clean.sh: avoid cleaning build directories while build is running

clean.sh

@@ -6,5 +6,21 @@ set -e -u
 test -f $HOME/.termuxrc && . $HOME/.termuxrc
 : ${TERMUX_TOPDIR:="$HOME/.termux-build"}
 
-[ -d "$TERMUX_TOPDIR" ] && chmod +w -R $TERMUX_TOPDIR
-rm -Rf /data/* $TERMUX_TOPDIR
+
+# Lock file. Same as used in build-package.sh.
+TERMUX_BUILD_LOCK_FILE="/tmp/.termux-build.lck"
+if [ ! -e "$TERMUX_BUILD_LOCK_FILE" ]; then
+	touch "$TERMUX_BUILD_LOCK_FILE"
+fi
+
+{
+	if ! flock -n 5; then
+		echo "Not cleaning build directory since you have unfinished build running."
+		exit 1
+	fi
+
+	if [ -d "$TERMUX_TOPDIR" ]; then
+		chmod +w -R $TERMUX_TOPDIR
+	fi
+	rm -Rf /data/* $TERMUX_TOPDIR
+} 5< "$TERMUX_BUILD_LOCK_FILE"