Re: Allowing printf("%m") only where it actually works

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. This
* 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/2782A2665E8342DF8695F396DBA80C88%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