submodule: do not copy unknown update mode from .gitmodules

When submodule.$name.update is given as hint from the upstream in
the .gitmodules file, we used to blindly copy it to .git/config,
unless there already is a value defined for the submodule.

However, there is no reason to expect that the update mode hinted by
the upstream is available in the version of Git the user is using,
and a really custom "!cmd" prepared by an upstream person running on
Linux may not even be available to a user on Windows.  It is simply
irresponsible to copy the setting blindly and to attempt to use it
during a later "submodule update" without validating it first.

Just show the suggested value to the diagnostic output, and set the
value to 'none' in the configuration, if it is not one of the ones
that are known to be supported by this version of Git.

Helped-by: Jens Lehmann <Jens.Lehmann@web.de>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -612,11 +612,21 @@ cmd_init()
 		fi
 
 		# Copy "update" setting when it is not set yet
-		upd="$(git config -f .gitmodules submodule."$name".update)"
-		test -z "$upd" ||
-		test -n "$(git config submodule."$name".update)" ||
-		git config submodule."$name".update "$upd" ||
-		die "$(eval_gettext "Failed to register update mode for submodule path '\$displaypath'")"
+		if upd="$(git config -f .gitmodules submodule."$name".update)" &&
+		   test -n "$upd" &&
+		   test -z "$(git config submodule."$name".update)"
+		then
+			case "$upd" in
+			rebase | merge | none)
+				;; # known modes of updating
+			*)
+				echo >&2 "warning: unknown update mode '$upd' suggested for submodule '$name'"
+				upd=none
+				;;
+			esac
+			git config submodule."$name".update "$upd" ||
+			die "$(eval_gettext "Failed to register update mode for submodule path '\$displaypath'")"
+		fi
 	done
 }
 

t/t7406-submodule-update.sh

@@ -323,6 +323,21 @@ test_expect_success 'submodule update - command in .git/config catches failure'
 	)
 '
 
+test_expect_success 'submodule init does not copy command into .git/config' '
+	(cd super &&
+	 H=$(git ls-files -s submodule | cut -d" " -f2) &&
+	 mkdir submodule1 &&
+	 git update-index --add --cacheinfo 160000 $H submodule1 &&
+	 git config -f .gitmodules submodule.submodule1.path submodule1 &&
+	 git config -f .gitmodules submodule.submodule1.url ../submodule &&
+	 git config -f .gitmodules submodule.submodule1.update !false &&
+	 git submodule init submodule1 &&
+	 echo "none" >expect &&
+	 git config submodule.submodule1.update >actual &&
+	 test_cmp expect actual
+	)
+'
+
 test_expect_success 'submodule init picks up rebase' '
 	(cd super &&
 	 git config -f .gitmodules submodule.rebasing.update rebase &&