Patchwork Ignoring errno makes QMP errors suck

login
register
mail settings
Submitter Anthony Liguori
Date March 26, 2012, 1:37 p.m.
Message ID <4F707118.1070200@codemonkey.ws>
Download mbox | patch
Permalink /patch/148742/
State New
Headers show

Comments

Anthony Liguori - March 26, 2012, 1:37 p.m.
On 03/26/2012 03:39 AM, Kevin Wolf wrote:
> Hi,
>
> I keep getting reports of problems, with nice error descriptions that
> usually look very similar to what I produced here:
>
> {"execute":"blockdev-snapshot-sync","arguments":{"device":"ide0-hd0","snapshot-file":"/tmp/backing.qcow2"}}
> {"error": {"class": "OpenFileFailed", "desc": "Could not open
> '/tmp/backing.qcow2'", "data": {"filename": "/tmp/backing.qcow2"}}}

This is not QMP's fault.  This is the block layers.  Specifically, you're missing:

> been the helpful additional information in this case)
>
> How should management tools ever be able to provide a helpful error
> message to their users if all they get is this useless "something went
> wrong" error?

You need to kill off error_report in the block layer and replace it with 
error_set.  The problem with error_report is that while you can understand what 
"Unknown file format 'qcow2'" means, management tools can't.  Responding that 
"the tool can just present that error to the user" implies that the management 
tool only provides an English-language interface which is not terribly friendly.

QMP provides all the infrastructure you need.   You just have to use it.

Regards,

Anthony Liguori

> Kevin
Kevin Wolf - March 26, 2012, 3:08 p.m.
Am 26.03.2012 15:37, schrieb Anthony Liguori:
> On 03/26/2012 03:39 AM, Kevin Wolf wrote:
>> Hi,
>>
>> I keep getting reports of problems, with nice error descriptions that
>> usually look very similar to what I produced here:
>>
>> {"execute":"blockdev-snapshot-sync","arguments":{"device":"ide0-hd0","snapshot-file":"/tmp/backing.qcow2"}}
>> {"error": {"class": "OpenFileFailed", "desc": "Could not open
>> '/tmp/backing.qcow2'", "data": {"filename": "/tmp/backing.qcow2"}}}
> 
> This is not QMP's fault.  This is the block layers.  Specifically, you're missing:
> 
> diff --git a/blockdev.c b/blockdev.c
> index 1a500b8..04c3a39 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -777,7 +777,11 @@ void qmp_transaction(BlockdevActionList *dev_list, Error **
>                                     states->old_bs->drv->format_name,
>                                     NULL, -1, flags);
>               if (ret) {
> -                error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
> +                if (ret == -EPERM) {
> +                    error_set(errp, QERR_PERMISSION_DENIED);
> +                } else {
> +                    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
> +                }
>                   goto delete_and_fail;
>               }
>           }
> 
> Which is handling:
> 
>              ret = bdrv_img_create(new_image_file, format,
>                                    states->old_bs->filename,
>                                    states->old_bs->drv->format_name,
>                                    NULL, -1, flags);

It really should be something like this:

-    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
+    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file, -ret);

And QERR_OPEN_FILE_FAILED would contain a conversion specifier for
errnos in qobject_from_jsonv().

> But it would be even better to push Error ** into bdrv_img_create().  There's 
> only two callers so it would be trivial to do that.  Then you could do:
> 
> diff --git a/block.c b/block.c
> index b88ee90..a7bf8a9 100644
> --- a/block.c
> +++ b/block.c
> @@ -3881,7 +3881,8 @@ bdrv_acct_done(BlockDriverState *bs, BlockAcctCookie *cook
> 
>   int bdrv_img_create(const char *filename, const char *fmt,
>                       const char *base_filename, const char *base_fmt,
> -                    char *options, uint64_t img_size, int flags)
> +                    char *options, uint64_t img_size, int flags,
> +                    Error **errp)
>   {
>       QEMUOptionParameter *param = NULL, *create_options = NULL;
>       QEMUOptionParameter *backing_fmt, *backing_file, *size;
> @@ -3893,14 +3894,14 @@ int bdrv_img_create(const char *filename, const char *fm
>       /* Find driver and parse its options */
>       drv = bdrv_find_format(fmt);
>       if (!drv) {
> -        error_report("Unknown file format '%s'", fmt);
> +        error_set(errp, QERR_INVALID_BLOCK_FORMAT, fmt);
>           ret = -EINVAL;
>           goto out;
>       }
> 
> Etc.

