Error handling

[Ed Tanous]

On Thu, Aug 31, 2023 at 4:47 AM chandu chanti <chandutmba at gmail.com> wrote:

> Hi team,
>
> We have drafted a proposal for  "Using default values in D-Bus properties
> to indicate errors"  as communicated in previous mail.
>
> Issue with Existing Implementation:
> Error handling is not implemented in the existing bmcweb source.
>

This statement is verifiably incorrect.  Many discussions have been had
about how we handle Dbus, json, and Redfish errors, and the handling
policies are written down in DBUS_USAGE.md.   Please familiarize yourself
with those previous discussions, and if you believe the conclusion was
incorrect, quote those in your response.


> The values fetched from D-Bus are used directly in the Redfish response,
> except for some properties.
>

While this is true for properties that are intended to be sent directly to
the user, like Asset information, it's not true of all properties.


> There was no way to represent error for a given data type on Redfish.
>

Please look at error_messages.hpp and error_messages.cpp


>
>
> New Implementation Proposal:
> We plan to handle errors from backend D-Bus services in bmcweb using a
> custom data type class. We will implement a helper class to create a new
> data type called mydatatype_t. An object of this class will be created with
> the D-Bus property value as input. This object will be assigned to the JSON
> response using the assignment operator. This assignment operation will
> invoke the to_json function with the mydatatype_t object, where D-Bus
> values will be validated based on their data types.
>

> Data Types and Default Values:
> The following values will be set on the D-Bus property in case of an
> error, based on the data type.
>

If the dbus data type is incorrect this already sets internal error (500).
Please look at sdbusplus::unpackPropertiesNoThrow usage in bmcweb.  Given
that the interface failed, this is the most specific error we can give the
user.  We do log the source of the error though.


> In Redfish, we will validate these properties. If the D-Bus property value
> matches the error value, we will update the property value as null in the
> Redfish JSON response; otherwise, the corresponding value on D-Bus will be
> used.
>

For properties that have a "null" or "invalid" value, this is already
implemented, generally with a simple branch.
Example from sensors:
https://github.com/openbmc/bmcweb/blob/99bf026224d6ea60be27bd36793ddfaea5537d2a/redfish-core/lib/sensors.hpp#L934




>
> *D-Bus*
>
> *data type*
>
> *Value of the D-Bus Property incase of Error *
>
> *Property value as seen on Redfish *
>
> INT16
>
> INT_MAX
>
> null
>
> UINT16
>
> UINT_MAX
>
> null
>
> INT32
>
> LONG_MAX
>
> null
>
> UINT32
>
> ULONG_MAX
>
> null
>
> INT64
>
> LLONG_MAX
>
> null
>
> UINT64
>
> ULLONG_MAX
>
> null
>
> DOUBLE
>
> std::numeric_limits
> <http://en.cppreference.com/w/cpp/types/numeric_limits><T>::quiet_NaN()
>
> null
>
> STRING
>
> BE_ERROR-XXX
>
> null
>
> BOOL
>
> uint8_t
> {0–false , 1–true,
>   others- error}
>
> null
>
>
This table does not match with the Redfish spec on error handling
behavior.  Please familiarize yourself with DSP0266
<https://www.dmtf.org/sites/default/files/standards/documents/DSP0266_1.18.0.pdf>,
and any updates you make, please reference that specification.

Also, I have no idea what "BE_ERROR-XXX" is supposed to represent.


> In case of error conditions Redfish will report "null" for a property
> irrespective  of the data type.
>
> For error validation, we will implement a custom class, which will
> facilitate error validation for D-Bus values. In instances of errors, a
> null value will be allocated to the JSON response. Example for the uint16_t
> data type.
>

As written, this seems considerably worse than what we have today, and goes
backwards on things like: https://github.com/openbmc/bmcweb/issues/176


>
> [image: image.png]
>
> [image: image.png]
>
> For more implementation details please refer the following gerrit link, we
> implemented it for redfish URI /redfish/v1/Systems/<str>/Memory/<str>/.
> https://gerrit.openbmc.org/c/openbmc/bmcweb/+/66368
>
> Your feedback and insights are greatly appreciated.
>
> Thanks,
> Chandrasekhar T.
>


Overall, I've read the code, and your email, and I really have no idea
what's being proposed that doesn't already exist in a much simpler form,
using the existing error handling mechanisms.  While your point that one
failing property can fail the whole call is correct, that's the intent.  If
an internal service fails, that needs propagated to the user, in the form
of an error code, per the Redfish specification.

