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

[Avi Kivity <avi@redhat.com>]

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".

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