Yes, but that's a completely independent problem.

>> Who can tell me what has happened here? Oh, yes, the command failed, I
>> would have guessed that from the "error" key. But the actual error
>> description is as useless as it gets. It doesn't tell me anything about
>> _why_ the snapshot couldn't be created. ("Permission denied" would have
>> been the helpful additional information in this case)
>>
>> How should management tools ever be able to provide a helpful error
>> message to their users if all they get is this useless "something went
>> wrong" error?
> 
> You need to kill off error_report in the block layer and replace it with 
> error_set.  The problem with error_report is that while you can understand what 
> "Unknown file format 'qcow2'" means, management tools can't.  Responding that 
> "the tool can just present that error to the user" implies that the management 
> tool only provides an English-language interface which is not terribly friendly.
> 
> QMP provides all the infrastructure you need.   You just have to use it.

It doesn't provide the portable way of reporting errno yet. I could add
tests for specific errors (like you suggested above) in every single
place that sets an error, but I'd rather not. It would make the code
verbose and the error reporting probably inconsistent, if not buggy.

Kevin
Anthony Liguori - March 26, 2012, 3:14 p.m.
On 03/26/2012 10:08 AM, Kevin Wolf wrote:
> Am 26.03.2012 15:37, schrieb Anthony Liguori:
>> On 03/26/2012 03:39 AM, Kevin Wolf wrote:
>>> Hi,
>>>
>>> I keep getting reports of problems, with nice error descriptions that
>>> usually look very similar to what I produced here:
>>>
>>> {"execute":"blockdev-snapshot-sync","arguments":{"device":"ide0-hd0","snapshot-file":"/tmp/backing.qcow2"}}
>>> {"error": {"class": "OpenFileFailed", "desc": "Could not open
>>> '/tmp/backing.qcow2'", "data": {"filename": "/tmp/backing.qcow2"}}}
>>
>> This is not QMP's fault.  This is the block layers.  Specifically, you're missing:
>>
>> diff --git a/blockdev.c b/blockdev.c
>> index 1a500b8..04c3a39 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -777,7 +777,11 @@ void qmp_transaction(BlockdevActionList *dev_list, Error **
>>                                      states->old_bs->drv->format_name,
>>                                      NULL, -1, flags);
>>                if (ret) {
>> -                error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>> +                if (ret == -EPERM) {
>> +                    error_set(errp, QERR_PERMISSION_DENIED);
>> +                } else {
>> +                    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>> +                }
>>                    goto delete_and_fail;
>>                }
>>            }
>>
>> Which is handling:
>>
>>               ret = bdrv_img_create(new_image_file, format,
>>                                     states->old_bs->filename,
>>                                     states->old_bs->drv->format_name,
>>                                     NULL, -1, flags);
>
> It really should be something like this:
>
> -    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
> +    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file, -ret);
>
> And QERR_OPEN_FILE_FAILED would contain a conversion specifier for
> errnos in qobject_from_jsonv().

No, it really shouldn't be.

Errors are verbs, not knows, you're treating the error as a noun "the operation 
open file" and looking to use errno as the verb.  This is wrong.  The noun is 
implied in the operation.

You could use error_set_from_errno(errp, -ret) which doesn't exist, but could. 
But errno on it's own lacks a lot of useful information so I wouldn't suggest 
always using such a function.

