maint: provide make target to update gnulib to latest

* Makefile.am (gnulib-sync, update-gnulib-to-latest): Add targets to
pull the latest commit of the gnulib submodule, and also to copy over
the files we keep in sync.
* doc/find-maint.texi (How to update Gnulib to latest): Add section.

Makefile.am

@@ -89,6 +89,27 @@ findutils-check-smells:
 	-print0 | \
 	xargs -0 python $(AUXDIR)/src-sniff.py
 
+# Update gnulib to latest, merging some additional files we take from there
+# as well.  This only works if the working tree of both findutils and gnulib
+# are clean (which is checked first).
+# The following is a good start to find additional candidates for copying:
+#  git ls-files \
+#    | sed 's,^.*/,,g; /^gnulib$/d; /^t-t$/d; /\.c$/d' \
+#    | grep -Ff - <( cd  gnulib && git ls-files )
+.PHONY: gnulib-sync update-gnulib-to-latest
+gnulib-sync update-gnulib-to-latest:
+	@( cd $(srcdir) \
+	    && { printf 'gnulib: ' && git -C gnulib describe --always --dirty \
+		  && printf 'findutils: ' &&  git describe --always --dirty \
+		  || echo dirty; \
+		  } | grep 'dirty$$' \
+			&& { echo "error: tree is dirty" >&2; exit 1; } || : \
+	    && git submodule foreach git pull origin master \
+	    && cp -v gnulib/doc/COPYINGv3 COPYING \
+	    && cp -v gnulib/build-aux/bootstrap bootstrap \
+	    && git status --short -- gnulib COPYING bootstrap \
+	)
+
 # Clean coverage files generated by running binaries built with gcc
 # -fprofile-arcs -ftest-coverage.  We touch subdirectories here
 # because the relevant Makefile.am files (which we would otherwise

doc/find-maint.texi

@@ -554,6 +554,23 @@ local diffs to be patched into gnulib beneath the
 @file{gnulib-local}.  Normally however, there is no need for this,
 since gnulib updates are very prompt.
 
+@section How to update Gnulib to latest
+With a non-dirty working tree, the command @code{make update-gnulib-to-latest}
+(or the shorter alias @code{make gnulib-sync} allows, well, to update the
+gnulib submodule.  In detail, that is:
+@enumerate
+@item Fetching the latest upstream gnulib reference.
+@item Copying the files which should stay in sync like
+@file{bootstrap} from gnulib into the findutils working tree.
+@item And finally showing the @code{git status} for the gnulib submodule
+and the above copied files.
+@end enumerate
+After that, the maintainer compares if all is correct, if the findutils build
+and run correct, and finally commits with the new gnulib version, e.g. via
+@code{git gui}.
+
+The @code{gnulib-sync} target can be run any time - after a @code{configure}
+run -, and only rejects to run if the working tree is dirty.
 
 @node Documentation
 @chapter Documentation