dist: add new "dist-no-built-sources" automake option.

Fixes automake bug https://debbugs.gnu.org/49317.

* bin/automake.in: implement new option "no-dist-built-sources" to
omit the dependency of distdir on $(BUILT_SOURCES).  (Allison's
original patch used the option name dist-pure; trivially renamed.)
* lib/am/distdir.am (distdir) [DIST_BUILT_SOURCES]: conditionalize
the dependency.
* lib/Automake/Options.pm (_is_valid_easy_option): list it.
* doc/automake.texi (List of Automake options): document it.
* NEWS: mention it.
* t/dist-no-built-sources.sh: test it.
* t/list-of-tests.mk (handwritten_TESTS): add it.

7 files changed, 107 insertions, 7 deletions

diff --git a/NEWS b/NEWS
index 0b11c2249..09ade1c58 100644
--- a/NEWS
+++ b/NEWS
@@ -7,20 +7,24 @@ New in ?.?.?:
 
 * New features added
 
+  - PYTHON_PREFIX and PYTHON_EXEC_PREFIX variables now set from Python's
+  sys.prefix and sys.exec_prefix; new configure options
+  --with-python_prefix and --with-python_exec_prefix supported,
+  to specify explicitly.
+
   - Common top-level files can be provided as .md; the non-md version is
   used if both are present:
       AUTHORS ChangeLog INSTALL NEWS README README-alpha THANKS
 
   - CTAGS, ETAGS, SCOPE variables can be set via configure.
 
-  - PYTHON_PREFIX and PYTHON_EXEC_PREFIX variables now set from Python's
-  sys.prefix and sys.exec_prefix; new configure options
-  --with-python_prefix and --with-python_exec_prefix supported,
-  to specify explicitly.
+  - new option "no-dist-built-sources" skips generating $(BUILT_SOURCES)
+  before building the tarball as part of "make dist", that is,
+  omits the dependency of $(distdir): $(BUILT_SOURCES).
 
 * Bugs fixed
 
-  - automake output reproducible.
+  - automake output more reproducible.
 
   - test-driver less likely to clash with tests writing to the same file.
 
diff --git a/bin/automake.in b/bin/automake.in
index 72c35793d..fde572cd1 100644
--- a/bin/automake.in
+++ b/bin/automake.in
@@ -3920,7 +3920,9 @@ sub handle_dist ()
   $output_rules .= file_contents ('distdir',
 				  new Automake::Location,
 				  %transform,
-				  FILENAME_FILTER => $filename_filter);
+				  FILENAME_FILTER => $filename_filter,
+				  DIST_BUILT_SOURCES
+				    => ! option 'no-dist-built-sources');
 }
 
 
diff --git a/doc/automake.texi b/doc/automake.texi
index f1deff813..c1c7b9f2b 100644
--- a/doc/automake.texi
+++ b/doc/automake.texi
@@ -10454,6 +10454,21 @@ disable automatic dependency tracking.
 Don't emit any code related to @code{dist} target.  This is useful
 when a package has its own method for making distributions.
 
+@item @option{no-dist-built-sources}
+@cindex Option, @option{no-dist-built-sources}
+@opindex no-dist-built-sources
+@cindex @command{help2man}, and @samp{dist} target
+@vindex BUILT_SOURCES, and @samp{dist} target
+Don't build @code{BUILT_SOURCES} as part of @code{dist}.  This option
+can be set if building the distribution only requires the source
+files, and doesn't compile anything as a side-effect.  The default is
+for @samp{$(distdir)} to depend on @samp{$(BUILT_SOURCES)} because it
+is common, at least among GNU packages, to want to build the program
+to generate man pages with @code{help2man} (@pxref{Errors with
+distclean}).  Admittedly the default behavior should perhaps be not to
+have the dependency, but to preserve compatibility, we don't want to
+change it now.
+
 @item @option{no-dist-gzip}
 @cindex Option, @option{no-dist-gzip}
 @opindex no-dist-gzip
diff --git a/lib/Automake/Options.pm b/lib/Automake/Options.pm
index b846fee87..6c07e5048 100644
--- a/lib/Automake/Options.pm
+++ b/lib/Automake/Options.pm
@@ -283,12 +283,14 @@ sub _is_valid_easy_option ($)
     no-define
     no-dependencies
     no-dist
+    no-dist-built-sources
     no-dist-gzip
     no-exeext
     no-installinfo
     no-installman
     no-texinfo.tex
     nostdinc
+    pure-dist
     readme-alpha
     serial-tests
     parallel-tests
diff --git a/lib/am/distdir.am b/lib/am/distdir.am
index 774d08b91..3b60b6e0b 100644
--- a/lib/am/distdir.am
+++ b/lib/am/distdir.am
@@ -1,5 +1,5 @@
 ## automake - create Makefile.in from Makefile.am
-## Copyright (C) 2001-2020 Free Software Foundation, Inc.
+## Copyright (C) 2001-2021 Free Software Foundation, Inc.
 
