Add zfs_gitrev.h to the distributed sources

Commit 109d2c9310 ("Move zfs_gitrev.h to build directory") stopped
distributing zfs_gitrev.h, as it is a generated file. Add it back, with
some changes in behavior.

Change the logic for gitrev as follows
- if the source tree is a git repository, the behavior for build is
  unchanged. For make dist, append -dist to the git tag in the
  distributed version of zfs_gitrev.h.
- otherwise, check if the source tree contains zfs_gitrev.h, and use it
  if so, falling back to "unknown" if it doesn't exist.
- clean it only in make maintainer-clean, so we don't remove it from the
  source tree on make clean or make distclean.

This allows disted sources to track what git tag they originally came
from, with the -dist suffix indicating that the code wasn't built
directly from git and so might contain additional changes beyond the git
tag.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10595

Makefile.am

@@ -50,7 +50,6 @@ EXTRA_DIST += module/zfs/THIRDPARTYLICENSE.cityhash.descrip
 @CODE_COVERAGE_RULES@
 
 GITREV = include/zfs_gitrev.h
-CLEANFILES = $(GITREV)
 
 PHONY = gitrev
 gitrev:
@@ -59,6 +58,9 @@ gitrev:
 all: gitrev
 
 # Double-colon rules are allowed; there are multiple independent definitions.
+maintainer-clean-local::
+	-$(RM) $(GITREV)
+
 distclean-local::
 	-$(RM) -R autom4te*.cache build
 	-find . \( -name SCCS -o -name BitKeeper -o -name .svn -o -name CVS \
@@ -76,6 +78,7 @@ all-local:
 	    ${top_builddir}/scripts/zfs-tests.sh -c
 
 dist-hook:
+	$(AM_V_GEN)$(top_srcdir)/scripts/make_gitrev.sh -D $(distdir) $(GITREV)
 	$(SED) ${ac_inplace} -e 's/Release:[[:print:]]*/Release:      $(RELEASE)/' \
 		$(distdir)/META
 

scripts/make_gitrev.sh

@@ -27,10 +27,21 @@
 
 set -e -u
 
+dist=no
+distdir=.
+while getopts D: flag
+do
+	case $flag in
+		\?) echo "Usage: $0 [-D distdir] [file]" >&2; exit 1;;
+		D)  dist=yes; distdir=${OPTARG};;
+	esac
+done
+shift $((OPTIND - 1))
+
 top_srcdir="$(dirname "$0")/.."
 GITREV="${1:-include/zfs_gitrev.h}"
 
-# GITREV should be a relative path (relative to top_builddir)
+# GITREV should be a relative path (relative to top_builddir or distdir)
 case "${GITREV}" in
 	/*) echo "Error: ${GITREV} should be a relative path" >&2
 	    exit 1;;
@@ -38,10 +49,27 @@ esac
 
 ZFS_GITREV=$({ cd "${top_srcdir}" &&
 	git describe --always --long --dirty 2>/dev/null; } || :)
+
+if [ "x${ZFS_GITREV}" = x ]
+then
+	# If the source directory is not a git repository, check if the file
+	# already exists (in the source)
+	if [ -f "${top_srcdir}/${GITREV}" ]
+	then
+		ZFS_GITREV="$(sed -n \
+			'1s/^#define[[:blank:]]ZFS_META_GITREV "\([^"]*\)"$/\1/p' \
+			"${top_srcdir}/${GITREV}")"
+	fi
+elif [ ${dist} = yes ]
+then
+	# Append -dist when creating distributed sources from a git repository
+	ZFS_GITREV="${ZFS_GITREV}-dist"
+fi
 ZFS_GITREV=${ZFS_GITREV:-unknown}
 
 GITREVTMP="${GITREV}~"
 printf '#define\tZFS_META_GITREV "%s"\n' "${ZFS_GITREV}" >"${GITREVTMP}"
+GITREV="${distdir}/${GITREV}"
 if cmp -s "${GITREV}" "${GITREVTMP}"
 then
 	rm -f "${GITREVTMP}"