David Steele <da...@pgmasters.net> writes:
> On 3/10/16 11:00 AM, Petr Jelinek wrote:
>> The comment above errhidefromclient says "Only log levels below ERROR
>> can be hidden from the client." but use of the errhidefromclient(true)
>> actually does hide the error message from client, client just gets
>> failed query without any message when used with ERROR level.

> Right you are.  The v3 patch adds this check.

> I also added the documentation to sources.sgml that Tom pointed out was
> missing.

I set to work on committing this, but was soon rather dissatisfied with
it, because as-implemented there is no way to short-circuit elog.c's
processing if the message is not to be sent to either the client or the
postmaster log.  Ideally this would be taken into account when errstart()
figures the output_to_client setting to begin with --- but of course we
can't do that with this API, because errhidefromclient() hasn't run yet.

The patch is buggy even without that consideration, because it would
potentially disable client output of messages even after they've been
promoted to FATAL by pg_re_throw.  We could fix that by clearing the flag
in pg_re_throw, but I think that's just another symptom of the shutoff
being done in the wrong place.

I wonder just what the expected usage scenario is for this function, and
whether it would not be better addressed by inventing some other API
rather than modeling it on errhidestmt(), which is by no means the same
kind of thing.

One idea is to invent a new elevel which is never sent to the client ---
analogously to COMMERROR, though probably much higher priority than that,
maybe the same priority as LOG.  If there actually is a use for a range of
elevels on errhidefromclient'd messages, that wouldn't work very well of
course.  Or we could consider having a flag bit that is OR'd into the
elevel, along the lines of

        ereport(LOG | ERR_HIDE_FROM_CLIENT, (errmsg(....)));

so that the information is available at errstart time.

                        regards, tom lane

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