From 69c9a2d20d8572dfead0486aa3a51bae1ac90250 Mon Sep 17 00:00:00 2001
From: Ryan VanGundy <ryan.vangundy+rmvangun@gmail.com>
Date: Sun, 12 Oct 2025 11:30:52 -0400
Subject: [PATCH] feature(blueprint): Mixed Feature interpolation

It is now possible to mix strings and interpolations in Feature inputs, i.e.,

```
domain: grafana.${dns.domain}
```
---
 pkg/blueprint/feature_evaluator.go      | 38 +++++++++-
 pkg/blueprint/feature_evaluator_test.go | 97 +++++++++++++++++++++++++
 2 files changed, 134 insertions(+), 1 deletion(-)

diff --git a/pkg/blueprint/feature_evaluator.go b/pkg/blueprint/feature_evaluator.go
index e747e7960..504e0425e 100644
--- a/pkg/blueprint/feature_evaluator.go
+++ b/pkg/blueprint/feature_evaluator.go
@@ -110,6 +110,9 @@ func (e *FeatureEvaluator) evaluateDefaultValue(value any, config map[string]any
 		if expr := e.extractExpression(v); expr != "" {
 			return e.EvaluateValue(expr, config)
 		}
+		if strings.Contains(v, "${") {
+			return e.interpolateString(v, config)
+		}
 		return v, nil
 
 	case map[string]any:
@@ -139,7 +142,7 @@ func (e *FeatureEvaluator) evaluateDefaultValue(value any, config map[string]any
 	}
 }
 
-// extractExpression checks if a string contains an expression in ${} syntax.
+// extractExpression checks if a string contains a single expression spanning the entire string.
 // If found, returns the expression content. Otherwise returns empty string.
 func (e *FeatureEvaluator) extractExpression(s string) string {
 	if !strings.Contains(s, "${") {
@@ -161,3 +164,36 @@ func (e *FeatureEvaluator) extractExpression(s string) string {
 
 	return ""
 }
+
+// interpolateString replaces all ${expression} occurrences in a string with their evaluated values.
+func (e *FeatureEvaluator) interpolateString(s string, config map[string]any) (string, error) {
+	result := s
+
+	for strings.Contains(result, "${") {
+		start := strings.Index(result, "${")
+		end := strings.Index(result[start:], "}")
+
+		if end == -1 {
+			return "", fmt.Errorf("unclosed expression in string: %s", s)
+		}
+
+		end += start
+		expr := result[start+2 : end]
+
+		value, err := e.EvaluateValue(expr, config)
+		if err != nil {
+			return "", fmt.Errorf("failed to evaluate expression '${%s}': %w", expr, err)
+		}
+
+		var replacement string
+		if value == nil {
+			replacement = ""
+		} else {
+			replacement = fmt.Sprintf("%v", value)
+		}
+
+		result = result[:start] + replacement + result[end+1:]
+	}
+
+	return result, nil
+}
diff --git a/pkg/blueprint/feature_evaluator_test.go b/pkg/blueprint/feature_evaluator_test.go
index c493b3884..320af15fd 100644
--- a/pkg/blueprint/feature_evaluator_test.go
+++ b/pkg/blueprint/feature_evaluator_test.go
@@ -517,6 +517,103 @@ func TestFeatureEvaluator_EvaluateDefaults(t *testing.T) {
 			t.Errorf("Expected literal_number to be 42, got %v", result["literal_number"])
 		}
 	})
+
+	t.Run("InterpolatesStringsWithSingleExpression", func(t *testing.T) {
+		defaults := map[string]any{
+			"domain": "grafana.${dns.domain}",
+		}
+
+		config := map[string]any{
+			"dns": map[string]any{
+				"domain": "example.com",
+			},
+		}
+
+		result, err := evaluator.EvaluateDefaults(defaults, config)
+		if err != nil {
+			t.Fatalf("Expected no error, got %v", err)
+		}
+
+		if result["domain"] != "grafana.example.com" {
+			t.Errorf("Expected domain to be 'grafana.example.com', got %v", result["domain"])
+		}
+	})
+
+	t.Run("InterpolatesStringsWithMultipleExpressions", func(t *testing.T) {
+		defaults := map[string]any{
+			"url": "${protocol}://${dns.domain}:${port}",
+		}
+
+		config := map[string]any{
+			"protocol": "https",
+			"dns": map[string]any{
+				"domain": "example.com",
+			},
+			"port": 8080,
+		}
+
+		result, err := evaluator.EvaluateDefaults(defaults, config)
+		if err != nil {
+			t.Fatalf("Expected no error, got %v", err)
+		}
+
+		if result["url"] != "https://example.com:8080" {
+			t.Errorf("Expected url to be 'https://example.com:8080', got %v", result["url"])
+		}
+	})
+
+	t.Run("InterpolatesStringsWithNumbers", func(t *testing.T) {
+		defaults := map[string]any{
+			"label": "worker-${cluster.workers.count}",
+		}
+
+		config := map[string]any{
+			"cluster": map[string]any{
+				"workers": map[string]any{
+					"count": 3,
+				},
+			},
+		}
+
+		result, err := evaluator.EvaluateDefaults(defaults, config)
+		if err != nil {
+			t.Fatalf("Expected no error, got %v", err)
+		}
+
+		if result["label"] != "worker-3" {
+			t.Errorf("Expected label to be 'worker-3', got %v", result["label"])
+		}
+	})
+
+	t.Run("FailsOnUnclosedInterpolationExpression", func(t *testing.T) {
+		defaults := map[string]any{
+			"bad": "prefix-${dns.domain",
+		}
+
+		config := map[string]any{
+			"dns": map[string]any{
+				"domain": "example.com",
+			},
+		}
+
+		_, err := evaluator.EvaluateDefaults(defaults, config)
+		if err == nil {
+			t.Fatal("Expected error for unclosed expression, got nil")
+		}
+	})
+
+	t.Run("FailsOnInvalidInterpolationExpression", func(t *testing.T) {
+		defaults := map[string]any{
+			"bad": "prefix-${invalid + }",
+		}
+
+		config := map[string]any{}
+
+		_, err := evaluator.EvaluateDefaults(defaults, config)
+		if err == nil {
+			t.Fatal("Expected error for invalid interpolation expression, got nil")
+		}
+	})
 }
 
 // =============================================================================