fix: Handle intermittent network errors

Intermittent network errors may happen when a module's source code is
downloaded over HTTP. That happened to me recently (an unexpected 502
from GitHub), which resulted in a missing module in my Docker image
(that was partially my fault, as the build process should stop in case
of any error).

This PR implements automatic retries (with backoff delay) using an
OS-agnostic `try_n_times` helper.

Author: jwadolowski

build_module.sh

@@ -253,6 +253,30 @@ while true; do
 	fi
 done
 
+#
+# A generic helper function to retry any command with a backoff strategy
+#
+try_n_times() {
+	MAX_ATTEMPTS=$1
+	CMD=$2
+	CLEAN_CMD=$3
+	i=0
+	WAIT_TIME=1
+	while ! $CMD; do
+		i=$(expr $i + 1)
+		if [ $i -le $MAX_ATTEMPTS ]; then
+			echo "Attempt $i failed! Waiting $WAIT_TIME seconds before retry..."
+			sleep $WAIT_TIME
+			test -n "$CLEAN_CMD" && $CLEAN_CMD
+			WAIT_TIME=$(expr $WAIT_TIME \* 2)
+		else
+			echo "$MAX_ATTEMPTS attempts failed!"
+			return 1
+		fi
+	done
+	return 0
+}
+
 #
 # Create temporary build area, with working copy of module source
 #
@@ -277,15 +301,15 @@ else
 			;;
 		"zip")
 			echo "$ME: INFO Downloading module source"
-			wget -O $BUILD_DIR/module.zip $1
+			try_n_times 3 "wget -O $BUILD_DIR/module.zip $1" "rm -f $BUILD_DIR/module.zip"
 			ARCHIVE_DIR=`zipinfo -1 $BUILD_DIR/module.zip | head -n 1 | cut -f1 -d/`
 			unzip $BUILD_DIR/module.zip -d $BUILD_DIR
 			mv $BUILD_DIR/$ARCHIVE_DIR $MODULE_DIR
 			;;
 		*)
 			echo "$ME: INFO Downloading module source"
 			# Assume tarball of some kind
-			wget -O $BUILD_DIR/module.tgz $1
+			try_n_times 3 "wget -O $BUILD_DIR/module.tgz $1" "rm -f $BUILD_DIR/module.tgz"
 			ARCHIVE_DIR=`tar tfz $BUILD_DIR/module.tgz | head -n 1 | cut -f1 -d/`
 			cd $BUILD_DIR
 			tar xfz module.tgz