Thread: Rename ShmemVariableCache and initialize it in more standard way

This came up in the "Refactoring backend fork+exec code" thread recently 
[0], but is independent of that work:

On 11/07/2023 01:50, Andres Freund wrote:
>> --- a/src/backend/storage/ipc/shmem.c
>> +++ b/src/backend/storage/ipc/shmem.c
>> @@ -144,6 +144,8 @@ InitShmemAllocation(void)
>>      /*
>>       * Initialize ShmemVariableCache for transaction manager. (This doesn't
>>       * really belong here, but not worth moving.)
>> +     *
>> +     * XXX: we really should move this
>>       */
>>      ShmemVariableCache = (VariableCache)
>>          ShmemAlloc(sizeof(*ShmemVariableCache));
> 
> Heh. Indeed. And probably just rename it to something less insane.

Here's a patch to allocate and initialize it with a pair of ShmemSize 
and ShmemInit functions, like all other shared memory structs.

+1 on renaming it too. It's not a cache, the values tracked in 
ShmemVariableCache are the authoritative source. Attached patch renames 
it to "TransamVariables", but I'm all ears for other suggestions.

[0] 
https://www.postgresql.org/message-id/20230710225043.svl7fqxecwshwc7a@awork3.anarazel.de

-- 
Heikki Linnakangas
Neon (https://neon.tech)

On Mon Dec 4, 2023 at 6:49 AM CST, Heikki Linnakangas wrote:
> This came up in the "Refactoring backend fork+exec code" thread recently
> [0], but is independent of that work:
>
> On 11/07/2023 01:50, Andres Freund wrote:
> >> --- a/src/backend/storage/ipc/shmem.c
> >> +++ b/src/backend/storage/ipc/shmem.c
> >> @@ -144,6 +144,8 @@ InitShmemAllocation(void)
> >>      /*
> >>       * Initialize ShmemVariableCache for transaction manager. (This doesn't
> >>       * really belong here, but not worth moving.)
> >> +     *
> >> +     * XXX: we really should move this
> >>       */
> >>      ShmemVariableCache = (VariableCache)
> >>          ShmemAlloc(sizeof(*ShmemVariableCache));
> >
> > Heh. Indeed. And probably just rename it to something less insane.
>
> Here's a patch to allocate and initialize it with a pair of ShmemSize
> and ShmemInit functions, like all other shared memory structs.
>

>  +        if (!IsUnderPostmaster)
>  +        {
>  +                Assert(!found);
>  +                memset(ShmemVariableCache, 0, sizeof(VariableCacheData));
>  +        }
>  +        else
>  +                Assert(found);

Should the else branch instead be a fatal log?

Patches look good to me.
--
Tristan Partin
Neon (https://neon.tech)

On 05/12/2023 05:40, Richard Guo wrote:
> On Tue, Dec 5, 2023 at 12:31 AM Tristan Partin <tristan@neon.tech> wrote:
>     On Mon Dec 4, 2023 at 6:49 AM CST, Heikki Linnakangas wrote:
>      > Here's a patch to allocate and initialize it with a pair of
>     ShmemSize
>      > and ShmemInit functions, like all other shared memory structs.
>      >
>      >  +        if (!IsUnderPostmaster)
>      >  +        {
>      >  +                Assert(!found);
>      >  +                memset(ShmemVariableCache, 0,
>     sizeof(VariableCacheData));
>      >  +        }
>      >  +        else
>      >  +                Assert(found);
> 
>> Should the else branch instead be a fatal log?
> 
> The Assert here seems OK to me.  We do the same when initializing
> commitTsShared/MultiXactState.  I think it would be preferable to adhere
> to this convention.

Right. I'm not 100% happy with that pattern either, but better be 
consistent.

There's a brief comment about this in CreateOrAttachShmemStructs():

>  * This is called by the postmaster or by a standalone backend.
>  * It is also called by a backend forked from the postmaster in the
>  * EXEC_BACKEND case.  In the latter case, the shared memory segment
>  * already exists and has been physically attached to, but we have to
>  * initialize pointers in local memory that reference the shared structures,
>  * because we didn't inherit the correct pointer values from the postmaster
>  * as we do in the fork() scenario.  The easiest way to do that is to run
>  * through the same code as before.  (Note that the called routines mostly
>  * check IsUnderPostmaster, rather than EXEC_BACKEND, to detect this case.
>  * This is a bit code-wasteful and could be cleaned up.)

The last sentence refers to this pattern.

>> Patches look good to me.
> 
> Also +1 to the patches.

Committed, thanks!

-- 
Heikki Linnakangas
Neon (https://neon.tech)