>
>> But it would be even better to push Error ** into bdrv_img_create().  There's
>> only two callers so it would be trivial to do that.  Then you could do:
>>
>> diff --git a/block.c b/block.c
>> index b88ee90..a7bf8a9 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -3881,7 +3881,8 @@ bdrv_acct_done(BlockDriverState *bs, BlockAcctCookie *cook
>>
>>    int bdrv_img_create(const char *filename, const char *fmt,
>>                        const char *base_filename, const char *base_fmt,
>> -                    char *options, uint64_t img_size, int flags)
>> +                    char *options, uint64_t img_size, int flags,
>> +                    Error **errp)
>>    {
>>        QEMUOptionParameter *param = NULL, *create_options = NULL;
>>        QEMUOptionParameter *backing_fmt, *backing_file, *size;
>> @@ -3893,14 +3894,14 @@ int bdrv_img_create(const char *filename, const char *fm
>>        /* Find driver and parse its options */
>>        drv = bdrv_find_format(fmt);
>>        if (!drv) {
>> -        error_report("Unknown file format '%s'", fmt);
>> +        error_set(errp, QERR_INVALID_BLOCK_FORMAT, fmt);
>>            ret = -EINVAL;
>>            goto out;
>>        }
>>
>> Etc.
>
> Yes, but that's a completely independent problem.

It's not really.  If you want high quality errors, you have to push the error 
handling up the stack.  That's the reason we have Error--to introduce a common 
error handling framework capable of generating high quality error information.

>>> Who can tell me what has happened here? Oh, yes, the command failed, I
>>> would have guessed that from the "error" key. But the actual error
>>> description is as useless as it gets. It doesn't tell me anything about
>>> _why_ the snapshot couldn't be created. ("Permission denied" would have
>>> been the helpful additional information in this case)
>>>
>>> How should management tools ever be able to provide a helpful error
>>> message to their users if all they get is this useless "something went
>>> wrong" error?
>>
>> You need to kill off error_report in the block layer and replace it with
>> error_set.  The problem with error_report is that while you can understand what
>> "Unknown file format 'qcow2'" means, management tools can't.  Responding that
>> "the tool can just present that error to the user" implies that the management
>> tool only provides an English-language interface which is not terribly friendly.
>>
>> QMP provides all the infrastructure you need.   You just have to use it.
>
> It doesn't provide the portable way of reporting errno yet.

I think what you'll find is that 90% of the time, the errno is being generated 
somewhere within QEMU code or that there's a system call that returns on one 
errno that we care about.  If you push error handling down to the source of the 
error, I'm sure you'll find that you almost never have to switch on errno.

Having an error_set_from_errno() would be a stop-gap to help bridge unconverted 
code, but if you want high quality errors, the right answer is to convert the 
existing code to use the Error infrastructure properly.

> I could add
> tests for specific errors (like you suggested above) in every single
> place that sets an error, but I'd rather not. It would make the code
> verbose and the error reporting probably inconsistent, if not buggy.

We have a lot of:

error_report("Some english string\n");
return -ERANDOMERRORCODE;

This idiom does not make for good on the wire errors.  You can replace these 
lines with a single error_set() call.  There's no need for switching.

