Re: [KVM-AUTOTEST PATCH v2 3/6] [RFC] Introduce exception context strings

[Avi Kivity <avi@redhat.com>]

On 01/05/2011 06:12 PM, Avi Kivity wrote:
> On 01/05/2011 05:45 PM, Michael Goldish wrote:
>> In complex tests (KVM) an exception string is often not informative 
>> enough and
>> the traceback and source code have to be examined in order to figure 
>> out what
>> caused the exception.  Context strings are a way for tests to provide
>> information about what they're doing, so that when an exception is 
>> raised, this
>> information will be embedded in the exception string.  The result is 
>> a concise
>> yet highly informative exception string, which should make it very 
>> easy to
>> figure out where/when the exception was raised.
>>
>> A typical example for a test where this may be useful is KVM's reboot 
>> test.
>> Some exceptions can be raised either before or after the VM is 
>> rebooted (e.g.
>> logging into the guest can fail) and whether they are raised before 
>> or after
>> is critical to the understanding of the failure.  Normally the 
>> traceback would
>> have to be examined, but the proposed method makes it easy to know 
>> where the
>> exception is raised without doing so.  To achieve this, the reboot 
>> test should
>> place calls to error.context() as follows:
>>
>> error.context("before reboot")
>> <carry out pre-reboot actions>
>> error.context("sending reboot command")
>> <send the reboot command>
>> error.context("after reboot")
>> <carry out post-reboot actions>
>>
>> If login fails in the pre-reboot section, the resulting exception 
>> string can
>> can have something like "context: before reboot" embedded in it.  
>> (The actual
>> embedding is done in the next patch in the series.)
>
> It would be nice to make the error context a stack, and to use the 
> with statement to manage the stack:
>
>
>    with error.context("main test"):
>        foo()
>        with error.context("before reboot"):
>            bar()
>
> If foo() throws an exception, the context would be "main test", while 
> if bar() throws an exception, the context would be "before reboot" in 
> "main test".
>

btw, you can have a decorator for enclosing an entire function in an 
error context:

    @function_error_context('migration test')
    def migration_test(...):
        ...

anything in migration_test() is enclosed in that context.  But we're 
just repeating the ordinary stack trace with something more readable.

-- 
error compiling committee.c: too many arguments to function