config: add file() function for reading file contents

config/interpolate_funcs.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"fmt"
+	"io/ioutil"
 	"strings"
 )
 
@@ -10,10 +11,28 @@ var Funcs map[string]InterpolationFunc
 
 func init() {
 	Funcs = map[string]InterpolationFunc{
+		"file":   interpolationFuncFile,
 		"lookup": interpolationFuncLookup,
 	}
 }
 
+// interpolationFuncFile implements the "file" function that allows
+// loading contents from a file.
+func interpolationFuncFile(
+	vs map[string]string, args ...string) (string, error) {
+	if len(args) != 1 {
+		return "", fmt.Errorf(
+			"file expects 1 arguments, got %d", len(args))
+	}
+
+	data, err := ioutil.ReadFile(args[0])
+	if err != nil {
+		return "", err
+	}
+
+	return string(data), nil
+}
+
 // interpolationFuncLookup implements the "lookup" function that allows
 // dynamic lookups of map types within a Terraform configuration.
 func interpolationFuncLookup(

config/interpolate_funcs_test.go

@@ -1,9 +1,58 @@
 package config
 
 import (
+	"io/ioutil"
+	"os"
 	"testing"
 )
 
+func TestInterpolateFuncFile(t *testing.T) {
+	tf, err := ioutil.TempFile("", "tf")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	path := tf.Name()
+	tf.Write([]byte("foo"))
+	tf.Close()
+	defer os.Remove(path)
+
+	cases := []struct {
+		Args   []string
+		Result string
+		Error  bool
+	}{
+		{
+			[]string{path},
+			"foo",
+			false,
+		},
+
+		// Invalid path
+		{
+			[]string{"/i/dont/exist"},
+			"",
+			true,
+		},
+
+		// Too many args
+		{
+			[]string{"foo", "bar"},
+			"",
+			true,
+		},
+	}
+
+	for i, tc := range cases {
+		actual, err := interpolationFuncFile(nil, tc.Args...)
+		if (err != nil) != tc.Error {
+			t.Fatalf("%d: err: %s", i, err)
+		}
+
+		if actual != tc.Result {
+			t.Fatalf("%d: bad: %#v", i, actual)
+		}
+	}
+}
 func TestInterpolateFuncLookup(t *testing.T) {
 	cases := []struct {
 		M      map[string]string