix error messages emitted by img_open()

[Qemu-devel] [PATCH] qemu-img: fix error messages emitted by img_open()

[Reda Sallahi]

img_open_file() and img_open_opts() were printing error messages with a
duplicate part because of a wrong use of error_reportf_err() (e.g.
qemu-img: Could not open 'foo': Could not open 'foo': No such file or directory)

This change uses error_report_err() instead to eliminate the duplicate part.

Signed-off-by: Reda Sallahi <fullmanet@gmail.com>
---
 qemu-img.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index 2e40e1f..dc6652d 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -268,7 +268,7 @@ static BlockBackend *img_open_opts(const char *optstr,
     options = qemu_opts_to_qdict(opts, NULL);
     blk = blk_new_open(NULL, NULL, options, flags, &local_err);
     if (!blk) {
-        error_reportf_err(local_err, "Could not open '%s': ", optstr);
+        error_report_err(local_err);
         return NULL;
     }
     blk_set_enable_write_cache(blk, !writethrough);
@@ -295,7 +295,7 @@ static BlockBackend *img_open_file(const char *filename,
 
     blk = blk_new_open(filename, NULL, options, flags, &local_err);
     if (!blk) {
-        error_reportf_err(local_err, "Could not open '%s': ", filename);
+        error_report(local_err);
         return NULL;
     }
     blk_set_enable_write_cache(blk, !writethrough);
-- 
2.9.0

Re: [Qemu-devel] [PATCH] qemu-img: fix error messages emitted by img_open()

[Stefan Hajnoczi]

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On Thu, Jul 21, 2016 at 10:41:53AM +0200, Reda Sallahi wrote:
> img_open_file() and img_open_opts() were printing error messages with a
> duplicate part because of a wrong use of error_reportf_err() (e.g.
> qemu-img: Could not open 'foo': Could not open 'foo': No such file or directory)
> 
> This change uses error_report_err() instead to eliminate the duplicate part.
> 
> Signed-off-by: Reda Sallahi <fullmanet@gmail.com>
> ---
>  qemu-img.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/qemu-img.c b/qemu-img.c
> index 2e40e1f..dc6652d 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -268,7 +268,7 @@ static BlockBackend *img_open_opts(const char *optstr,
>      options = qemu_opts_to_qdict(opts, NULL);
>      blk = blk_new_open(NULL, NULL, options, flags, &local_err);
>      if (!blk) {
> -        error_reportf_err(local_err, "Could not open '%s': ", optstr);
> +        error_report_err(local_err);
>          return NULL;
>      }
>      blk_set_enable_write_cache(blk, !writethrough);
> @@ -295,7 +295,7 @@ static BlockBackend *img_open_file(const char *filename,
>  
>      blk = blk_new_open(filename, NULL, options, flags, &local_err);
>      if (!blk) {
> -        error_reportf_err(local_err, "Could not open '%s': ", filename);
> +        error_report(local_err);
>          return NULL;
>      }
>      blk_set_enable_write_cache(blk, !writethrough);

The duplication happens in the "Could not open 'foo'" case, but other
error cases do not include the filename in the error message.

We would lose information in those error cases since the filename is no
longer included by qemu-img.c in the error message.

I'm not aware of a clean way to distinguish Error objects.  Maybe
someone else can suggest one.  Otherwise it may be best to leave the
code as it is.

Stefan

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Re: [Qemu-devel] [PATCH] qemu-img: fix error messages emitted by img_open()

[Markus Armbruster]

Stefan Hajnoczi <stefanha@redhat.com> writes:

> On Thu, Jul 21, 2016 at 10:41:53AM +0200, Reda Sallahi wrote:
>> img_open_file() and img_open_opts() were printing error messages with a
>> duplicate part because of a wrong use of error_reportf_err() (e.g.
>> qemu-img: Could not open 'foo': Could not open 'foo': No such file or directory)
>> 
>> This change uses error_report_err() instead to eliminate the duplicate part.
>> 
>> Signed-off-by: Reda Sallahi <fullmanet@gmail.com>
>> ---
>>  qemu-img.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/qemu-img.c b/qemu-img.c
>> index 2e40e1f..dc6652d 100644
>> --- a/qemu-img.c
>> +++ b/qemu-img.c
>> @@ -268,7 +268,7 @@ static BlockBackend *img_open_opts(const char *optstr,
>>      options = qemu_opts_to_qdict(opts, NULL);
>>      blk = blk_new_open(NULL, NULL, options, flags, &local_err);
>>      if (!blk) {
>> -        error_reportf_err(local_err, "Could not open '%s': ", optstr);
>> +        error_report_err(local_err);
>>          return NULL;
>>      }
>>      blk_set_enable_write_cache(blk, !writethrough);
>> @@ -295,7 +295,7 @@ static BlockBackend *img_open_file(const char *filename,
>>  
>>      blk = blk_new_open(filename, NULL, options, flags, &local_err);
>>      if (!blk) {
>> -        error_reportf_err(local_err, "Could not open '%s': ", filename);
>> +        error_report(local_err);
>>          return NULL;
>>      }
>>      blk_set_enable_write_cache(blk, !writethrough);
>
> The duplication happens in the "Could not open 'foo'" case, but other
> error cases do not include the filename in the error message.
>
> We would lose information in those error cases since the filename is no
> longer included by qemu-img.c in the error message.

