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Re: try/catch macros for Postgres backend

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Jeroen T(dot) Vermeulen" <jtv(at)xs4all(dot)nl>
Cc: Alvaro Herrera Munoz <alvherre(at)dcc(dot)uchile(dot)cl>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: try/catch macros for Postgres backend
Date: 2004-07-29 13:58:54
Message-ID: 13617.1091109534@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
"Jeroen T. Vermeulen" <jtv(at)xs4all(dot)nl> writes:
> ...Or I could try.  Yes, the "finally" block is executed after executing
> the "catch" block if an exception was caught, or when leaving the "try"
> block if there wasn't.  That includes both normal completion and uncaught
> exceptions.

Right.  The last bit (FINALLY executes whether or not a CATCH block
re-throws) seemed too messy to handle in my little macros, so I'm
planning on leaving it out.  But I'm open to the idea if anyone has
a clever implementation thought.

What I have turning over at the moment is

/*----------
 * API for catching ereport(ERROR) exits.  Use these macros like so:
 *
 *		PG_TRY();
 *		{
 *			... code that might throw ereport(ERROR) ...
 *		}
 *		PG_CATCH();
 *		{
 *			... error recovery code ...
 *		}
 *		PG_END_TRY();
 *
 * (The braces are not actually necessary, but are recommended so that
 * pg_indent will indent the construct nicely.)  The error recovery code
 * can optionally do PG_RE_THROW() to propagate the same error outwards.
 *
 * Note: while the system will correctly propagate any new ereport(ERROR)
 * occurring in the recovery section, there is a small limit on the number
 * of levels this will work for.  It's best to keep the error recovery
 * section simple enough that it can't generate any new errors, at least
 * not before popping the error stack.
 *----------
 */
#define PG_TRY()  \
	do { \
		sigjmp_buf *save_exception_stack = PG_exception_stack; \
		ErrorContextCallback *save_context_stack = error_context_stack; \
		sigjmp_buf local_sigjmp_buf; \
		if (sigsetjmp(local_sigjmp_buf, 1) == 0) \
		{ \
			PG_exception_stack = &local_sigjmp_buf

#define PG_CATCH()  \
		} \
		else \
		{ \
			PG_exception_stack = save_exception_stack; \
			error_context_stack = save_context_stack

#define PG_END_TRY()  \
		} \
		PG_exception_stack = save_exception_stack; \
		error_context_stack = save_context_stack; \
	} while (0)

#define PG_RE_THROW()  \
	siglongjmp(*PG_exception_stack, 1)

extern DLLIMPORT sigjmp_buf *PG_exception_stack;


It's passing regression tests but I have some loose ends to fix before
committing.

			regards, tom lane

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