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<== Date ==> <== Thread ==>

Subject: Re: dbGetPdbAddrFromLink dropped from 3.15 again
From: Michael Davidsaver <[email protected]>
To: Andrew Johnson <[email protected]>
Cc: EPICS Techtalk <[email protected]>
Date: Mon, 6 Aug 2018 11:26:04 -0700
On 08/06/2018 10:01 AM, Andrew Johnson wrote:
> On 08/06/2018 11:15 AM, Michael Davidsaver wrote:
>> On 08/04/2018 08:43 AM, Johnson, Andrew N. wrote:
>>> I would be wary of just handing out the dbAddr* pointer though,
>>> since the link could get changed without any notice, so I would
>>> like to add a little more enforcement. Some link types could even
>>> use this to provide access to their raw data buffers, so I’m
>>> wondering about an API like doLocked() where the caller gives us a
>>> function pointer which we call back with a dbAddr*.
>>
>> My thinking has been something like:
>>
>>> int dbGetPdbAddrFromLink(DBLINK *plink, DBADDR *pout);
>>
>> which would be used like
>>
>>> DBLINK *plink = ...;
>>> DBADDR addr;
>>> dbScanLock(plink->precord);
>>> if(dbGetPdbAddrFromLink(plink, &addr)==0) {
>>>    // do something with addr
>>
>> Though this would look different in the context of an aSub function,
>> the point is that this avoids returning a pointer to the internal
>> struct dbAddr by making a copy.  This copy remains valid in
>> perpetuity, although the assurance that both records are covered by
>> the same lock expires as soon as the source record is unlocked.
> 
> I don't believe it's legal to call any current dbLink.h function unless
> you're already holding the lock for the record that owns that link, so
> IMO your dbScanLock() should be superfluous.

Yup, I was just being explicit.

> Copying a dbAddr also copies its pfield member pointer. If the target
> field of the link is using double-buffering that pointer isn't going to
> be valid after the next buffer-switch, but your API doesn't indicate that.

True.

> That's why I'd prefer to have the user provide a callback function; it
> should be obvious that the information provided is only valid for the
> duration of the call (we can also avoid making a copy of the dbAddr):
> 
> typedef long (*dbLinkDbAddrCallback)(const dbAddr *paddr, void *priv);
> long dbLinkDoDbAddr(struct link *plink, dbLinkDbAddrCallback rtn,
>         void *priv);

I don't see how this is significantly different than copying the dbAddr.
As you point out, the complexity comes from handling dbAddr*.  And this
can only be avoided by extracting the relevant information.  Of course
this requires a better idea of what "relevant" is for Till.

If we want to go the callback route, maybe the better way to handle this
is to take the signature of lset::getValue() as a starting point?  Passing
a triple of buffer pointer, DBF code, and element count.

> This would call a new lset routine which only DB links would implement;
> other link types would just return an error (unless they are willing to
> give out access to their local buffer for the link data, in which case
> they could create a dbAddr for that buffer).

Can we defer talk of an API change until there is a second link type which
wants to do this?  DB links are already handled as a special case in other
ways (locking).

> In dbDbLink.c the implementation is just:
> 
> static long doDbAddr(struct link *plink, dbLinkDbAddrCallback rtn,
>         void *priv)
> {
>     DBADDR *paddr = (DBADDR *) plink->value.pv_link.pvt;
> 
>     return rtn(paddr, priv);
> }
> 
> 
> Till, nothing more is likely to happen on this topic without your
> feedback, would this API be sufficient to implement what you need?

I'm not planning to take any immediate action.

Replies:
Re: dbGetPdbAddrFromLink dropped from 3.15 again Till Straumann
References:
dbGetPdbAddrFromLink dropped from 3.15 again Till Straumann
Re: dbGetPdbAddrFromLink dropped from 3.15 again Michael Davidsaver
Re: dbGetPdbAddrFromLink dropped from 3.15 again Johnson, Andrew N.
Re: dbGetPdbAddrFromLink dropped from 3.15 again Michael Davidsaver
Re: dbGetPdbAddrFromLink dropped from 3.15 again Andrew Johnson

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