Fixed replacement of instance_pool_id in databricks_cluster (#850)

Added corner case to fix issue #824 where `driver_instance_pool_id` was not explicitly specified and old driver instance pool sent in cluster update request

Author: nfx

CHANGELOG.md

@@ -8,6 +8,7 @@
 * Already deleted `databricks_token` don't fail the apply ([#808](https://github.com/databrickslabs/terraform-provider-databricks/pull/808))
 * Default `terraform-mount` clusters created for mounting for `databricks_aws_s3_mount`, `databricks_azure_adls_gen1_mount`, `databricks_azure_adls_gen2_mount`, and `databricks_azure_blob_mount` have now `spark.scheduler.mode` as `FIFO` ([#828](https://github.com/databrickslabs/terraform-provider-databricks/pull/828))
 * Fixed crash when using non-Azure authentication to mount Azure resources ([#831](https://github.com/databrickslabs/terraform-provider-databricks/issues/831))
+* Fixed replacement of `instance_pool_id` in `databricks_cluster`, when `driver_instance_pool_id` was not explicitly specified ([#824](https://github.com/databrickslabs/terraform-provider-databricks/issues/824))
 * Multiple documentation improvements
 
 **Deprecations**

compute/resource_cluster.go

@@ -250,6 +250,17 @@ func hasClusterConfigChanged(d *schema.ResourceData) bool {
 	return false
 }
 
+// https://github.com/databrickslabs/terraform-provider-databricks/issues/824
+func fixInstancePoolChangeIfAny(d *schema.ResourceData, cluster *Cluster) {
+	oldInstancePool, newInstancePool := d.GetChange("instance_pool_id")
+	oldDriverPool, newDriverPool := d.GetChange("driver_instance_pool_id")
+	if oldInstancePool != newInstancePool &&
+	   oldDriverPool == oldInstancePool && 
+	   oldDriverPool == newDriverPool {
+		cluster.DriverInstancePoolID = cluster.InstancePoolID
+	}
+}
+
 func resourceClusterUpdate(ctx context.Context, d *schema.ResourceData, c *common.DatabricksClient) error {
 	clusters := NewClustersAPI(ctx, c)
 	clusterID := d.Id()
@@ -266,6 +277,7 @@ func resourceClusterUpdate(ctx context.Context, d *schema.ResourceData, c *commo
 			return err
 		}
 		modifyClusterRequest(&cluster)
+		fixInstancePoolChangeIfAny(d, &cluster)
 		clusterInfo, err = clusters.Edit(cluster)
 		if err != nil {
 			return err