automate setting package version

this avoids the need for bumping the version, which is particularly
helpful if one doesn't know yet whether the next release will be a
patch, minor, or major.

we cache the version extracted from git, which also provides a fallback
for the case of somebody rebuilding configure from a tar-ball.

note that it's impossible to determine the version at configure time, so
after git-tagging you need to remember to run version.sh (or autoconf)
prior to rolling a tar-ball.

.gitignore

@@ -1,6 +1,7 @@
 /.autoconf_trace
 /ChangeLog
 /INSTALL
+/VERSION
 /autom4te.cache/
 /aclocal.m4
 /autodefs.h

Makefile.am

@@ -4,7 +4,7 @@
 
 SUBDIRS = src
 bin_SCRIPTS = mbsync-get-cert
-EXTRA_DIST = LICENSES debian isync.spec $(bin_SCRIPTS)
+EXTRA_DIST = LICENSES VERSION debian isync.spec $(bin_SCRIPTS)
 
 LOG_PL = \
     use POSIX qw(strftime); \

configure.ac

@@ -2,7 +2,8 @@
 # SPDX-FileCopyrightText: 2002-2022 Oswald Buddenhagen <ossi@users.sf.net>
 dnl SPDX-License-Identifier: GPL-2.0-or-later
 
-AC_INIT([isync], [1.5.0])
+m4_syscmd([./version.sh])
+AC_INIT([isync], m4_include([VERSION]))
 AC_CONFIG_HEADERS([autodefs.h])
 
 AC_CANONICAL_TARGET

version.sh

@@ -0,0 +1,36 @@
+#!/bin/sh
+# SPDX-FileCopyrightText: (C) 2024 Oswald Buddenhagen <ossi@users.sf.net>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+cd $(dirname $0)
+
+test -e .git || exit
+
+mb=$(git merge-base HEAD "@{upstream}" 2> /dev/null)
+if test -z "$mb"; then
+	# we presume that a failure to find a merge base means no upstream.
+	# and no upstream may mean detached head in the middle of a rebase
+	br=$(git branch | sed -n -e 's/^\* (no branch, rebasing \([^\)]*\))$/\1/p')
+	if test -n "$br"; then
+		mb=$(git merge-base HEAD "$br@{upstream}" 2> /dev/null)
+	fi
+fi
+if test -z "$mb"; then
+	# still no upstream, so just describe HEAD as-is.
+	gver=$(git describe --tags HEAD)
+else
+	# find out whether we have local work, and if so, collapse it into
+	# a single suffix. otherwise, we'd cause pointless rebuilds during
+	# development.
+	gver=$(git describe --tags $mb)
+	lcl=$(git rev-list -n 1 $mb..HEAD)
+	if test -n "$lcl"; then
+		gver="$gver-plus"
+	fi
+fi
+gver=${gver#v}
+pgver=$(cat VERSION 2> /dev/null)
+if [ "x$gver" != "x$pgver" ]; then
+	echo "$gver" > VERSION
+fi
+