Regards,

Anthony Liguori

> Kevin
Kevin Wolf - March 26, 2012, 3:34 p.m.
Am 26.03.2012 17:14, schrieb Anthony Liguori:
> On 03/26/2012 10:08 AM, Kevin Wolf wrote:
>> Am 26.03.2012 15:37, schrieb Anthony Liguori:
>>> On 03/26/2012 03:39 AM, Kevin Wolf wrote:
>>>> Hi,
>>>>
>>>> I keep getting reports of problems, with nice error descriptions that
>>>> usually look very similar to what I produced here:
>>>>
>>>> {"execute":"blockdev-snapshot-sync","arguments":{"device":"ide0-hd0","snapshot-file":"/tmp/backing.qcow2"}}
>>>> {"error": {"class": "OpenFileFailed", "desc": "Could not open
>>>> '/tmp/backing.qcow2'", "data": {"filename": "/tmp/backing.qcow2"}}}
>>>
>>> This is not QMP's fault.  This is the block layers.  Specifically, you're missing:
>>>
>>> diff --git a/blockdev.c b/blockdev.c
>>> index 1a500b8..04c3a39 100644
>>> --- a/blockdev.c
>>> +++ b/blockdev.c
>>> @@ -777,7 +777,11 @@ void qmp_transaction(BlockdevActionList *dev_list, Error **
>>>                                      states->old_bs->drv->format_name,
>>>                                      NULL, -1, flags);
>>>                if (ret) {
>>> -                error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>>> +                if (ret == -EPERM) {
>>> +                    error_set(errp, QERR_PERMISSION_DENIED);
>>> +                } else {
>>> +                    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>>> +                }
>>>                    goto delete_and_fail;
>>>                }
>>>            }
>>>
>>> Which is handling:
>>>
>>>               ret = bdrv_img_create(new_image_file, format,
>>>                                     states->old_bs->filename,
>>>                                     states->old_bs->drv->format_name,
>>>                                     NULL, -1, flags);
>>
>> It really should be something like this:
>>
>> -    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>> +    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file, -ret);
>>
>> And QERR_OPEN_FILE_FAILED would contain a conversion specifier for
>> errnos in qobject_from_jsonv().
> 
> No, it really shouldn't be.
> 
> Errors are verbs, not knows, you're treating the error as a noun "the operation 
> open file" and looking to use errno as the verb.  This is wrong.  The noun is 
> implied in the operation.
> 
> You could use error_set_from_errno(errp, -ret) which doesn't exist, but could. 
> But errno on it's own lacks a lot of useful information so I wouldn't suggest 
> always using such a function.

I couldn't care less about nouns and verbs and stuff.

I want to transfer the information that a "permission denied" error has
happened and on which file it has happened. The existing OpenFileFailed
error doesn't allow to specify that the missing permission was the
problem, and a hypothetical PermissionDenied error wouldn't allow me to
specify the file name because it would be too generic.

This is my problem, and nothing else.

>> Yes, but that's a completely independent problem.
> 
> It's not really.  If you want high quality errors, you have to push the error 
> handling up the stack.  That's the reason we have Error--to introduce a common 
> error handling framework capable of generating high quality error information.

Yes, but if there is no appropriate error, then even if I added Error
support to the Linux syscalls they couldn't generate the right error
message. This is why I still think it's completely independent.

>>>> Who can tell me what has happened here? Oh, yes, the command failed, I
>>>> would have guessed that from the "error" key. But the actual error
>>>> description is as useless as it gets. It doesn't tell me anything about
>>>> _why_ the snapshot couldn't be created. ("Permission denied" would have
>>>> been the helpful additional information in this case)
>>>>
>>>> How should management tools ever be able to provide a helpful error
>>>> message to their users if all they get is this useless "something went
>>>> wrong" error?
>>>
>>> You need to kill off error_report in the block layer and replace it with
>>> error_set.  The problem with error_report is that while you can understand what
>>> "Unknown file format 'qcow2'" means, management tools can't.  Responding that
>>> "the tool can just present that error to the user" implies that the management
>>> tool only provides an English-language interface which is not terribly friendly.
>>>
>>> QMP provides all the infrastructure you need.   You just have to use it.
>>
>> It doesn't provide the portable way of reporting errno yet.
> 
> I think what you'll find is that 90% of the time, the errno is being generated 
> somewhere within QEMU code or that there's a system call that returns on one 
> errno that we care about.  If you push error handling down to the source of the 
> error, I'm sure you'll find that you almost never have to switch on errno.

I'm looking for a solution that works now and not only in five years
when all of qemu has been rewritten. I'm also not quite sure if we
really want to drag Errors through coroutines and AIO code in the block
layer...