 ## This program is free software; you can redistribute it and/or modify
 ## it under the terms of the GNU General Public License as published by
@@ -75,7 +75,11 @@ if %?SUBDIRS%
 AM_RECURSIVE_TARGETS += distdir distdir-am
 endif %?SUBDIRS%
 
+if %?DIST_BUILT_SOURCES%
+distdir:
+else !%?DIST_BUILT_SOURCES%
 distdir: $(BUILT_SOURCES)
+endif !%?DIST_BUILT_SOURCES%
 	$(MAKE) $(AM_MAKEFLAGS) distdir-am
 
 distdir-am: $(DISTFILES)
diff --git a/t/dist-no-built-sources.sh b/t/dist-no-built-sources.sh
new file mode 100644
index 000000000..8d2d8a099
--- /dev/null
+++ b/t/dist-no-built-sources.sh
@@ -0,0 +1,72 @@
+#! /bin/sh
+# Copyright (C) 2021 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2, or (at your option)
+# any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+# Test the presence and absence of the option no-dist-built-sources.
+
+. test-init.sh
+
+# the tests are almost the same, so do a loop with a couple conditionals.
+for testopt in no-built-sources dist-built-sources; do
+
+  if test "$testopt" = no-built-sources; then
+    sed -e 's/AM_INIT_AUTOMAKE/AM_INIT_AUTOMAKE([no-dist-built-sources])/' \
+        configure.ac >configure.tmp
+    cmp configure.ac configure.tmp && fatal_ 'failed to edit configure.ac'
+    mv -f configure.tmp configure.ac
+  fi
+
+  cat >> configure.ac << 'END'
+AC_OUTPUT
+END
+
+  cat > Makefile.am <<EOF
+BUILT_SOURCES = x.c
+EXTRA_DIST = y.c
+
+x.c:
+	touch \$@
+
+y.c:
+	cp x.c y.c # simulate 'undetectable' dependency on x.c
+EOF
+
+  if test "$testopt" = no-built-sources; then
+    touch y.c # no-built-sources dist needs to have all files already there
+  else
+    : # with default $(BUILT_SOURCES) dep, will try to build y.c by the rule
+  fi
+
+  $ACLOCAL
+  $AUTOMAKE
+  $AUTOCONF
+  ./configure
+  run_make dist
+
+  # In any case, the tarball should contain y.c, but not x.c.
+  # The tarball is always named based on $0, regardless of our options.
+  pkg_ver=$me-1.0
+  ! tar tf "${pkg_ver}".tar.gz "${pkg_ver}"/x.c
+  tar tf "${pkg_ver}".tar.gz "${pkg_ver}"/y.c
+
+  # And x.c should only have been built for the built-sources version.
+  if test "$testopt" = no-built-sources; then
+    # no-built-sources should not have generated this
+    ! test -e x.c
+  else
+    # built-sources build should have it
+    test -e x.c
+  fi
+done
diff --git a/t/list-of-tests.mk b/t/list-of-tests.mk
index 934930b50..277e330a8 100644
--- a/t/list-of-tests.mk
+++ b/t/list-of-tests.mk
@@ -410,6 +410,7 @@ t/dist-missing-m4.sh \
 t/dist-readonly.sh \
 t/dist-repeated.sh \
 t/dist-pr109765.sh \
+t/dist-no-built-sources.sh \
 t/dist-vs-built-sources.sh \
 t/distcleancheck.sh \
 t/distcom2.sh \