Date: Tue, 3 Dec 2019 18:17:39 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
Cc: Alexey Bashtanov <bashtanov@imap.cc>,
	Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: log bind parameter values on error
Message-ID: <20191203211739.GA12872@alvherre.pgsql>
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On 2019-Dec-03, Alvaro Herrera wrote:

> Now, I have to say that this doesn't make me terribly happy, because I
> would like the additional ability to limit the printed values to N
> bytes.  This means the new function would have to have an additional
> argument to indicate the maximum length (pass 0 to print all args
> whole) ... and the logic then because more involved because we need
> logic to stop printing early.

So after actually writing this, it seems that it's better to have a
separate path for this; otherwise you need to scan the whole input
string (strchr or friends), which might be long.  I tried to see how to
make strnlen() work for us, but I found no simple way to print the "..."
at the end.  So this just runs once per char.  I think that's okay,
since the intended usage is to print a few dozen bytes, like
ExecBuildSlotValueDescription does (64 bytes is what all its callers
do).

Another point: this implementation is not strict about how many
input characters it prints.  It counts up to maxlen *output* characters,
which means if it duplicates a ', that's one less input char printed.
(Given the usage of this function, this seems a feature rather than a
bug.  The first implementation did the opposite, and on the whole it
seemed worse.)

If anyone can suggest a smarter implementation, I'm all ears.

(Maybe do strnlen(maxlen), then count strnlen(1) starting at that point
-- so if that returns >=1, print the "..."?)

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services