From aeb085c5f0e98ca4c5648335d2ad2ead6219432d Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <mitchell.hashimoto@gmail.com>
Date: Tue, 22 Jul 2014 06:51:02 -0700
Subject: [PATCH] config: error if variable interpolation can't find variable

---
 config/interpolate.go      |  8 +++++++-
 config/interpolate_test.go | 15 +++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/config/interpolate.go b/config/interpolate.go
index 1c6aeefc9..77bc15117 100644
--- a/config/interpolate.go
+++ b/config/interpolate.go
@@ -193,7 +193,13 @@ func (i *VariableInterpolation) FullString() string {
 
 func (i *VariableInterpolation) Interpolate(
 	vs map[string]string) (string, error) {
-	return vs[i.key], nil
+	v, ok := vs[i.key]
+	if !ok {
+		return "", fmt.Errorf(
+			"%s: value for variable '%s' not found", v)
+	}
+
+	return v, nil
 }
 
 func (i *VariableInterpolation) Variables() map[string]InterpolatedVariable {
diff --git a/config/interpolate_test.go b/config/interpolate_test.go
index 87c2be0ca..f9ccb619f 100644
--- a/config/interpolate_test.go
+++ b/config/interpolate_test.go
@@ -273,3 +273,18 @@ func TestVariableInterpolation(t *testing.T) {
 		t.Fatalf("bad: %#v", actual)
 	}
 }
+
+func TestVariableInterpolation_missing(t *testing.T) {
+	uv, err := NewUserVariable("var.foo")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	i := &VariableInterpolation{Variable: uv, key: "var.foo"}
+	_, err = i.Interpolate(map[string]string{
+		"var.bar": "bar",
+	})
+	if err == nil {
+		t.Fatal("should error")
+	}
+}