fix(snap): Install Unit of Measure config file on upgrade (#4125)

* fix(snap): Install Unit of Measure config file on upgrade

* refactor: rename snap options for clarity

* fix: Correct legacy snap option name

Signed-off-by: Farshid Tavakolizadeh <farshid.tavakolizadeh@canonical.com>

snap/hooks/post-refresh

@@ -1,4 +1,28 @@
 #!/bin/bash -e
 
+TAG=$SNAP_INSTANCE_NAME.$(basename "$0")
+
+# read revision/version of the previous installation, set in the pre-refresh hook
+pre_rev=$(snapctl get pre-refresh.revision)
+pre_ver=$(snapctl get pre-refresh.version)
+# read the revision from the legacy lastrev option (EdgeX <2.3)
+[ -z "$pre_rev" ] && pre_rev=$(snapctl get lastrev)
+
+logger --tag $TAG "Refreshing from $pre_ver ($pre_rev) to $SNAP_VERSION ($SNAP_REVISION)"
+
 # set up postgres, if we are upgrading it
 $SNAP/bin/kong-postgres-setup.sh "post-refresh"
+
+# Install the Unit of Measure config file when upgrading from an old version
+# UoM was added in v2.3.0-dev.45 (snap revision 3900):
+# https://github.com/edgexfoundry/edgex-go/pull/4119
+if (( pre_rev < 3900 )); then
+    uom="config/core-metadata/res/uom.toml"
+    logger --tag $TAG "Installing $SNAP/$uom"
+    if [ -f "$SNAP/$uom" ]; then
+        # --no-clobber: copy if missing from target
+        cp --no-clobber "$SNAP/$uom" "$SNAP_DATA/$uom"
+    else
+        logger --stderr --tag $TAG "$SNAP/$uom does not exit."
+    fi
+fi

snap/hooks/pre-refresh

@@ -1,11 +1,13 @@
 #!/bin/bash -e
 
-# save this revision for when we run again in the post-refresh
-snapctl set lastrev="$SNAP_REVISION"
+# save these for use in the post-refresh hook
+snapctl set pre-refresh.revision=$SNAP_REVISION
+snapctl set pre-refresh.version=$SNAP_VERSION
+
+# unset legacy EdgeX <2.3 options
+snapctl unset lastrev release
 
-snapctl set release="jakarta"
-
  # back up the Kong database, so that if the new snap contains a newer
  # postgresql, then the configuration can be imported into the new database
  # in the post-refresh hook.
-$SNAP/bin/kong-postgres-setup.sh "pre-refresh"
+$SNAP/bin/kong-postgres-setup.sh "pre-refresh"