From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@enterprisedb.com>
cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Allowing printf("%m") only where it actually works
In-reply-to: <1437.1534100930@sss.pgh.pa.us>
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Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Sun, 12 Aug 2018 15:08:50 -0400"
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Date: Sat, 18 Aug 2018 16:34:50 -0400
Message-ID: <13183.1534624490@sss.pgh.pa.us>
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I wrote:
> Consider the following approach:
> 1. Teach src/port/snprintf.c about %m.  While I've not written a patch
> for this, it looks pretty trivial.
> 2. Teach configure to test for %m and if it's not there, use the
> replacement snprintf.  (Note: we're already forcing snprintf replacement
> in cross-compiles, so the added run-time test isn't losing anything.)
> 3. Get rid of elog.c's hand-made substitution of %m strings, and instead
> just let it pass the correct errno value down.  (We'd likely need to do
> some fooling in appendStringInfoVA and related functions to preserve
> call-time errno, but that's not complicated, nor expensive.)
> 4. (Optional) Get rid of strerror(errno) calls in favor of %m, even in
> frontend code.

So I started to hack on this, and soon noticed that actually, what elog.c
replaces %m with is *not* the result of strerror(), it's the result of
useful_strerror().  Which, primarily, does this:

    /*
     * Some strerror()s return an empty string for out-of-range errno.  Th=
is
     * is ANSI C spec compliant, but not exactly useful.  Also, we may get
     * back strings of question marks if libc cannot transcode the message=
 to
     * the codeset specified by LC_CTYPE.  If we get nothing useful, first=
 try
     * get_errno_symbol(), and if that fails, print the numeric errno.
     */

I don't know offhand whether glibc's implementation delivers anything
useful for out-of-range errno, but I do know that we've seen the
transcoding problem with it, cf commit 8e68816cc which arose from
this discussion:

https://www.postgresql.org/message-id/flat/2782A2665E8342DF8695F396DBA80C8=
8%40maumau

We could easily move useful_strerror() into snprintf.c, I think
(might need to move pgwin32_socket_strerror there too).  But then
we'd lose its functionality when using glibc.

So now I'm about ready to propose that we just *always* use
snprintf.c, and forget all of the related configure probing.
This'd have some advantages, notably that we'd get the
useful_strerror() behavior in frontend as well as backend,
assuming we converted all our frontend code to use %m.
And we'd not exactly be the first project to decide that.
But it's kind of a big move from where we are today.

Thoughts?

			regards, tom lane