fix: update vmss on user_data change (CLOUD-3215)

Author: kristina-solovyova

functions.tf

@@ -32,17 +32,19 @@ locals {
   lb_backend_pool_ids  = var.create_lb ? [azurerm_lb_backend_address_pool.lb_backend_pool[0].id] : []
 
   vmss_config = jsonencode({
-    name                            = "${var.prefix}-${var.cluster_name}-vmss"
-    location                        = data.azurerm_resource_group.rg.location
-    zones                           = var.zone != null ? [var.zone] : []
-    resource_group_name             = var.rg_name
-    sku                             = var.instance_type
-    upgrade_mode                    = "Manual"
-    health_probe_id                 = local.vmss_health_probe_id
-    admin_username                  = var.vm_username
-    ssh_public_key                  = local.public_ssh_key
-    computer_name_prefix            = "${var.prefix}-${var.cluster_name}-backend"
-    custom_data                     = ""
+    name                 = "${var.prefix}-${var.cluster_name}-vmss"
+    location             = data.azurerm_resource_group.rg.location
+    zones                = var.zone != null ? [var.zone] : []
+    resource_group_name  = var.rg_name
+    sku                  = var.instance_type
+    upgrade_mode         = "Manual"
+    health_probe_id      = local.vmss_health_probe_id
+    admin_username       = var.vm_username
+    ssh_public_key       = local.public_ssh_key
+    computer_name_prefix = "${var.prefix}-${var.cluster_name}-backend"
+    # NOTE: as custom data (to keep it short but unique) we use the hash of the function app code + user_data input hash
+    # this way we can detect both changes in the function app code and user input and trigger vmss update
+    custom_data                     = base64encode(join("", [local.function_app_code_hash, sha256(var.user_data)]))
     disable_password_authentication = true
     proximity_placement_group_id    = local.placement_group_id
     single_placement_group          = var.vmss_single_placement_group