Re: BUG #17767: psql: tab-completion causes warnings when standard_conforming_strings = off

I wrote:
> PG Bug reporting form <noreply@postgresql.org> writes:
>> Step to reproduce:
>> 1. Set standard_conforming_strings = off
>> 2. Type `\d _` and press Tab key

>> Actual result:
>> Got `WARNING:  nonstandard use of \\ in a string literal`

> Yeah, that should be coded a bit more robustly.  Will fix, thanks
> for the report!

Actually, that seems to be considerably more easily said than done.
I'd thought it'd only require a localized fix, but the issue is
that tab-complete.c is depending on PQescapeStringConn which
only cares about generating a legal literal, not about avoiding
the escape_string_warning warning.  To fix, we'd need to

(1) modify what tab-complete.c's escape_string() does, which isn't hard;

(2) modify every query string that incorporates an escape_string result,
along the lines of

-" WHERE d.datname LIKE '%s' "\
+" WHERE d.datname LIKE E'%s' "\

It might be better to change to

+" WHERE d.datname LIKE %s "\

and make escape_string() responsible for adding the E and quotes,
so that it's less likely someone forgets the E.  In any case,
step (2) is going to be enormously invasive to tab-complete.c,
and there'd be substantial risk of introducing new bugs.

This problem has been there a long time, though it doesn't manifest
in exactly this way before v15 changed tab-complete's implementation
of name matching.  In v14, I can get it to happen with

\d a\b<TAB>

which is less likely to be something somebody would try.

I'm having a hard time getting excited about making such a change,
TBH.  Why is it that you are running with
standard_conforming_strings = off and escape_string_warning = on
anyway?  If you haven't yet converted your apps to support
standard-conforming strings, you probably aren't intent on
doing so in the near future, so you might as well turn off
escape_string_warning.  We last worried about silencing such
warnings in our client programs more than a dozen years ago;
I'm not sure we should still be worried in 2023.

            regards, tom lane