MKDIR_P now falls back on mkdir -p

* lib/autoconf/programs.m4 (AC_PROG_MKDIR_P):
Fall back on mkdir -p instead of on a relative path to
install-sh, as the latter now seems to be more of a problem
than the former.

NEWS

@@ -27,7 +27,6 @@ GNU Autoconf NEWS - User visible changes.
   This matters only for uses that, contrary to the documentation
   and despite warnings, use m4_divert with numbered diversions.
 
-
 ** New features
 
 *** New macro AC_YEAR2038 for 64-bit time_t.
@@ -57,6 +56,18 @@ GNU Autoconf NEWS - User visible changes.
   and ‘false’ work anyway.  This is for compatibility with C 2023 and
   with C++.
 
+*** AC_PROG_MKDIR_P now falls back on plain 'mkdir -p'.
+
+  When AC_PROG_MKDIR_P cannot find a mkdir implementation that is
+  known to lack race condition bugs, it now falls back on 'mkdir -p'
+  instead of falling back on a relative path to install-sh, as the
+  relative paths now seem to be a more important problem than the
+  problems of ancient mkdir implementations with race condition bugs.
+  See <https://savannah.gnu.org/support/?110740>.  The only ancient
+  mkdir still in common use is Solaris 10 /usr/bin/mkdir, and for that
+  platform AC_PROG_MKDIR_P falls back on /opt/sfw/bin/mkdir which
+  should work.
+
 * Noteworthy changes in release 2.71 (2021-01-28) [stable]
 
 ** Bug fixes, including:

doc/autoconf.texi

@@ -2178,7 +2178,7 @@ without any backslash-newlines.
 @prindex @command{install-sh}
 
 Some Autoconf macros require auxiliary scripts.  @code{AC_PROG_INSTALL}
-and @code{AC_PROG_@w{MKDIR_P}} (@pxref{Particular Programs}) require a
+(@pxref{Particular Programs}) requires a
 fallback implementation of @command{install} called @file{install-sh},
 and the @code{AC_CANONICAL} macros (@pxref{Manual Configuration})
 require the system-identification scripts @file{config.sub} and
@@ -4272,16 +4272,10 @@ and its parent directories if needed, and without race conditions when
 two instances of the program attempt to make the same directory at
 nearly the same time.
 
-This macro uses the @samp{mkdir -p} command if possible.  Otherwise, it
-falls back on invoking @command{install-sh} with the @option{-d} option,
-so your package should
-contain @file{install-sh} as described under @code{AC_PROG_INSTALL}.
-An @file{install-sh} file that predates Autoconf 2.60 or Automake 1.10
-is vulnerable to race conditions, so if you want to support parallel
-installs from
-different packages into the same directory you need to make sure you
-have an up-to-date @file{install-sh}.  In particular, be careful about
-using @samp{autoreconf -if} if your Automake predates Automake 1.10.
+This macro uses the equivalent of the @samp{mkdir -p} command.  Ancient
+versions of @command{mkdir} are vulnerable to race conditions, so if you
+want to support parallel installs from different packages into the same
+directory you should use a non-ancient @command{mkdir}.
 
 This macro is related to the @code{AS_MKDIR_P} macro (@pxref{Programming
 in M4sh}), but it sets an output variable intended for use in other
@@ -4291,11 +4285,6 @@ but @code{MKDIR_P} supports the @option{-m} option, e.g., a makefile
 might invoke @code{$(MKDIR_P) -m 0 dir} to create an inaccessible
 directory, and conversely a makefile should use @code{$(MKDIR_P) --
 $(FOO)} if @var{FOO} might yield a value that begins with @samp{-}.
-Finally, @code{AS_MKDIR_P} does not check for race condition
-vulnerability, whereas @code{AC_PROG_MKDIR_P} does.
-
-@samp{@@MKDIR_P@@} is special, as its value may vary for different
-configuration files.
 
 The result of the test can be overridden by setting the variable
 @code{MKDIR_P} or the cache variable @code{ac_cv_path_mkdir}.
@@ -14208,15 +14197,8 @@ use @code{AC_REQUIRE} directly or indirectly (@pxref{Prerequisite Macros}).
 @defmac AS_MKDIR_P (@var{file-name})
 @asindex{MKDIR_P}
 Make the directory @var{file-name}, including intervening directories
-as necessary.  This is equivalent to @samp{mkdir -p -- @var{file-name}},
-except that it is portable to older versions of @command{mkdir} that
-lack support for the @option{-p} option or for the @option{--}
-delimiter (@pxref{mkdir, , Limitations of Usual Tools}).  Also,
-@code{AS_MKDIR_P}
-succeeds if @var{file-name} is a symbolic link to an existing directory,
-even though Posix is unclear whether @samp{mkdir -p} should
-succeed in that case.  If creation of @var{file-name} fails, exit the
-script.
+as necessary.  This is equivalent to @samp{mkdir -p -- @var{file-name}}.
+If creation of @var{file-name} fails, exit the script.
 
 Also see the @code{AC_PROG_MKDIR_P} macro (@pxref{Particular Programs}).
 @end defmac
@@ -19739,11 +19721,6 @@ through @code{uniq}.
 @c ------------------
 @prindex @command{mkdir}
 @cindex Making directories
-No @command{mkdir} option is portable to older systems.  Instead of
-@samp{mkdir -p @var{file-name}}, you should use
-@code{AS_MKDIR_P(@var{file-name})} (@pxref{Programming in M4sh})
-or @code{AC_PROG_MKDIR_P} (@pxref{Particular Programs}).
-
 Combining the @option{-m} and @option{-p} options, as in @samp{mkdir -m
 go-w -p @var{dir}}, often leads to trouble.  FreeBSD
 @command{mkdir} incorrectly attempts to change the permissions of
@@ -19753,26 +19730,21 @@ any newly-created parents of @var{dir}.
 
