Add a Terraform pitfalls section to the docs (fixes #58) (#59)

Author: ThoSap

README.md

@@ -64,6 +64,10 @@ Further documentation of each Custom Resource can be found here:
 - [Grant](docs/grant.md) - Manage privileges.
 - [DefaultPrivilege](docs/default-privilege.md) - Manage default privileges.
 
+If you manage the Custom Resources with Terraform, please also read:
+
+- [Terraform](docs/terraform.md) - Pitfalls when using the `kubernetes_manifest` resource.
+
 ### Declarative Management
 
 The Operator leverages the power of Kubernetes Custom Resource Definitions (CRDs) to manage PostgreSQL resources declaratively.  

docs/terraform.md

@@ -0,0 +1,103 @@
+# Terraform
+
+The Custom Resources of this Operator can be managed with the
+[`kubernetes_manifest`](https://registry.terraform.io/providers/hashicorp/kubernetes/latest/docs/resources/manifest)
+resource of the `hashicorp/kubernetes` provider.
+
+## Never set optional fields to `null`
+
+**Omit** optional spec fields from the manifest instead of setting them to `null`. A field that is present but `null` produces a permanent in-place update on every single plan:
+
+```hcl
+# module.postgresql_role.kubernetes_manifest.postgresql_role_cr will be updated in-place
+~ resource "kubernetes_manifest" "postgresql_role_cr" {
+    ~ object = {
+        ~ spec = {
+            + comment = (known after apply)
+            ~ flags = {
+                + validUntil = (known after apply)
+              }
+          }
+      }
+  }
+```
+
+### Why this happens
+
+While planning, the provider fills every field with the CRD schema that the configuration does not set with an unknown value,  
+and then takes the value from the prior state again - unless the field was present in the previous configuration.  
+In that case it keeps the value unknown, to give the API server a chance to default it.
+
+A `null` counts as "present" here. As this Operator does not default these fields, the applied object never contains them, the refreshed state holds `null` again,
+and the next plan repeats the same `(known after apply)`. The plan never converges.
+
+A field that is **absent** from the configuration produces no diff at all.
+
+This is a known limitation of the provider, see [hashicorp/terraform-provider-kubernetes#2669](https://github.com/hashicorp/terraform-provider-kubernetes/issues/2669).
+
+### How to avoid it
+
+Build the `spec` with [`merge`](https://developer.hashicorp.com/terraform/language/functions/merge)
+and only add optional attributes when they actually have a value:
+
+```hcl
+variable "comment" {
+  type = string
+  default = null
+  nullable = true
+}
+
+variable "valid_until" {
+  type = string
+  default = null
+  nullable = true
+}
+
+resource "kubernetes_manifest" "postgresql_role_cr" {
+  manifest = {
+    apiVersion = "postgresql.aboutbits.it/v1"
+    kind = "Role"
+
+    metadata = {
+      namespace = var.namespace
+      name = var.name
+    }
+
+    spec = merge(
+      {
+        clusterRef = {
+          namespace = var.cluster_ref_namespace
+          name = var.cluster_ref_name
+        }
+        name = var.role
+        flags = merge(
+          {
+            createdb = var.flag_createdb
+          },
+          var.valid_until == null ? {} : {
+            validUntil = var.valid_until
+          },
+        )
+      },
+      var.comment == null ? {} : {
+        comment = var.comment
+      },
+    )
+  }
+}
+```
+
+For variables that have a non-`null` default, declaring them as `nullable = false` additionally makes Terraform fall back to the default whenever a caller passes `null` explicitly.
+
+### Affected fields
+
+Every optional field of every Custom Resource is affected, in particular:
+
+| Custom Resource     | Optional fields                                                                                |
+|---------------------|------------------------------------------------------------------------------------------------|
+| `ClusterConnection` | `parameters`, `adminSecretRef.namespace`                                                       |
+| `Database`          | `owner`, `reclaimPolicy`, `clusterRef.namespace`                                               |
+| `Schema`            | `owner`, `reclaimPolicy`, `clusterRef.namespace`                                               |
+| `Role`              | `comment`, `passwordSecretRef`, `flags` (including `flags.validUntil`), `clusterRef.namespace` |
+| `Grant`             | `schema`, `objects`, `clusterRef.namespace`                                                    |
+| `DefaultPrivilege`  | `schema`, `clusterRef.namespace`                                                               |