Function to limit interpolation to specific path (#127)

New function `IncludeLookupsInPath` is counterpart to
`ExcludeLookupsInPath`.

bundle/config/interpolation/lookup.go

@@ -20,16 +20,29 @@ func DefaultLookup(path string, lookup map[string]string) (string, error) {
 	return v, nil
 }
 
+func pathPrefixMatches(prefix []string, path string) bool {
+	parts := strings.Split(path, Delimiter)
+	return len(parts) >= len(prefix) && slices.Compare(prefix, parts[0:len(prefix)]) == 0
+}
+
 // ExcludeLookupsInPath is a lookup function that skips lookups for the specified path.
 func ExcludeLookupsInPath(exclude ...string) LookupFunction {
 	return func(path string, lookup map[string]string) (string, error) {
-		parts := strings.Split(path, Delimiter)
-
-		// Skip interpolation of this path.
-		if len(parts) >= len(exclude) && slices.Compare(exclude, parts[0:len(exclude)]) == 0 {
+		if pathPrefixMatches(exclude, path) {
 			return fmt.Sprintf("${%s}", path), nil
 		}
 
 		return DefaultLookup(path, lookup)
 	}
 }
+
+// IncludeLookupsInPath is a lookup function that limits lookups to the specified path.
+func IncludeLookupsInPath(include ...string) LookupFunction {
+	return func(path string, lookup map[string]string) (string, error) {
+		if pathPrefixMatches(include, path) {
+			return DefaultLookup(path, lookup)
+		}
+
+		return fmt.Sprintf("${%s}", path), nil
+	}
+}

bundle/config/interpolation/lookup_test.go

@@ -7,12 +7,14 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestExcludePath(t *testing.T) {
-	tmp := struct {
-		A map[string]string `json:"a"`
-		B map[string]string `json:"b"`
-		C map[string]string `json:"c"`
-	}{
+type interpolationFixture struct {
+	A map[string]string `json:"a"`
+	B map[string]string `json:"b"`
+	C map[string]string `json:"c"`
+}
+
+func fixture() interpolationFixture {
+	return interpolationFixture{
 		A: map[string]string{
 			"x": "1",
 		},
@@ -24,7 +26,10 @@ func TestExcludePath(t *testing.T) {
 			"bx": "${b.x}",
 		},
 	}
+}
 
+func TestExcludePath(t *testing.T) {
+	tmp := fixture()
 	m := interpolate{
 		fn: ExcludeLookupsInPath("a"),
 	}
@@ -37,3 +42,18 @@ func TestExcludePath(t *testing.T) {
 	assert.Equal(t, "${a.x}", tmp.C["ax"])
 	assert.Equal(t, "2", tmp.C["bx"])
 }
+
+func TestIncludePath(t *testing.T) {
+	tmp := fixture()
+	m := interpolate{
+		fn: IncludeLookupsInPath("a"),
+	}
+
+	err := m.expand(&tmp)
+	require.NoError(t, err)
+
+	assert.Equal(t, "1", tmp.A["x"])
+	assert.Equal(t, "2", tmp.B["x"])
+	assert.Equal(t, "1", tmp.C["ax"])
+	assert.Equal(t, "${b.x}", tmp.C["bx"])
+}