> Having an error_set_from_errno() would be a stop-gap to help bridge unconverted 
> code, but if you want high quality errors, the right answer is to convert the 
> existing code to use the Error infrastructure properly.

Only if it can be used properly. That is, if I can somehow create an
error message that contains _both_ the file name and the error cause.

>> I could add
>> tests for specific errors (like you suggested above) in every single
>> place that sets an error, but I'd rather not. It would make the code
>> verbose and the error reporting probably inconsistent, if not buggy.
> 
> We have a lot of:
> 
> error_report("Some english string\n");
> return -ERANDOMERRORCODE;
> 
> This idiom does not make for good on the wire errors.  You can replace these 
> lines with a single error_set() call.  There's no need for switching.

But this is not the case I have asked for.

Kevin
Anthony Liguori - March 26, 2012, 3:38 p.m.
On 03/26/2012 10:34 AM, Kevin Wolf wrote:
> Am 26.03.2012 17:14, schrieb Anthony Liguori:
>> On 03/26/2012 10:08 AM, Kevin Wolf wrote:
>>> Am 26.03.2012 15:37, schrieb Anthony Liguori:
>>>> On 03/26/2012 03:39 AM, Kevin Wolf wrote:
>>>>> Hi,
>>>>>
>>>>> I keep getting reports of problems, with nice error descriptions that
>>>>> usually look very similar to what I produced here:
>>>>>
>>>>> {"execute":"blockdev-snapshot-sync","arguments":{"device":"ide0-hd0","snapshot-file":"/tmp/backing.qcow2"}}
>>>>> {"error": {"class": "OpenFileFailed", "desc": "Could not open
>>>>> '/tmp/backing.qcow2'", "data": {"filename": "/tmp/backing.qcow2"}}}
>>>>
>>>> This is not QMP's fault.  This is the block layers.  Specifically, you're missing:
>>>>
>>>> diff --git a/blockdev.c b/blockdev.c
>>>> index 1a500b8..04c3a39 100644
>>>> --- a/blockdev.c
>>>> +++ b/blockdev.c
>>>> @@ -777,7 +777,11 @@ void qmp_transaction(BlockdevActionList *dev_list, Error **
>>>>                                       states->old_bs->drv->format_name,
>>>>                                       NULL, -1, flags);
>>>>                 if (ret) {
>>>> -                error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>>>> +                if (ret == -EPERM) {
>>>> +                    error_set(errp, QERR_PERMISSION_DENIED);
>>>> +                } else {
>>>> +                    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>>>> +                }
>>>>                     goto delete_and_fail;
>>>>                 }
>>>>             }
>>>>
>>>> Which is handling:
>>>>
>>>>                ret = bdrv_img_create(new_image_file, format,
>>>>                                      states->old_bs->filename,
>>>>                                      states->old_bs->drv->format_name,
>>>>                                      NULL, -1, flags);
>>>
>>> It really should be something like this:
>>>
>>> -    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>>> +    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file, -ret);
>>>
>>> And QERR_OPEN_FILE_FAILED would contain a conversion specifier for
>>> errnos in qobject_from_jsonv().
>>
>> No, it really shouldn't be.
>>
>> Errors are verbs, not knows, you're treating the error as a noun "the operation
>> open file" and looking to use errno as the verb.  This is wrong.  The noun is
>> implied in the operation.
>>
>> You could use error_set_from_errno(errp, -ret) which doesn't exist, but could.
>> But errno on it's own lacks a lot of useful information so I wouldn't suggest
>> always using such a function.
>
> I couldn't care less about nouns and verbs and stuff.
>
> I want to transfer the information that a "permission denied" error has
> happened and on which file it has happened. The existing OpenFileFailed
> error doesn't allow to specify that the missing permission was the
> problem, and a hypothetical PermissionDenied error wouldn't allow me to
> specify the file name because it would be too generic.
>
> This is my problem, and nothing else.

