hcl2template: add filebase64 function

The filebase64 function aims to read and encode a file's contents into
base64.
This is mostly to support reading the content of a file that is not
valid UTF-8, as is the case with the `file` function.

hcl2template/function/filebase64.go

@@ -0,0 +1,40 @@
+package function
+
+import (
+	"encoding/base64"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/function"
+)
+
+var Filebase64 = function.New(&function.Spec{
+	Params: []function.Parameter{
+		function.Parameter{
+			Name:        "path",
+			Description: "Read a file and encode it as a base64 string",
+			Type:        cty.String,
+		},
+	},
+	Type:         function.StaticReturnType(cty.String),
+	RefineResult: refineNotNull,
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		path := args[0].AsString()
+		content, err := os.ReadFile(path)
+		if err != nil {
+			return cty.NullVal(cty.String), fmt.Errorf("failed to read file %q: %s", path, err)
+		}
+
+		out := &strings.Builder{}
+		enc := base64.NewEncoder(base64.StdEncoding, out)
+		_, err = enc.Write(content)
+		if err != nil {
+			return cty.NullVal(cty.String), fmt.Errorf("failed to write file %q as base64: %s", path, err)
+		}
+		_ = enc.Close()
+
+		return cty.StringVal(out.String()), nil
+	},
+})

hcl2template/function/filebase64_test.go

@@ -0,0 +1,62 @@
+package function
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/zclconf/go-cty/cty"
+)
+
+func TestFilebase64(t *testing.T) {
+	tests := []struct {
+		name           string
+		file           string
+		expectedOutput string
+		expectError    bool
+	}{
+		{
+			"file exists, return base64'd contents, no error",
+			"./testdata/list.tmpl",
+			"JXsgZm9yIHggaW4gbGlzdCB+fQotICR7eH0KJXsgZW5kZm9yIH59Cg==",
+			false,
+		},
+		{
+			"file doesn't exist, return nilval and an error",
+			"./testdata/no_file",
+			"",
+			true,
+		},
+		{
+			"directory passed as arg, should error",
+			"./testdata",
+			"",
+			true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			res, err := Filebase64.Call([]cty.Value{
+				cty.StringVal(tt.file),
+			})
+
+			if tt.expectError && err == nil {
+				t.Fatal("succeeded; want error")
+			}
+
+			if !tt.expectError && err != nil {
+				t.Fatalf("unexpected error: %s", err)
+			}
+
+			if err != nil {
+				return
+			}
+
+			retVal := res.AsString()
+			diff := cmp.Diff(retVal, tt.expectedOutput)
+			if diff != "" {
+				t.Errorf("expected output and returned are different: %s", diff)
+			}
+		})
+	}
+}

hcl2template/functions.go

@@ -60,6 +60,7 @@ func Functions(basedir string) map[string]function.Function {
 		"distinct":           stdlib.DistinctFunc,
 		"element":            stdlib.ElementFunc,
 		"file":               filesystem.MakeFileFunc(basedir, false),
+		"filebase64":         pkrfunction.Filebase64,
 		"fileexists":         filesystem.MakeFileExistsFunc(basedir),
 		"fileset":            filesystem.MakeFileSetFunc(basedir),
 		"flatten":            stdlib.FlattenFunc,