Re: try/catch macros for Postgres backend

Joe Conway <mail(at)joeconway(dot)com> writes:
> This is especially a problem when the cleanup needs to be done inside
> the embedded interpreter. I found that with R, I had to throw an error
> in the R interpreter in order to allow the interpreter to clean up its
> own state. That left me with code like this:
> [ snip ]
> Looks good to me, but I worry about being able to do what I've described
> above. Basically I found that if I don't allow R to clean up after
> itself by propagating the SPI call generated error into R, before
> throwing a Postgres ERROR, I wind up with core dumps.

You could still do that, and perhaps even a bit more cleanly:

sqlErrorOccurred = false;
PG_TRY();
{
ans = R_tryEval(call, R_GlobalEnv, &errorOccurred);
}
PG_CATCH();
{
sqlErrorOccurred = true;
/* push PG error into R machinery */
error("%s", "error executing SQL statement");
}
PG_END_TRY();

if (sqlErrorOccurred)
PG_RE_THROW();
if (errorOccurred)
ereport(ERROR, "report R error here");

(The ereport will trigger only for errors originating in R, not for
PG errors propagated out, which exit via the RE_THROW.)

However I wonder whether either of these really work. What happens
inside R's "error()" routine, exactly? A longjmp? It seems like this
structure is relying on the stack not to get clobbered between elog.c's
longjmp and R's. Which would usually work, except when you happened to
get a signal during those few instructions...

It seems like what you really need is a TRY inside each of the functions
you offer as callbacks from R to PG. These would catch errors, return
them as failures to the R level, which would in turn fail out to the
tryEval call, and from there you could RE_THROW the original error
(which will still be patiently waiting in elog.c).

regards, tom lane