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Re: libpq object hooks

From: Andrew Chernow <ac(at)esilo(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Merlin Moncure <mmoncure(at)gmail(dot)com>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, "Patches (PostgreSQL)" <pgsql-patches(at)postgresql(dot)org>
Subject: Re: libpq object hooks
Date: 2008-05-15 17:50:29
Message-ID: 482C77E5.1060500@esilo.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Tom Lane wrote:
> "Merlin Moncure" <mmoncure(at)gmail(dot)com> writes:
>> The problem is the functions PQhookData(conn, hookname) and
>> PQresultHookData(result, hookName).  We need these to work in
>> functions that are not callbacks.  If we eliminate hookname
>> completely, there is no way for libpq to know which private state we
>> are asking for.
> 
> Well, depending on a hook name for this is broken-by-design anyway,
> because there is no way for two independently written libraries to
> be sure they don't choose conflicting hook names.  So the need for
> a hook name has to go away.
> 
> It might work to use the address of the hook callback function as
> a key for retrieving the associated void * pointer.  You'd need to
> not register the same callback function more than once per object,
> but from what I gather here you don't need to.
> 
> 			regards, tom lane
> 
> 

There can be cases to use the same callbacks, although unlikely.  To 
completely avoid collisions, the below would work:

Use the address of a static, maybe an 'int', as a hook hanlde.  Provide 
the user with a macro that can make a hook handle.

typedef void *PGhookHandle;

// Declare an int and point "tokname" at it.  The value doesn't
// matter, its the pointer address we are interested in.
#define PQ_MAKE_HOOK_HANDLE(tokname) \
   static int hh__ ## tokname = 0; \
   static const PGhookHandle tokname = &hh__ ## tokname

As an example, here is what libpqtypes would do:

// libpqtypes hooks.c
PQ_MAKE_HOOK_HANDLE(pqthookhandle);

Now the handle replaces the hookName.  The "const char *hookName" member 
of the PQobjectHooks structure is changed to "const PGhookHanlde 
hookHandle".  This allows for all the flexibility of a const char * w/o 
the collision issues.

// these function prototypes change as well
void *PQhookData(PGconn *, const PGhookHandle);
void *PQresultHookData(PGresult *, const PGhookHandle);

We will send in an updated patch.

-- 
Andrew Chernow
eSilo, LLC
every bit counts
http://www.esilo.com/

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