proposal: catch warnings

Hello,

PostgreSQL allow only catch exception (elevel ERROR). SQL/PSM requires that
warnings are catchable too. Simply solution's is adding one callback to
error's processing of errors on level WARNING.

typedef struct WarningHandlerCallback
{
bool (*callback) (void *arg, ErrorData *edata);
void *arg;
} WarningHandlerCallback;

extern DLLIMPORT WarningHandlerCallback *warning_handler;

Callback function returns true if accept warning and process it. This
function is called from errfinish()

/*
* Emit the message to the right places. If warning_handler is
defined,
* try use warning_handler. Emit message only if handler don't
accept
* message (returns false). Warning handlers are used in PL/pgPSM
language.
*/
if (elevel == WARNING)
{
bool handled = false;

if (warning_handler)
handled =
(*warning_handler->callback)(warning_handler->arg,edata);

if (!handled)
EmitErrorReport();
}
else
EmitErrorReport();

It's propably usable only for SQL/PSM implementation, and it's one from two
necessery hacks to core for this PL (second is scrollable cursor's support).
But without this hook I cannot simply distribute plpgpsm.

Any comments?

Best regards

Pavel Stehule

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