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Re: [Qemu-devel] [Qemu-block] [PATCH] block: clarify error message for q
From: |
John Snow |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH] block: clarify error message for qmp-eject |
Date: |
Wed, 18 May 2016 10:21:23 -0400 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 |
On 05/18/2016 01:36 AM, Markus Armbruster wrote:
> 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.
>
> [...]
>
Basically, in some contexts certain callers *may* consider certain
error/success conditions as an error, and in others they may not.
For instance, when using qmp_blockdev_open_tray directly via QMP, it has
a few outcomes:
1) Tray is unlocked, force parameter is irrelevant, command succeeds.
2) Tray is locked, force is false, command "fails," but Guest VM is
notified of a desire to open the tray and may or may not respect the
command. We have "successfully" applied a best effort to opening the tray.
3) Tray is locked, force is true, command succeeds.
It's the behavior of #2 that I am trying to clarify here.
When used indirectly via qmp_eject, #2 is definitely an error -- the
full routine of eject will NOT succeed (we cannot remove the medium) so
the error reported to the user should be from Eject's perspective, not
medium_remove's.
To this end, I thought I'd add error codes into the helper to help
callers differentiate hard errors from "soft errors."
If this is too iffy for people, I can:
- Leave all error codes set to -ERRNO if we set errp, and
- Change all "maybe error codes" +ERRNO if we don't set errp. (Eric's
suggestion.)
Good?
Re: [Qemu-devel] [PATCH] block: clarify error message for qmp-eject, Eric Blake, 2016/05/17