Then extend PermissionDenied to include a filename.  Problem solved.

>>> Yes, but that's a completely independent problem.
>>
>> It's not really.  If you want high quality errors, you have to push the error
>> handling up the stack.  That's the reason we have Error--to introduce a common
>> error handling framework capable of generating high quality error information.
>
> Yes, but if there is no appropriate error, then even if I added Error
> support to the Linux syscalls they couldn't generate the right error
> message. This is why I still think it's completely independent.

Than add/improve the existing errors.

>>>> QMP provides all the infrastructure you need.   You just have to use it.
>>>
>>> It doesn't provide the portable way of reporting errno yet.
>>
>> I think what you'll find is that 90% of the time, the errno is being generated
>> somewhere within QEMU code or that there's a system call that returns on one
>> errno that we care about.  If you push error handling down to the source of the
>> error, I'm sure you'll find that you almost never have to switch on errno.
>
> I'm looking for a solution that works now and not only in five years
> when all of qemu has been rewritten. I'm also not quite sure if we
> really want to drag Errors through coroutines and AIO code in the block
> layer...

You can bridge this all today.  I showed you in patches how to do it.  It's not 
difficult.

>> Having an error_set_from_errno() would be a stop-gap to help bridge unconverted
>> code, but if you want high quality errors, the right answer is to convert the
>> existing code to use the Error infrastructure properly.
>
> Only if it can be used properly. That is, if I can somehow create an
> error message that contains _both_ the file name and the error cause.

You can add parameters to Errors in a fully compatible fashion, so just add an 
filename parameter to PermissionDenied.  Problem solved.

Regards,

Anthony Liguori
Kevin Wolf - March 26, 2012, 3:59 p.m.
Am 26.03.2012 17:38, schrieb Anthony Liguori:
> On 03/26/2012 10:34 AM, Kevin Wolf wrote:
>> Am 26.03.2012 17:14, schrieb Anthony Liguori:
>>> On 03/26/2012 10:08 AM, Kevin Wolf wrote:
>>>> Am 26.03.2012 15:37, schrieb Anthony Liguori:
>>>>> On 03/26/2012 03:39 AM, Kevin Wolf wrote:
>>>>>> Hi,
>>>>>>
>>>>>> I keep getting reports of problems, with nice error descriptions that
>>>>>> usually look very similar to what I produced here:
>>>>>>
>>>>>> {"execute":"blockdev-snapshot-sync","arguments":{"device":"ide0-hd0","snapshot-file":"/tmp/backing.qcow2"}}
>>>>>> {"error": {"class": "OpenFileFailed", "desc": "Could not open
>>>>>> '/tmp/backing.qcow2'", "data": {"filename": "/tmp/backing.qcow2"}}}
>>>>>
>>>>> This is not QMP's fault.  This is the block layers.  Specifically, you're missing:
>>>>>
>>>>> diff --git a/blockdev.c b/blockdev.c
>>>>> index 1a500b8..04c3a39 100644
>>>>> --- a/blockdev.c
>>>>> +++ b/blockdev.c
>>>>> @@ -777,7 +777,11 @@ void qmp_transaction(BlockdevActionList *dev_list, Error **
>>>>>                                       states->old_bs->drv->format_name,
>>>>>                                       NULL, -1, flags);
>>>>>                 if (ret) {
>>>>> -                error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>>>>> +                if (ret == -EPERM) {
>>>>> +                    error_set(errp, QERR_PERMISSION_DENIED);
>>>>> +                } else {
>>>>> +                    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>>>>> +                }
>>>>>                     goto delete_and_fail;
>>>>>                 }
>>>>>             }
>>>>>
>>>>> Which is handling:
>>>>>
>>>>>                ret = bdrv_img_create(new_image_file, format,
>>>>>                                      states->old_bs->filename,
>>>>>                                      states->old_bs->drv->format_name,
>>>>>                                      NULL, -1, flags);
>>>>
>>>> It really should be something like this:
>>>>
>>>> -    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
>>>> +    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file, -ret);
>>>>
>>>> And QERR_OPEN_FILE_FAILED would contain a conversion specifier for
>>>> errnos in qobject_from_jsonv().
>>>
>>> No, it really shouldn't be.
>>>
>>> Errors are verbs, not knows, you're treating the error as a noun "the operation
>>> open file" and looking to use errno as the verb.  This is wrong.  The noun is
>>> implied in the operation.
>>>
>>> You could use error_set_from_errno(errp, -ret) which doesn't exist, but could.
>>> But errno on it's own lacks a lot of useful information so I wouldn't suggest
>>> always using such a function.
>>
>> I couldn't care less about nouns and verbs and stuff.
>>
>> I want to transfer the information that a "permission denied" error has
>> happened and on which file it has happened. The existing OpenFileFailed
>> error doesn't allow to specify that the missing permission was the
>> problem, and a hypothetical PermissionDenied error wouldn't allow me to
>> specify the file name because it would be too generic.
>>
>> This is my problem, and nothing else.
> 
> Then extend PermissionDenied to include a filename.  Problem solved.

