Merge pull request 'fix: preserve empty list vs null for optional list attributes' (#1) from benvin/fix-empty-list-null-inconsistency into main

Reviewed-on: #1

internal/provider/helpers.go

@@ -16,7 +16,7 @@ func listToStrings(ctx context.Context, l types.List) []string {
 }
 
 func stringsToList(ctx context.Context, ss []string) types.List {
-	if len(ss) == 0 {
+	if ss == nil {
 		return types.ListNull(types.StringType)
 	}
 	elems := make([]types.String, len(ss))
@@ -26,3 +26,14 @@ func stringsToList(ctx context.Context, ss []string) types.List {
 	list, _ := types.ListValueFrom(ctx, types.StringType, elems)
 	return list
 }
+
+// preserveListNullEmptySemantics keeps the prior null/empty distinction when
+// the API returns null for a field that was previously an empty list.
+// Without this, OpenTofu reports "inconsistent result after apply" because
+// the plan/state had [] but the provider returned null.
+func preserveListNullEmptySemantics(prior, current types.List) types.List {
+	if current.IsNull() && !prior.IsNull() && len(prior.Elements()) == 0 {
+		return prior
+	}
+	return current
+}

internal/provider/helpers_test.go

@@ -63,8 +63,11 @@ func TestListToStrings_WithValues(t *testing.T) {
 func TestStringsToList_EmptySlice(t *testing.T) {
 	ctx := context.Background()
 	result := stringsToList(ctx, []string{})
-	if !result.IsNull() {
-		t.Fatalf("expected null list for empty slice, got %v", result)
+	if result.IsNull() {
+		t.Fatalf("expected empty list for empty slice, got null")
+	}
+	if len(result.Elements()) != 0 {
+		t.Fatalf("expected 0 elements, got %d", len(result.Elements()))
 	}
 }
 
@@ -103,3 +106,42 @@ func TestStringsToList_SingleValue(t *testing.T) {
 		t.Fatalf("expected [\"solo\"], got %v", back)
 	}
 }
+
+func TestPreserveListNullEmptySemantics(t *testing.T) {
+	ctx := context.Background()
+	emptyList, _ := types.ListValueFrom(ctx, types.StringType, []types.String{})
+	nullList := types.ListNull(types.StringType)
+	populatedList := stringsToList(ctx, []string{"a"})
+
+	t.Run("prior empty + current null → preserves empty", func(t *testing.T) {
+		result := preserveListNullEmptySemantics(emptyList, nullList)
+		if result.IsNull() {
+			t.Fatal("expected empty list, got null")
+		}
+		if len(result.Elements()) != 0 {
+			t.Fatalf("expected 0 elements, got %d", len(result.Elements()))
+		}
+	})
+
+	t.Run("prior null + current null → stays null", func(t *testing.T) {
+		result := preserveListNullEmptySemantics(nullList, nullList)
+		if !result.IsNull() {
+			t.Fatal("expected null, got non-null")
+		}
+	})
+
+	t.Run("prior empty + current populated → uses current", func(t *testing.T) {
+		result := preserveListNullEmptySemantics(emptyList, populatedList)
+		elems := listToStrings(ctx, result)
+		if len(elems) != 1 || elems[0] != "a" {
+			t.Fatalf("expected [a], got %v", elems)
+		}
+	})
+
+	t.Run("prior populated + current null → stays null", func(t *testing.T) {
+		result := preserveListNullEmptySemantics(populatedList, nullList)
+		if !result.IsNull() {
+			t.Fatal("expected null (prior was populated, not empty — no preservation)")
+		}
+	})
+}

internal/provider/resource_remote.go

@@ -181,6 +181,7 @@ func (r *remoteResource) Create(ctx context.Context, req resource.CreateRequest,
 	}
 
 	state := r.apiToModel(ctx, created)
+	reconcileOptionalLists(&plan, &state)
 	resp.Diagnostics.Append(resp.State.Set(ctx, state)...)
 }
 
@@ -203,6 +204,7 @@ func (r *remoteResource) Read(ctx context.Context, req resource.ReadRequest, res
 	}
 
 	newState := r.apiToModel(ctx, remote)
+	reconcileOptionalLists(&state, &newState)
 	resp.Diagnostics.Append(resp.State.Set(ctx, newState)...)
 }
 
@@ -223,6 +225,7 @@ func (r *remoteResource) Update(ctx context.Context, req resource.UpdateRequest,
 	}
 
 	state := r.apiToModel(ctx, updated)
+	reconcileOptionalLists(&plan, &state)
 	resp.Diagnostics.Append(resp.State.Set(ctx, state)...)
 }
 
@@ -243,6 +246,14 @@ func (r *remoteResource) ImportState(ctx context.Context, req resource.ImportSta
 	resource.ImportStatePassthroughID(ctx, path.Root("name"), req, resp)
 }
 
+func reconcileOptionalLists(prior, current *remoteResourceModel) {
+	current.Patterns = preserveListNullEmptySemantics(prior.Patterns, current.Patterns)
+	current.Blocklist = preserveListNullEmptySemantics(prior.Blocklist, current.Blocklist)
+	current.MutablePatterns = preserveListNullEmptySemantics(prior.MutablePatterns, current.MutablePatterns)
+	current.ImmutablePatterns = preserveListNullEmptySemantics(prior.ImmutablePatterns, current.ImmutablePatterns)
+	current.BanTags = preserveListNullEmptySemantics(prior.BanTags, current.BanTags)
+}
+
 func (r *remoteResource) modelToAPI(ctx context.Context, m remoteResourceModel) remoteAPI {
 	api := remoteAPI{
 		Name:              m.Name.ValueString(),