configure providing function (Was: Severe performance problem and proposed solution)

[Akim Demaille]

| > From: Akim Demaille <address@hidden>
| > Date: 27 Feb 2002 10:05:42 +0100
| 
| > What we want, is to find a reliable `echo', or any means that would
| > help us remove the thousands of heredocs we have.
| 
| > elif test "X`($echo '\t') 2>/dev/null`" = 'X\t'; then
| >   # Yippee, $echo works!
| 
| Unfortunately this approach won't work in the long run.
| POSIX 1003.2-1992 allows echo '\t' to output backslash-t, but
| POSIX 1003.1-2001 requires that it must output a tab character.
| Eventually shells will head in that direction, albeit reluctantly.

Good to know, thanks!

| Here's an idea.  How about if we use expr instead of cat?  That is
| allowed by the GNU coding standards, and it handles backslash
| correctly.  That is, instead of doing this:
| 
|       cat <<__ACEOF
|       some very
|       long string
|       with funny characters "'\\
|       and with $SHELL expansions in it
|       __ACEOF
| 
| we do this:
| 
|       expr "Xsome very
|       long string
|       with funny characters \"'\\
|       and with $SHELL expansions in it" : 'X\(.*\)'
| 
| If __ACEOF is quoted, the expr version should use ' instead of ",
| and should quote ' rather than " of course.
| 
| The problems I see with this approach are:
| 
| * expr may be limited in the length of the string that it can handle.
|   But we already have this problem with some extent with __ACEOF;
|   and we already use expr in similar ways so there is some precedent.

I find this risky, but why not.

| * " is tricky, e.g.
| 
|       cat <<__ACEOF
|       ${foo+"abc"}
|       __ACEOF
| 
|   will be mishandled if we blindly escape all "s in the expr version.
|   But that is a problem no matter what program we substitute for
|   __ACEOF.

Arg :(

| * expr exits with nonzero status if the string is empty.  But we're
|   already ignoring the exit status of cat, so I don't see this as a
|   real problem.



Actually, all this should be done after an initial step, which is
equipping configure with means to provide replacement functions.  I'm
think about something like

        ./configure --replacement-function=ln -s from to

        ./configure --replacement-function=echo "Hello world"

and so forth.

This mean equipping M4sh which such a feature.  Using diversions,
that's very easy to do.  But my problem is that I would like to be
sure of the order we want to follow between the different tests.

Basically, we have several blocks to order:

- sanitizing the shell (``emulate sh'' etc.).
- re-execution (based on LINENO tests currently)
- analysis of various tool limitations (e.g., as_expr=expr or
  as_expr=false).

where do we want to add:
- command replacement

Paul, I'm especially interested in your opinion.  Currently, I'd
suggest that you toy with autom4te to see how M4sh works.  For
instance:

/tmp % cat script.as                                             nostromo 10:40
AS_INIT
AS_SHELL_SANITIZE
echo `AS_DIRNAME(//1/2)`
/tmp % autom4te -l m4sh script.as -o script                      nostromo 10:40
/tmp % ./script                                                  nostromo 10:40
//1


then, where would you place the handling of --replacement-function in
script?

Thanks!