Could you give an example of such an information loss?

> I'm not aware of a clean way to distinguish Error objects.  Maybe
> someone else can suggest one.  Otherwise it may be best to leave the
> code as it is.

If you need to distinguish different kinds of errors to conditionally
rewrite the error message so it makes actual sense, chances are the
error messages that need the rewriting should be improved instead.

A more legitimate case is when a caller needs to handle different errors
differently.  Doesn't occur all that often.

There are two techniques for callers to distinguish different kinds of
Errors:

* ErrorClass, use error_get_class() to retrieve it.  This is actually a
  remnant of the failed "rich" error object idea.  Almost always
  ERROR_CLASS_GENERIC_ERROR, so this is unlikely to help.

* Error code separate from the Error object, e.g. the function returns
  -errno in addition to an Error object.

Re: [Qemu-devel] [PATCH] qemu-img: fix error messages emitted by img_open()

[Stefan Hajnoczi]

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On Mon, Jul 25, 2016 at 05:58:54PM +0200, Markus Armbruster wrote:
> Stefan Hajnoczi <stefanha@redhat.com> writes:
> 
> > On Thu, Jul 21, 2016 at 10:41:53AM +0200, Reda Sallahi wrote:
> >> img_open_file() and img_open_opts() were printing error messages with a
> >> duplicate part because of a wrong use of error_reportf_err() (e.g.
> >> qemu-img: Could not open 'foo': Could not open 'foo': No such file or directory)
> >> 
> >> This change uses error_report_err() instead to eliminate the duplicate part.
> >> 
> >> Signed-off-by: Reda Sallahi <fullmanet@gmail.com>
> >> ---
> >>  qemu-img.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/qemu-img.c b/qemu-img.c
> >> index 2e40e1f..dc6652d 100644
> >> --- a/qemu-img.c
> >> +++ b/qemu-img.c
> >> @@ -268,7 +268,7 @@ static BlockBackend *img_open_opts(const char *optstr,
> >>      options = qemu_opts_to_qdict(opts, NULL);
> >>      blk = blk_new_open(NULL, NULL, options, flags, &local_err);
> >>      if (!blk) {
> >> -        error_reportf_err(local_err, "Could not open '%s': ", optstr);
> >> +        error_report_err(local_err);
> >>          return NULL;
> >>      }
> >>      blk_set_enable_write_cache(blk, !writethrough);
> >> @@ -295,7 +295,7 @@ static BlockBackend *img_open_file(const char *filename,
> >>  
> >>      blk = blk_new_open(filename, NULL, options, flags, &local_err);
> >>      if (!blk) {
> >> -        error_reportf_err(local_err, "Could not open '%s': ", filename);
> >> +        error_report(local_err);
> >>          return NULL;
> >>      }
> >>      blk_set_enable_write_cache(blk, !writethrough);
> >
> > The duplication happens in the "Could not open 'foo'" case, but other
> > error cases do not include the filename in the error message.
> >
> > We would lose information in those error cases since the filename is no
> > longer included by qemu-img.c in the error message.
> 
> Could you give an example of such an information loss?

The issue is that while this patch eliminates duplication in:

  qemu-img: Could not open 'foo': Could not open 'foo': No such file or directory

It loses the name from:

  qemu-img: Driver 'bar' is not whitelisted

or any other error message in block.c that doesn't include the filename.

This is probably the reason why qemu-img.c prepends "Could not open
'%s'".

> > I'm not aware of a clean way to distinguish Error objects.  Maybe
> > someone else can suggest one.  Otherwise it may be best to leave the
> > code as it is.
> 
> If you need to distinguish different kinds of errors to conditionally
> rewrite the error message so it makes actual sense, chances are the
> error messages that need the rewriting should be improved instead.
> 
> A more legitimate case is when a caller needs to handle different errors
> differently.  Doesn't occur all that often.
> 
> There are two techniques for callers to distinguish different kinds of
> Errors:
> 
> * ErrorClass, use error_get_class() to retrieve it.  This is actually a
>   remnant of the failed "rich" error object idea.  Almost always
>   ERROR_CLASS_GENERIC_ERROR, so this is unlikely to help.
> 
> * Error code separate from the Error object, e.g. the function returns
>   -errno in addition to an Error object.

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