> You can add parameters to Errors in a fully compatible fashion, so just add an 
> filename parameter to PermissionDenied.  Problem solved.

So your error types will end up accumulating optional parameters for all
contexts in which they can occur?

In the long run, PermissionDenied would have an optional file name (for
raw), optional host name, port, sheepdog volume ID (for sheepdog),
optional source and destination block devices (for blkmirror), remote
host and port, local address and port (for UDP chardevs)...

I could go on forever. Does this really make sense?

Kevin
Anthony Liguori - March 26, 2012, 4:01 p.m.
On 03/26/2012 10:59 AM, Kevin Wolf wrote:
> Am 26.03.2012 17:38, schrieb Anthony Liguori:
>> You can add parameters to Errors in a fully compatible fashion, so just add an
>> filename parameter to PermissionDenied.  Problem solved.
>
> So your error types will end up accumulating optional parameters for all
> contexts in which they can occur?
>
> In the long run, PermissionDenied would have an optional file name (for
> raw), optional host name, port, sheepdog volume ID (for sheepdog),
> optional source and destination block devices (for blkmirror), remote
> host and port, local address and port (for UDP chardevs)...

I don't see the generalization.

>
> I could go on forever. Does this really make sense?

What's the alternative proposal?  If you just add errno to OPEN_FILE_FAILED, 
then you end up with errors for SHEEP_DOG_ATTACH_VOLUME_FAILED, 
NBD_CONNECT_FAILED, etc.

The proper way to design an error is to make it a verb, and have parameters that 
correspond to the direct object and/or indirect object.  The subject is implied 
by the command itself.

So in the case of block_snapshot_sync, the failure is:

The block_snapshot_sync command failed due to insufficient permission when 
creating foo.img.

So the error is "PERMISSION_DENIED", with the data "filename=foo.img"

In the case of NBD, the error would be:

The block_snapshot_sync command failed because the host localhost:42 could not 
be contacted.

The error is "CONNECTION_REFUSED", with the data "hostname=localhost,port=42".

It the block layer returns both of these as EACCESS today, then this is a good 
argument to refactor the block layer to take an Error object instead of 
overloading the meaning of EACCESS.

Regards,

Anthony Liguori

>
> Kevin
Paolo Bonzini - April 11, 2012, 9:05 p.m.
Il 26/03/2012 18:01, Anthony Liguori ha scritto:
> On 03/26/2012 10:59 AM, Kevin Wolf wrote:
>> Am 26.03.2012 17:38, schrieb Anthony Liguori:
>>> You can add parameters to Errors in a fully compatible fashion, so
>>> just add an
>>> filename parameter to PermissionDenied.  Problem solved.
>>
>> So your error types will end up accumulating optional parameters for all
>> contexts in which they can occur?
>>
>> In the long run, PermissionDenied would have an optional file name (for
>> raw), optional host name, port, sheepdog volume ID (for sheepdog),
>> optional source and destination block devices (for blkmirror), remote
>> host and port, local address and port (for UDP chardevs)...
> 
> I don't see the generalization.

