Обсуждение: Simplifying the interface of UpdateMinRecoveryPoint

Hi all,

As of now UpdateMinRecoveryPoint() is using two arguments:
- lsn, to check if the minimum recovery point should be updated to that
- force, a boolean flag to decide if the update should be enforced or not.
However those two arguments are correlated. If lsn is
InvalidXlogRecPtr, the minimum recovery point update will be enforced.
Hence why not simplifying its interface and remove the force flag? See
attached.

Thanks,
--
Michael

On Wed, Jul 13, 2016 at 9:19 AM, Michael Paquier
<michael.paquier@gmail.com> wrote:
> Hi all,
>
> As of now UpdateMinRecoveryPoint() is using two arguments:
> - lsn, to check if the minimum recovery point should be updated to that
> - force, a boolean flag to decide if the update should be enforced or not.
> However those two arguments are correlated. If lsn is
> InvalidXlogRecPtr, the minimum recovery point update will be enforced.

Right.

> Hence why not simplifying its interface and remove the force flag?

One point to note is that the signature and usage of
UpdateMinRecoveryPoint() is same as it was when it got introduced in
commit-cdd46c76.  Now the only reasons that come to my mind for
introducing the force parameter was (a) it looks cleaner that way to
committer (b) they have some usecase for the same in mind (c) it got
have overlooked.  Now, if it got introduced due to (c), then your
patch does the right thing by removing it.  Personally, I feel
overloading the parameter for multiple purposes makes code less
maintainable, so retaining as it is in HEAD has some merits.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

On Wed, Jul 13, 2016 at 8:27 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Wed, Jul 13, 2016 at 9:19 AM, Michael Paquier
> <michael.paquier@gmail.com> wrote:
>> Hence why not simplifying its interface and remove the force flag?
>
> One point to note is that the signature and usage of
> UpdateMinRecoveryPoint() is same as it was when it got introduced in
> commit-cdd46c76.  Now the only reasons that come to my mind for
> introducing the force parameter was (a) it looks cleaner that way to
> committer (b) they have some usecase for the same in mind (c) it got
> have overlooked.  Now, if it got introduced due to (c), then your
> patch does the right thing by removing it.  Personally, I feel
> overloading the parameter for multiple purposes makes code less
> maintainable, so retaining as it is in HEAD has some merits.

There is no way to tell what that is, but perhaps Heikki recalls
something on the matter. I am just adding him in CC.
-- 
Michael

On Wed, Jul 13, 2016 at 10:31 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> On 12 July 2016 at 23:49, Michael Paquier <michael.paquier@gmail.com> wrote:
>> Hence why not simplifying its interface and remove the force flag?
>
> Is this change needed as part of a feature? If not, I see no reason for
> change.
>
> If we all work towards meaningful features the code can be cleaned up as we
> go.

That's just refactoring. The interactions between the two arguments of
this routine is plain.
-- 
Michael

On 07/13/2016 04:25 PM, Michael Paquier wrote:
> On Wed, Jul 13, 2016 at 8:27 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> On Wed, Jul 13, 2016 at 9:19 AM, Michael Paquier
>> <michael.paquier@gmail.com> wrote:
>>> Hence why not simplifying its interface and remove the force flag?
>>
>> One point to note is that the signature and usage of
>> UpdateMinRecoveryPoint() is same as it was when it got introduced in
>> commit-cdd46c76.  Now the only reasons that come to my mind for
>> introducing the force parameter was (a) it looks cleaner that way to
>> committer (b) they have some usecase for the same in mind (c) it got
>> have overlooked.  Now, if it got introduced due to (c), then your
>> patch does the right thing by removing it.  Personally, I feel
>> overloading the parameter for multiple purposes makes code less
>> maintainable, so retaining as it is in HEAD has some merits.
>
> There is no way to tell what that is, but perhaps Heikki recalls
> something on the matter. I am just adding him in CC.

No, I don't remember. Maybe the function originally used the 
caller-supplied 'lsn' value as the value to force-update 
minRecoveryPoint to. Or I anticipated that some callers might want to do 
that in the future.

If we were to do this, it might be better to still have a 'force' 
variable inside the function, to keep the if()s slighltly more readable, 
like:
  bool force = XLogRecPtrIsInvalid(lsn);

But even then, I don't think this makes it really any more readable 
overall. Not worse either, but it's a wash. I'll just mark this as 
rejected in the commitfest, let's move on.

- Heikki