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m4sh: Require shell to support $(...) command substitution.

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As of the 2020-11-07 update, config.sub and config.guess
unconditionally use $(...) command substitution; see
<https://lists.gnu.org/archive/html/config-patches/2020-11/msg00011.html>.

Therefore, add this to the set of required shell features, searched
for by _AS_DETECT_BETTER_SHELL.  On a system where /bin/sh doesn’t
support $(...), $CONFIG_SHELL will be set to one that does (and the
primary configure script will be re-executed using that shell).
AC_CANONICAL_* use $CONFIG_SHELL to execute config.guess/sub, so they
will keep working.  This also means that configure scripts and
third-party macros that use $(...) will quietly start working
correctly on such ancient systems.

The test code is simple, but sufficient to weed out Solaris 10’s
/bin/sh, which doesn’t support $(...) but *does* support shell
functions.

I’m not going to touch any of the existing uses of `...` command
substitution in Autoconf proper for now, but it might make sense to
bulk upgrade them early in the 2.71 release cycle; if nothing else,
it would remove a major obstacle to running shellcheck over our
scripts.

* lib/m4sugar/m4sh.m4 (_AS_MODERN_CMDSUBST_WORKS): New macro.
  (AS_INIT, AS_SHELL_SANITIZE): Call _AS_DETECT_REQUIRED for
  _AS_MODERN_CMDSUBST_WORKS.
* NEWS: Mention the requirement for $(...).
This commit is contained in:
Zack Weinberg 2020-11-09 15:15:23 -05:00
parent d1a65dc043
commit 3b411849a0
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13
NEWS
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@ -116,6 +116,19 @@ GNU Autoconf NEWS - User visible changes.
old, and unlikely to be able to handle modern C programs for other
reasons (e.g. not having a C90-compliant compiler at all).
*** Configure scripts require support for $( ... ) command substitution.
This POSIX shell feature is approximately the same age as
user-defined functions, but there do exist shells that support
functions and not $( ... ), such as Solaris 10 /bin/sh.
Configure scripts will automatically locate a shell that supports
this feature and re-execute themselves with it, if necessary, so
the new requirement should be transparent to most users.
In this release, most of Autoconf’s code still uses the older `...`
notation for command substitution.
*** AC_INIT now trims extra white space from its arguments.
For instance, AC_INIT([ GNU Hello ], [1.0]) will set PACKAGE_NAME

11
lib/m4sugar/m4sh.m4
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@ -468,6 +468,15 @@ AS_IF([( set x; as_fn_ret_success y && test x = "[$]1" )], [],
test x$exitcode = x0[]])# _AS_SHELL_FN_WORK
# _AS_MODERN_CMDSUBST_WORKS
# -------------------------
# This is a spy to detect "in the wild" shells that do not support
# the newer $(...) form of command substitutions.
m4_define([_AS_MODERN_CMDSUBST_WORKS],
[blah=$(echo $(echo blah))
test x"$blah" = xblah])
# _AS_SHELL_SANITIZE
# ------------------
# This is the prolog that is emitted by AS_INIT and AS_INIT_GENERATED;
@ -544,6 +553,7 @@ m4_define([AS_SHELL_SANITIZE],
m4_provide_if([AS_INIT], [],
[m4_provide([AS_INIT])
_AS_DETECT_REQUIRED([_AS_SHELL_FN_WORK])
_AS_DETECT_REQUIRED([_AS_MODERN_CMDSUBST_WORKS])
_AS_DETECT_REQUIRED([_AS_TEST_X_WORKS])
_AS_DETECT_BETTER_SHELL
_AS_UNSET_PREPARE
@ -2177,6 +2187,7 @@ m4_divert_text([M4SH-INIT-FN], [m4_text_box([M4sh Shell Functions.])])
m4_divert([BODY])dnl
m4_text_box([Main body of script.])
_AS_DETECT_REQUIRED([_AS_SHELL_FN_WORK])dnl
_AS_DETECT_REQUIRED([_AS_MODERN_CMDSUBST_WORKS])dnl
_AS_DETECT_REQUIRED([_AS_TEST_X_WORKS])dnl
AS_REQUIRE([_AS_UNSET_PREPARE], [], [M4SH-INIT-FN])dnl
])