subst: allow implementor to pick which variables it substitutes

subst.go

@@ -47,7 +47,22 @@ var reGroup = regexp.MustCompile(`\\([0-9]+)`)
 //  - ${variable/re/subst/} expands to the variable, with a regexp replacement.
 //    for instance, ${variable/^([^:]*):/\1/}, where variable=foo:bar, expands
 //    to foo.
+//
+// If the passed VariableMap implements CanSubstitute(key string) bool, then
+// the method is called to determine whether the variable is to be substituted.
+// If the method returns false, the variable is left untouched and is output
+// as-is into the result.
 func Substitute(s string, vars VariableMap) (string, error) {
+
+	type CanSubstitute interface {
+		CanSubstitute(key string) bool
+	}
+
+	cansubst := func(key string) bool { return true }
+	if cs, ok := vars.(CanSubstitute); ok {
+		cansubst = cs.CanSubstitute
+	}
+
 	var out strings.Builder
 	start := 0
 outer:
@@ -104,6 +119,11 @@ outer:
 				break outer
 			}
 
+			if !cansubst(name) {
+				i += delim + 1
+				continue
+			}
+
 			out.WriteString(s[start:subsStart])
 			value, present := vars.Get(name)
 

subst_test.go

@@ -6,9 +6,27 @@
 package shutil
 
 import (
+	"strings"
 	"testing"
 )
 
+type PrefixedVarMap struct {
+	Prefix string
+	Values map[string]string
+}
+
+func (m *PrefixedVarMap) Get(key string) (value string, present bool) {
+	if !strings.HasPrefix(key, m.Prefix) {
+		return "", false
+	}
+	value, present = m.Values[key[len(m.Prefix):]]
+	return
+}
+
+func (m *PrefixedVarMap) CanSubstitute(key string) bool {
+	return strings.HasPrefix(key, m.Prefix)
+}
+
 func TestSubstitute(t *testing.T) {
 
 	t.Run("Simple", func(t *testing.T) {
@@ -69,4 +87,33 @@ func TestSubstitute(t *testing.T) {
 		}
 	})
 
+	t.Run("Partial", func(t *testing.T) {
+
+		tcases := []struct {
+			In, Expected string
+			Error bool
+		}{
+			{`${substitute.variable}`, "value", false},
+			{`${substitute.undefined}`, "", true},
+			{`${nosubstitute.variable}`, `${nosubstitute.variable}`, false},
+		}
+
+		vals := &PrefixedVarMap{
+			Prefix: "substitute.",
+			Values: map[string]string{"variable": "value"},
+		}
+
+		for _, tc := range tcases {
+			t.Run(tc.In, func(t *testing.T) {
+				actual, err := Substitute(tc.In, vals)
+				if tc.Error != (err != nil) {
+					t.Fatalf("unexpected error or success: %v", err)
+				}
+				if actual != tc.Expected {
+					t.Fatalf("expected %q, got %q", tc.Expected, actual)
+				}
+			})
+		}
+	})
+
 }