 Posix does not clearly specify whether @samp{mkdir -p foo}
 should succeed when @file{foo} is a symbolic link to an already-existing
-directory.  The GNU Core Utilities 5.1.0 @command{mkdir}
-succeeds, but Solaris @command{mkdir} fails.
+directory.  The GNU @command{mkdir}
+succeeds, but Solaris 10 @command{mkdir} fails.
 
 Traditional @code{mkdir -p} implementations suffer from race conditions.
 For example, if you invoke @code{mkdir -p a/b} and @code{mkdir -p a/c}
 at the same time, both processes might detect that @file{a} is missing,
 one might create @file{a}, then the other might try to create @file{a}
-and fail with a @code{File exists} diagnostic.  The GNU Core
-Utilities (@samp{fileutils} version 4.1), FreeBSD 5.0,
-NetBSD 2.0.2, and OpenBSD 2.4 are known to be
-race-free when two processes invoke @code{mkdir -p} simultaneously, but
-earlier versions are vulnerable.  Solaris @command{mkdir} is still
-vulnerable as of Solaris 10, and other traditional Unix systems are
+and fail with a @code{File exists} diagnostic.  Solaris 10 @command{mkdir}
+is vulnerable, and other traditional Unix systems are
 probably vulnerable too.  This possible race is harmful in parallel
 builds when several Make rules call @code{mkdir -p} to
 construct directories.  You may use
-@code{install-sh -d} as a safe replacement, provided this script is
-recent enough; the copy shipped with Autoconf 2.60 and Automake 1.10 is
-OK, but copies from older versions are vulnerable.
-
+@code{install-sh -d} as a safe replacement, for example by setting
+@samp{MKDIR_P='/path/to/install-sh -d'} in the environment of
+@command{configure}, assuming the package distributes @file{install-sh}.
 
 @item @command{mkfifo}
 @itemx @command{mknod}

lib/autoconf/programs.m4

@@ -653,36 +653,30 @@ AC_SUBST(INSTALL_DATA)dnl
 # Do not use -m 0755 and let people choose whatever they expect by
 # setting umask.
 #
-# We cannot accept any implementation of 'mkdir' that recognizes '-p'.
-# Some implementations (such as Solaris 8's) are vulnerable to race conditions:
+# Some implementations (such as Solaris 10's) are vulnerable to race conditions:
 # if a parallel make tries to run 'mkdir -p a/b' and 'mkdir -p a/c'
 # concurrently, both version can detect that a/ is missing, but only
-# one can create it and the other will error out.  Consequently we
-# restrict ourselves to known race-free implementations.
+# one can create it and the other will error out.  Users of these
+# implementations should install and use GNU mkdir instead;
+# on Solaris 10, this is /opt/sfw/bin/mkdir.
 #
 # Automake used to define mkdir_p as 'mkdir -p .', in order to
 # allow $(mkdir_p) to be used without argument.  As in
 #   $(mkdir_p) $(somedir)
 # where $(somedir) is conditionally defined.  However we don't do
 # that for MKDIR_P.
-#  1. before we restricted the check to GNU mkdir, 'mkdir -p .' was
-#     reported to fail in read-only directories.  The system where this
-#     happened has been forgotten.
-#  2. in practice we call $(MKDIR_P) on directories such as
+#  * in practice we call $(MKDIR_P) on directories such as
 #       $(MKDIR_P) "$(DESTDIR)$(somedir)"
 #     and we don't want to create $(DESTDIR) if $(somedir) is empty.
 #     To support the latter case, we have to write
 #       test -z "$(somedir)" || $(MKDIR_P) "$(DESTDIR)$(somedir)"
 #     so $(MKDIR_P) always has an argument.
 #     We will have better chances of detecting a missing test if
 #     $(MKDIR_P) complains about missing arguments.
-#  3. $(MKDIR_P) is named after 'mkdir -p' and we don't expect this
+#   * $(MKDIR_P) is named after 'mkdir -p' and we don't expect this
 #     to accept no argument.
-#  4. having something like 'mkdir .' in the output is unsightly.
+#   * having something like 'mkdir .' in the output is unsightly.
 #
-# On NextStep and OpenStep, the 'mkdir' command does not
-# recognize any option.  It will interpret all options as
-# directories to create.
 AN_MAKEVAR([MKDIR_P], [AC_PROG_MKDIR_P])
 AC_DEFUN_ONCE([AC_PROG_MKDIR_P],
 [AC_REQUIRE_AUX_FILE([install-sh])dnl
@@ -706,11 +700,9 @@ if test -z "$MKDIR_P"; then
   if test ${ac_cv_path_mkdir+y}; then
     MKDIR_P="$ac_cv_path_mkdir -p"
   else
-    # As a last resort, use the slow shell script.  Don't cache a
-    # value for MKDIR_P within a source directory, because that will
-    # break other packages using the cache if that directory is
-    # removed, or if the value is a relative name.
-    MKDIR_P="$ac_install_sh -d"
+    # As a last resort, use plain mkdir -p,
+    # in the hope it doesn't have the bugs of ancient mkdir.
+    MKDIR_P='mkdir -p'
   fi
 fi
 dnl status.m4 does special magic for MKDIR_P instead of AC_SUBST,