Re: [HACKERS] Autovacuum launcher occurs error when cancelled by SIGINT

On Thu, Jun 22, 2017 at 4:29 AM, Kuntal Ghosh
<kuntalghosh.2007@gmail.com> wrote:
> On Wed, Jun 21, 2017 at 7:52 PM, Dilip Kumar <dilipbalaut@gmail.com> wrote:
>> On Wed, Jun 21, 2017 at 7:47 PM, Kuntal Ghosh
>> <kuntalghosh.2007@gmail.com> wrote:
>>>> IMHO, It's not a good idea to use DSM call to verify the DSA handle.
>>>>
>>> Okay. Is there any particular scenario you've in mind where this may fail?
>>
>> It's not about failure, but about the abstraction.  When we are using
>> the DSA we should not directly access the DSM which is under DSA.
>>
> Okay. I thought that I've found at least one usage of
> dsm_find_mapping() in the code. :-)
>
> But, I've some more doubts.
> 1. When should we use dsm_find_mapping()? (The first few lines of
> dsm_attach is same as dsm_find_mapping().)
> 2. As a user of dsa, how should we check whether my dsa handle is
> already attached? I guess this is required because, if a user tries to
> re-attach a dsa handle,  it's punishing the user by throwing an error
> and the user wants to avoid such errors.

I thought about this when designing the DSA API.  I couldn't think of
any good reason to provide an 'am-I-already-attached?' function
equivalent to dsm_find_mapping.  It seemed to me that the client code
shouldn't ever be in any doubt about whether it's attached, and that
wilfully or absent-mindedly throwing away dsa_area pointers and having
to ask for them again doesn't seem like a very good design.  I suspect
the same applies to dsm_find_mapping, and I don't see any callers in
the source tree or indeed anywhere on the internet (based on a quick
Google search).  But I could be missing something.

-- 
Thomas Munro
http://www.enterprisedb.com