Made the source tarball reproducible

Makefile.am

@@ -54,10 +54,43 @@ SUBDIRS = \
 	contrib/vagrant-ci/centos-9s-x64
 
 
-# Hide the buildsystem's username, at least with GNU tar.
-TAR_OPTIONS = --owner=0 --group=0
+# Normalize tar header fields so two builds of the same source tree produce a
+# byte-identical tarball, following the GNU tar reproducibility guidance:
+# https://www.gnu.org/software/tar/manual/html_section/Reproducibility.html
+#   --format=posix      stable, version-independent header encoding (configure.ac
+#                       selects tar-pax so $(am__tar) emits posix)
+#   --pax-option=...    keep tar's PID out of extended-header names and omit
+#                       atime/ctime, leaving the archive in the ustar subset
+#   --sort=name         stable member order
+#   --numeric-owner     do not record buildslave user/group names
+#   --owner=0 --group=0 deterministic ownership
+#   --mode=go+u,go-w    deterministic permissions
+# mtime clamping (the manual's --clamp-mtime --mtime) is handled by the
+# touch -d @$$SOURCE_DATE_EPOCH call in dist-hook below.
+TAR_OPTIONS = \
+	--format=posix \
+	--pax-option=exthdr.name=%d/PaxHeaders/%f \
+	--pax-option=delete=atime,delete=ctime \
+	--sort=name \
+	--numeric-owner --owner=0 --group=0 \
+	--mode=go+u,go-w
 export TAR_OPTIONS
 
+# The same guidance says to "run GNU tar in the C locale" alongside the options
+# above; its example invokes "LC_ALL=C tar ...".
+LC_ALL = C
+export LC_ALL
+
+# --no-name strips the mtime from the gzip header. --best is automake's default
+GZIP_ENV = "--best --no-name"
+
+# Clamp every mtime in the source tarball to SOURCE_DATE_EPOCH, so that it
+# follows the commit the tarball was made from rather than the time it was built.
+dist-hook:
+	if [ -n "$$SOURCE_DATE_EPOCH" ]; then \
+	    find $(distdir) -exec touch -d @$$SOURCE_DATE_EPOCH {} + ; \
+	fi
+
 
 EXTRA_DIST = CHANGELOG.md INSTALL README.md LICENSE CFVERSION
 

configure.ac

@@ -69,7 +69,9 @@ m4_define(SERIAL_TESTS, m4_bmatch(AUTOMAKE_VERSION, [^1\.\([0-9]\|1[0-1]\)\(\.\|
 AC_MSG_RESULT(AUTOMAKE_VERSION)
 
 
-AM_INIT_AUTOMAKE([tar-ustar] SERIAL_TESTS)
+dnl tar-pax rather than tar-ustar so that $(am__tar) emits the posix format the
+dnl reproducibility options in Makefile.am ask for, as in masterfiles.
+AM_INIT_AUTOMAKE([tar-pax] SERIAL_TESTS)
 AM_MAINTAINER_MODE([enable])
 
 m4_divert_text([DEFAULTS], [: "${AR_FLAGS=cr}"])

tests/unit/tar_portability_test.sh

@@ -6,8 +6,10 @@ then
     exit 77
 fi
 
-cd "$(dirname $0)/../.."
+cd "$(dirname "$0")"/../.. || exit 1
 
-tar --exclude="tests/acceptance/workdir" --format=ustar -cf /dev/null *
+# Clear TAR_OPTIONS: Makefile.am sets it for "make dist", and the --pax-option
+# in there only works on posix archives, not on the ustar one made here.
+TAR_OPTIONS='' tar --exclude="tests/acceptance/workdir" --format=ustar -cf /dev/null *
 
 exit $?