feat: add downgrade target for 2.1 (#23029)

cmd/influxd/downgrade/downgrade.go

@@ -30,8 +30,11 @@ type migrationTarget struct {
 	kvMigration, sqlMigration int
 }
 
+// migrationTarget int values mean "migrate to this migration number". For example, kvMigration of 15 will result in the
+// downgraded database including migration 15, but migration 15 will not be undone.
 var downgradeMigrationTargets = map[string]migrationTarget{
 	"2.0": {kvMigration: 15, sqlMigration: 0},
+	"2.1": {kvMigration: 18, sqlMigration: 3},
 }
 
 func NewCommand(ctx context.Context, v *viper.Viper) (*cobra.Command, error) {

scripts/ci/test-downgrade.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -exo pipefail
 
-declare -a DOWNGRADE_TARGETS=('2.0')
+declare -a DOWNGRADE_TARGETS=('2.0' '2.1')
 
 declare -r TEST_ORG=testorg
 declare -r TEST_TOKEN=supersecretfaketoken
@@ -17,6 +17,10 @@ function download_older_binary () {
             dl_url=https://dl.influxdata.com/influxdb/releases/influxdb2-2.0.9-linux-amd64.tar.gz
             dl_sha=64b9cfea1b5ca07479a16332056f9e7f806ad71df9c6cc0ec70c4333372b9d26
             ;;
+        2.1)
+            dl_url=https://dl.influxdata.com/influxdb/releases/influxdb2-2.1.1-linux-amd64.tar.gz
+            dl_sha=1688e3afa7f875d472768e4f4f5a909b357287a45a8f28287021e4184a185927
+            ;;
         *)
             >&2 echo Error: Unknown downgrade target "'$target_version'"
             exit 1