abuild: implement locking of downloaded files

We need locking Since the build servers use a shared download dir
and multiple vservers might want download same file at same time.

fixes #873

abuild.in

@@ -86,6 +86,9 @@ cleanup() {
 		msg "Uninstalling dependencies..."
 		$SUDO $APK del --quiet $apk_opt_wait $uninstall_after
 	fi
+	if [ -n "$CLEANUP_FILES" ]; then
+		rm -f $CLEANUP_FILES
+	fi
 }
 
 die() {
@@ -258,7 +261,8 @@ uri_fetch() {
 	local d="${uri##*/}"	# $(basename $uri)
 	local opts
 	[ -n "$quiet" ] && opts="-s"
-	[ -f "$SRCDEST/$d" ] && return 0
+
+	local lockfile="$SRCDEST/$d".lock
 
 	# fix saveas-*://* URIs
 	case "$uri" in
@@ -272,6 +276,14 @@ uri_fetch() {
 	esac
 
 	mkdir -p "$SRCDEST"
+
+	CLEANUP_FILES="$CLEANUP_FILES $lockfile"
+	(
+	flock -n -x 200 || msg "Waiting for ${lockfile##*/}..."
+	flock -w 600 -x 200
+
+	[ -f "$SRCDEST/$d" ] && exit 0 # use exit since its a subshell
+
 	if [ -f "$SRCDEST/$d.part" ]; then
 		msg "Partial download found. Trying to resume"
 		opts="$opts -C -"
@@ -289,6 +301,12 @@ uri_fetch() {
 
 	$fetcher $opts -o "$SRCDEST/$d.part" "$uri" \
 		&& mv "$SRCDEST/$d.part" "$SRCDEST/$d"
+
+	) 200>$lockfile
+
+	local rc=$?
+	rm -f "$lockfile"
+	return $rc
 }
 
 is_remote() {