MM-31699 Don't downgrade plugins with feature flag for on-prem (#16639)

* Don't downgrade plugins with feature flag for on-prem

* Update app/plugin.go

Co-authored-by: Christopher Poile <cpoile@gmail.com>

* Short circut for plugin doesn't exist.

Co-authored-by: Christopher Poile <cpoile@gmail.com>

app/plugin.go

@@ -909,11 +909,33 @@ func (a *App) installFeatureFlagPlugins() {
 
 		// Check if we already installed this version as InstallMarketplacePlugin can't handle re-installs well.
 		pluginStatus, err := a.Srv().GetPluginStatus(pluginId)
-		if err == nil && pluginStatus.Version == version {
+		pluginExists := err == nil
+		if pluginExists && pluginStatus.Version == version {
 			continue
 		}
 
 		if version != "" && version != "control" {
+			// If we are on-prem skip installation if this is a downgrade
+			license := a.Srv().License()
+			inCloud := license != nil && *license.Features.Cloud
+			if !inCloud && pluginExists {
+				parsedVersion, err := semver.Parse(version)
+				if err != nil {
+					a.Log().Debug("Bad version from feature flag", mlog.String("plugin_id", pluginId), mlog.Err(err), mlog.String("version", version))
+					return
+				}
+				parsedExistingVersion, err := semver.Parse(pluginStatus.Version)
+				if err != nil {
+					a.Log().Debug("Bad version from plugin manifest", mlog.String("plugin_id", pluginId), mlog.Err(err), mlog.String("version", pluginStatus.Version))
+					return
+				}
+
+				if parsedVersion.LTE(parsedExistingVersion) {
+					a.Log().Debug("Skip installation because given version was a downgrade and on-prem installations should not downgrade.", mlog.String("plugin_id", pluginId), mlog.Err(err), mlog.String("version", pluginStatus.Version))
+					return
+				}
+			}
+
 			_, err := a.InstallMarketplacePlugin(&model.InstallMarketplacePluginRequest{
 				Id:      pluginId,
 				Version: version,