Re: [Qemu-block] [Qemu-devel] [PATCH] block: clarify error message for qmp-eject

[Markus Armbruster]

Fam Zheng <address@hidden> writes:

> On Tue, 05/17 20:42, John Snow wrote:
>> If you use HMP's eject but the CDROM tray is locked, you may get a
>> confusing error message informing you that the "tray isn't open."
>> 
>> As this is the point of eject, we can do a little better and help
>> clarify that the tray was locked and that it (might) open up later,
>> so try again.
>> 
>> It's not ideal, but it makes the semantics of the (legacy) eject
>> command more understandable to end users when they try to use it.
>> 
>> Signed-off-by: John Snow <address@hidden>
>> ---
>>  blockdev.c | 36 +++++++++++++++++++++++++++++-------
>>  1 file changed, 29 insertions(+), 7 deletions(-)
>> 
>> diff --git a/blockdev.c b/blockdev.c
>> index 1892b8e..feb8484 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -2290,16 +2290,26 @@ exit:
>>      block_job_txn_unref(block_job_txn);
>>  }
>>  
>> +static int do_open_tray(const char *device, bool has_force, bool force,
>> +                        Error **errp);
>> +
>>  void qmp_eject(const char *device, bool has_force, bool force, Error **errp)
>>  {
>>      Error *local_err = NULL;
>> +    int rc;
>>  
>> -    qmp_blockdev_open_tray(device, has_force, force, &local_err);
>> +    rc = do_open_tray(device, has_force, force, &local_err);
>>      if (local_err) {
>>          error_propagate(errp, local_err);
>>          return;
>>      }
>>  
>> +    if (rc == -EINPROGRESS) {
>> +        error_setg(errp, "Device '%s' is locked and force was not 
>> specified, "
>> +                   "wait for tray to open and try again", device);
>> +        return;
>> +    }
>> +
>>      qmp_x_blockdev_remove_medium(device, errp);
>>  }
>>  
>> @@ -2327,8 +2337,8 @@ void qmp_block_passwd(bool has_device, const char 
>> *device,
>>      aio_context_release(aio_context);
>>  }
>>  
>> -void qmp_blockdev_open_tray(const char *device, bool has_force, bool force,
>> -                            Error **errp)
>> +static int do_open_tray(const char *device, bool has_force, bool force,
>> +                        Error **errp)
>
> Personally I feel the has_force and force could be merged as one parameter.

For qmp_blockdev_open_tray(), the signature is dictated by
scripts/qapi-commands.py.  To make has_FOO go away, you need to make the
FOO non-optional.

You have to duplicate the cumbersome has_FOO, FOO couple in your helper
functions only when an absent value (has_FOO=false) has special meaning
you can't get with any present value.  Not my favorite interface design,
by the way.

We've discussed two improvements to the QAPI language and generators:

* Optional with default: has_FOO goes away, and instead FOO assumes the
  default value declared in the schema when it's absent.  Optional
  without default stays at it is, i.e. has_FOO tells whether it's
  present.

* Use null pointer for absent when it can't be a value.

If Eric stops flooding me with QAPI patches, I might even get to
implement them :)

>>  {
>>      BlockBackend *blk;
>>      bool locked;
>> @@ -2341,21 +2351,21 @@ void qmp_blockdev_open_tray(const char *device, bool 
>> has_force, bool force,
>>      if (!blk) {
>>          error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
>>                    "Device '%s' not found", device);
>> -        return;
>> +        return -ENODEV;
>>      }
>>  
>>      if (!blk_dev_has_removable_media(blk)) {
>>          error_setg(errp, "Device '%s' is not removable", device);
>> -        return;
>> +        return -ENOTSUP;
>>      }
>>  
>>      if (!blk_dev_has_tray(blk)) {
>>          /* Ignore this command on tray-less devices */
>> -        return;
>> +        return -ENOSYS;
>
> I'm not sure how acceptable it is to leave errp untouched while setting ret
> code to non-zero. Markus?

It's questionable style, becaue it gives the two plausible ways to check
for errors different meaning:

    if (do_open_tray(...) < 0) ...

and

    Error *err = NULL;
    do_open_tray(..., &err);
    if (err) ...

I find this confusing.

The former way lets me pass a null Error * argument, which is convenient
when I'm not interested in error details.

Whenever practical, separate an Error-setting function's values into
distinct error and success sets.  Example: when a function looks up
something, return pointer to it on success, set error and return null on
failure.

This isn't always practical, for instance, when a pointer-valued
function can legitimately return null.  That causes confusion, too.  We
fixed a few bugs around such functions.

Whether it isn't practical for *this* function I can't say without
developing a better understanding of its purpose and context.

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