PG_RE_THROW is mandatory (was Re: jsonpath)

In https://postgr.es/m/1676.1548726280@sss.pgh.pa.us Tom Lane wrote:

> Sure: every errcode we have is unsafe to treat this way.
>
> The backend coding rule from day one has been that a thrown error requires
> (sub)transaction cleanup to be done to make sure that things are back in a
> good state. You can *not* just decide that it's okay to ignore that,
> especially not when invoking code outside the immediate area of what
> you're doing.

elog.h claims that PG_RE_THROW is "optional":

/*----------
* 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
* pgindent will indent the construct nicely.) The error recovery code
* can optionally do PG_RE_THROW() to propagate the same error outwards.

This is obviously wrong; while we have a couple of codesites that omit
it, it's not a generally available coding pattern. I think we should
amend that comment. I propose: "The error recovery code must normally
do PG_RE_THROW() to propagate the error outwards; failure to do so may
leave the system in an inconsistent state for further processing."

Other wording proposals welcome, but I don't want to leave it as is.

--
Álvaro Herrera https://www.2ndQuadrant.com/
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