If you use an extremely generic error such as PermissionDenied, you have
no idea whether it happened in the guts of the implementation (internal
error in QEMU or management), or it is due to a comparatively trivial
OS-level error.

If you use PermissionDenied for internal errors such as QOM property
accesses, and a more high-level error such as OpenFileFailed, you
clearly separate the two cases.  Having the same error for something
internal and something related to user configuration sucks.

>>
>> I could go on forever. Does this really make sense?
> 
> What's the alternative proposal?  If you just add errno to
> OPEN_FILE_FAILED, then you end up with errors for
> SHEEP_DOG_ATTACH_VOLUME_FAILED, NBD_CONNECT_FAILED, etc.
> 
> The proper way to design an error is to make it a verb, and have
> parameters that correspond to the direct object and/or indirect object. 
> The subject is implied by the command itself.
> 
> So in the case of block_snapshot_sync, the failure is:
> 
> The block_snapshot_sync command failed due to insufficient permission
> when creating foo.img.
> 
> So the error is "PERMISSION_DENIED", with the data "filename=foo.img"

No, the error is OPEN_FILE_FAILED, and the data is
"errno=QEMU_ERRNO_EACCES,<blah>" where <blah> is some kind of union that
would be the same type you pass to a hypothetical QMP command
blockdev_add.  Let's define our own enum so that errno values can be
mapped to a platform-independent value.

Paolo

Patch

diff --git a/blockdev.c b/blockdev.c
index 1a500b8..04c3a39 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -777,7 +777,11 @@  void qmp_transaction(BlockdevActionList *dev_list, Error **
                                    states->old_bs->drv->format_name,
                                    NULL, -1, flags);
              if (ret) {
-                error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
+                if (ret == -EPERM) {
+                    error_set(errp, QERR_PERMISSION_DENIED);
+                } else {
+                    error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
+                }
                  goto delete_and_fail;
              }
          }

Which is handling:

             ret = bdrv_img_create(new_image_file, format,
                                   states->old_bs->filename,
                                   states->old_bs->drv->format_name,
                                   NULL, -1, flags);

But it would be even better to push Error ** into bdrv_img_create().  There's 
only two callers so it would be trivial to do that.  Then you could do:

diff --git a/block.c b/block.c
index b88ee90..a7bf8a9 100644
--- a/block.c
+++ b/block.c
@@ -3881,7 +3881,8 @@  bdrv_acct_done(BlockDriverState *bs, BlockAcctCookie *cook

  int bdrv_img_create(const char *filename, const char *fmt,
                      const char *base_filename, const char *base_fmt,
-                    char *options, uint64_t img_size, int flags)
+                    char *options, uint64_t img_size, int flags,
+                    Error **errp)
  {
      QEMUOptionParameter *param = NULL, *create_options = NULL;
      QEMUOptionParameter *backing_fmt, *backing_file, *size;
@@ -3893,14 +3894,14 @@  int bdrv_img_create(const char *filename, const char *fm
      /* Find driver and parse its options */
      drv = bdrv_find_format(fmt);
      if (!drv) {
-        error_report("Unknown file format '%s'", fmt);
+        error_set(errp, QERR_INVALID_BLOCK_FORMAT, fmt);
          ret = -EINVAL;
          goto out;
      }

Etc.

> Who can tell me what has happened here? Oh, yes, the command failed, I
> would have guessed that from the "error" key. But the actual error
> description is as useless as it gets. It doesn't tell me anything about
> _why_ the snapshot couldn't be created. ("Permission denied" would have