One thing I don't understand is what code is actually failing in this
case?  Can you point to a place where an error is returned that you'd like
handled differently?  Speaking in concrete terms would help to make the
generalized change you're attempting here.


>
> On Fri, Aug 4, 2023 at 12:37 PM Shakeeb Pasha <shakeebbk at gmail.com> wrote:
>
>> Thanks Joseph for the response.
>> >> I didn't see anyone else answer this.  Is your question about how to
>> >> architect or design your D-Bus interfaces?  Is the reference the BMCWeb
>> >> merely to provide context as a consumer of these D-Bus services?  I
>> >> assume so.
>> Question is two pronged, one we wanted to know the best way to
>> propagate error from dbus to bmcweb.
>> Options we tried:
>> 1. Have handlers of each property throw an exception if there was an
>> error. But this did not work well with current bmcweb implementation(get
>> all method call), as it would lead to the entire resource not getting
>> populated even if we have one property failing.
>> 2. Implement one additional associated rc property - and do the error
>> handling bmcweb by looking at that, but this doubled up the number of
>> properties on dbus.
>> 3. Use "default values" per property data type, but this would fail for
>> extreme cases like if the property is u32, and if the default value used
>> was INT_MAX, then it breaks when the property could take value 0xFFFFFFFF
>> as valid value.
>> So the query here was to check if any preference from above options or
>> hear any new approaches as well.
>>
>> The second query was about how to represent this error(per property
>> error) on redfish, our proposal for now is to return "NULL" for any
>> property that might be failing at the backend. Any thoughts on this
>> approach?
>>
>> >> I don't have any special insights.  Are you looking to follow a design
>> >> pattern?  Are you looking for direction from the BMCWeb maintainers?
>>
>> Yes, we are working on a design pattern proposal and will publish it here
>> for review.
>> It would be great if we could get some directions/inputs from the
>> maintainers as well.
>>
>> Thanks,
>> Shakeeb
>>
>> On Thu, Aug 3, 2023 at 10:33 PM Joseph Reynolds <jrey at linux.ibm.com>
>> wrote:
>>
>>> On 7/20/23 2:04 AM, chandu chanti wrote:
>>> > Hi Team, We are planning to handle errors from backend services in bmc
>>> > web. We are considering the following approaches to implement it, but
>>> > we are facing some issues. please provide your inputs. 1 . Using
>>> > exceptions in our backend D-Bus service
>>> > ZjQcmQRYFpfptBannerStart
>>> > This Message Is From an Untrusted Sender
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>>> >
>>> > ZjQcmQRYFpfptBannerEnd
>>> >
>>> > Hi Team,
>>> >
>>> > We are planning to handle errors from backend services in bmc web. We
>>> > are considering the following approaches to implement it, but we are
>>> > facing some issues. please provide your inputs.
>>> >
>>>
>>> I didn't see anyone else answer this.  Is your question about how to
>>> architect or design your D-Bus interfaces?  Is the reference the BMCWeb
>>> merely to provide context as a consumer of these D-Bus services?  I
>>> assume so.
>>>
>>> I don't have any special insights.  Are you looking to follow a design
>>> pattern?  Are you looking for direction from the BMCWeb maintainers?
>>>
>>> Joseph
>>>
>>> > 1 . Using exceptions in our backend D-Bus service by throwing
>>> > exceptions in the D-Bus property get handlers. It works fine for the
>>> > Get property method call. However, when using the Get All method call,
>>> > if one property fails, an error is returned without checking the other
>>> > properties. Is there a way to implement exceptions in GetAll so that
>>> > even if one property fails, we can still fetch the remaining
>>> properties.
>>> >
>>> > 2. Using default values in D-Bus properties to indicate errors. Is
>>> > there a reference implementation available for setting default values
>>> > for string and integer data types in bmc web, similar to the
>>> > implementation of NaN (default value) for the double data type in
>>> > cable.hpp.
>>> >
>>> > 3. Implement associated return code per property on dbus, but this
>>> > would be very inefficient in terms of double the properties on dbus,
>>> > something we would like to avoid
>>> >
>>> > Thanks
>>> > Chandrasekhar T.
>>> >
>>>
>>>
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