Re: try/catch macros for Postgres backend

Tom Lane wrote:

> Joe Conway <mail@joeconway.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).
> 

For what it's worth, I think this looks really good. Especially when 
combined with the proposal discussed in the "Sketch of extending error 
handling for subtransactions in functions". PL/Java makes heavy use 
(almost all calls) of TRY/CATCH macros today so any performance 
increase, even a small one, might be significant. And the ability to 
catch an error and actually handle it, hear, hear!

Kind regards,

Thomas Hallgren