deploy script now can delete old version and can update html directory using debug mode for temporary update.

deploy.sh

-#/bin/bash
-
-rm -rf /tmp/goweb
-mkdir -p /tmp/goweb
-cp -a . /tmp/goweb/
-rm -rf /tmp/goweb/html/*
-rsync -avh /mnt/hgfs/workspace/2021-07-31-BiukopWeb/ /tmp/goweb/html/
-
-cd /tmp/goweb
-gcloud app deploy

deploy/.vscode/launch.json

+{
+    // Use IntelliSense to learn about possible attributes.
+    // Hover to view descriptions of existing attributes.
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "type": "bashdb",
+            "request": "launch",
+            "name": "Bash-Debug (simplest configuration)",
+            "program": "${file}"
+        }
+    ]
+}

deploy/config.sh

+#!/bin/bash
+PROJ_DIR="/home/sp/go/src/goweb"
+
+UPDATE_HTML="update-go-web-html"
+HTML_WORKDIR="/tmp/$UPDATE_HTML"
+REMOTE_WORK="/home/sp/$UPDATE_HTML"
+

deploy/deploy.sh

+#/bin/bash
+SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
+PROJ_DIR="$SCRIPT_DIR/../"
+
+rm -rf /tmp/goweb
+mkdir -p /tmp/goweb
+rsync  -a  $PROJ_DIR /tmp/goweb/
+rm -rf /tmp/goweb/html/*
+rsync -avh /mnt/hgfs/workspace/2021-07-31-BiukopWeb/ /tmp/goweb/html/
+
+cd /tmp/goweb
+#gcloud app deploy
+
+source $SCRIPT_DIR/gcp_version.sh
+del_old_instances

deploy/gcp_version.sh

+#!/bin/bash
+
+GCP_VERSIONS=()
+LAST_VERSION_ID=""
+INSTANCE_ID=""
+
+get_instance_id() {
+#  SERVICE  VERSION          ID                                VM_STATUS  VM_LIVENESS  DEBUG_MODE
+#  default  20210807t221459  aef-default-20210807t221459-1nqz  RUNNING    HEALTHY      YES
+    echo "Getting gcloud instance id ..."
+    if [ "$INSTANCE_ID" == "" ]
+    then
+      output=` gcloud app instances list | grep RUNNING`    # get the running version only
+      INSTANCE_ID=(` echo $output | awk -F' ' '{print $3}' `)
+      LAST_VERSION_ID=(` echo $output | awk -F' ' '{print $2}' `)
+    fi
+}
+
+del_old_instances(){
+  get_instance_id
+  echo "RUNNING VERSION: " $LAST_VERSION_ID
+
+  GCP_VERSIONS=(` gcloud app versions list | awk -F' ' '{print $2}' `)
+  LENGTH=${#GCP_VERSIONS[@]}
+  for i in $(seq 1 1 $(expr $LENGTH - 1) )
+  do
+    if [ "${GCP_VERSIONS[$i]}" != "$LAST_VERSION_ID" ] ; then
+      echo "delete old version ${GCP_VERSIONS[$i]} "
+      del_instance_by_version ${GCP_VERSIONS[$i]}
+    else
+      echo "KEEP RUNNING VERSION ${GCP_VERSIONS[$i]}"
+    fi
+  done
+}
+
+del_instance_by_version() {
+  versionId=$1
+  if [ "$versionId" == "" ]; then
+    return
+  fi
+  gcloud app versions delete --service=default $versionId ;
+
+#  while true; do
+#      read -p "Do you wish to delete version $versionId ?" yn
+#      case $yn in
+#          [Yy]* )
+#            echo gcloud app instances delete $INSTANCE_ID --service=default --version=$versionId ;
+#            break;;
+#          [Nn]* ) exit;;
+#          * ) echo "Please answer yes or no.";;
+#      esac
+#  done
+
+}
+

deploy/r_sync_html.sh

+#!/bin/bash
+SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
+
+source $SCRIPT_DIR/config.sh 
+NOW=$(date +"%m-%d-%Y")
+MERGED404PATH=`find /var/lib/docker/overlay2/ -name '404.html' | grep merged `
+SRC="$SCRIPT_DIR/../html/"
+DEST="$(dirname ${MERGED404PATH})/"
+
+#clear
+rm -rf $SRC 
+#untar recreate SRC
+cd "$SCRIPT_DIR/.."
+tar -xvf "$SCRIPT_DIR/../html.tar.gz"
+echo "======= check source directory ========= "
+ls -R $SRC
+
+# command looks like
+# "rsync --delete -avz /home/sp/goweb/html/
+# /var/lib/docker/overlay2/511128e41cf1c2df2f270176d6f8de647ef5bcffe44e0d2e2317a4914123e03c/merged/app/html/
+# >> /home/sp/goweb/rsync-html-$NOW.log"
+echo "=====   rsync --delete -avz $SRC $DEST >> ${SCRIPT_DIR}/rsync-html-$NOW.log  ======"
+rsync --delete -avz $SRC $DEST >> ${SCRIPT_DIR}/rsync-html-$NOW.log
+echo "--- remote log for rsync ---- "
+cat ${SCRIPT_DIR}/rsync-html-$NOW.log
+

deploy/update_html.sh

+#!/bin/bash
+
+source config.sh
+source gcp_version.sh
+
+get_instance_id
+echo INSTANCE ID: $INSTANCE_ID
+echo VERSION: $LAST_VERSION_ID
+
+rm    -rf $HTML_WORKDIR
+mkdir -p  $HTML_WORKDIR
+
+rsync -a $PROJ_DIR/deploy $HTML_WORKDIR
+mkdir -p "$HTML_WORKDIR/html"
+rsync -a /mnt/hgfs/workspace/2021-07-31-BiukopWeb/* "$HTML_WORKDIR/html/"
+#create compressed archive
+echo tar -zcvf -C $HTML_WORKDIR $HTML_WORKDIR/html.tar.gz "$HTML_WORKDIR/html/"
+tar -C $HTML_WORKDIR -zcvf $HTML_WORKDIR/html.tar.gz   "html"
+rm -rf "$HTML_WORKDIR/html/"
+
+rm -f "/home/sp/.ssh/google_compute_known_hosts"
+
+# copy deploy script and html to destination
+echo "===copy $HTML_WORKDIR to remote instance ======"
+gcloud app instances scp --recurse "$HTML_WORKDIR"  $INSTANCE_ID:/home/sp/  --service=default --version=$LAST_VERSION_ID
+# copy html
+echo "===execute remote sync======"
+gcloud app instances ssh $INSTANCE_ID --service=default --version=$LAST_VERSION_ID -- sudo bash $REMOTE_WORK/deploy/r_sync_html.sh

html/404.html

-custom 404
+<html>
+<body>
+custom 404
+<img src="favicon.ico" />
+</body>
+</html>

http_handler.go

 
 func fileSystem404(w http.ResponseWriter, r *http.Request) (doDefaultFileServe bool) {
 	//if not found redirect to /
-	r.URL.Path = "/404.html"
-	return true
+	// r.URL.Path = "/404.html" //not working as some directorys may not be feasible.
+	// return true
+
+	http.Redirect(w, r, "/404.html", http.StatusSeeOther)
+	return false
 }