Add base64gzip function support to Packer template

hcl2template/function/encoding.go

@@ -0,0 +1,54 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: BUSL-1.1
+
+package function
+
+import (
+	"bytes"
+	"compress/gzip"
+	"encoding/base64"
+	"fmt"
+
+	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/function"
+)
+
+// Base64GzipFunc constructs a function that compresses a string with gzip and then encodes the result in
+// Base64 encoding.
+var Base64GzipFunc = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name: "str",
+			Type: cty.String,
+		},
+	},
+	Type: function.StaticReturnType(cty.String),
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		s := args[0].AsString()
+
+		var b bytes.Buffer
+		gz := gzip.NewWriter(&b)
+		if _, err := gz.Write([]byte(s)); err != nil {
+			return cty.UnknownVal(cty.String), fmt.Errorf("failed to write gzip raw data: %w", err)
+		}
+		if err := gz.Flush(); err != nil {
+			return cty.UnknownVal(cty.String), fmt.Errorf("failed to flush gzip writer: %w", err)
+		}
+		if err := gz.Close(); err != nil {
+			return cty.UnknownVal(cty.String), fmt.Errorf("failed to close gzip writer: %w", err)
+		}
+		return cty.StringVal(base64.StdEncoding.EncodeToString(b.Bytes())), nil
+	},
+})
+
+// Base64Gzip compresses a string with gzip and then encodes the result in
+// Base64 encoding.
+//
+// Packer uses the "standard" Base64 alphabet as defined in RFC 4648 section 4.
+//
+// Strings in the Packer language are sequences of unicode characters rather
+// than bytes, so this function will first encode the characters from the string
+// as UTF-8, then apply gzip compression, and then finally apply Base64 encoding.
+func Base64Gzip(str cty.Value) (cty.Value, error) {
+	return Base64GzipFunc.Call([]cty.Value{str})
+}

hcl2template/function/encoding_test.go

@@ -0,0 +1,49 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: BUSL-1.1
+
+package function
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/zclconf/go-cty/cty"
+)
+
+func TestBase64Gzip(t *testing.T) {
+	tests := []struct {
+		String cty.Value
+		Want   cty.Value
+		Err    bool
+	}{
+		{
+			cty.StringVal("test"),
+			cty.StringVal("H4sIAAAAAAAA/ypJLS4BAAAA//8BAAD//wx+f9gEAAAA"),
+			false,
+		},
+		{
+			cty.StringVal("helloworld"),
+			cty.StringVal("H4sIAAAAAAAA/8pIzcnJL88vykkBAAAA//8BAAD//60g6/kKAAAA"),
+			false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("base64gzip(%#v)", test.String), func(t *testing.T) {
+			got, err := Base64Gzip(test.String)
+
+			if test.Err {
+				if err == nil {
+					t.Fatal("succeeded; want error")
+				}
+				return
+			} else if err != nil {
+				t.Fatalf("unexpected error: %s", err)
+			}
+
+			if !got.RawEquals(test.Want) {
+				t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, test.Want)
+			}
+		})
+	}
+}

hcl2template/functions.go

@@ -38,6 +38,7 @@ func Functions(basedir string) map[string]function.Function {
 		"basename":           filesystem.BasenameFunc,
 		"base64decode":       encoding.Base64DecodeFunc,
 		"base64encode":       encoding.Base64EncodeFunc,
+		"base64gzip":         pkrfunction.Base64GzipFunc,
 		"bcrypt":             crypto.BcryptFunc,
 		"can":                tryfunc.CanFunc,
 		"ceil":               stdlib.CeilFunc,

website/content/docs/templates/hcl_templates/functions/encoding/base64gzip.mdx

@@ -0,0 +1,32 @@
+---
+page_title: base64gzip - Functions - Configuration Language
+description: The base64encode function compresses the given string with gzip and then
+  encodes the result in Base64.
+---
+
+# `base64gzip` Function
+
+`base64gzip` compresses a string with gzip and then encodes the result in
+Base64 encoding.
+
+Packer uses the "standard" Base64 alphabet as defined in
+[RFC 4648 section 4](https://tools.ietf.org/html/rfc4648#section-4).
+
+Strings in the Packer language are sequences of unicode characters rather
+than bytes, so this function will first encode the characters from the string
+as UTF-8, and then apply Base64 encoding to the result.
+
+The Packer language applies Unicode normalization to all strings, and so
+passing a string through `base64decode` and then `base64encode` may not yield
+the original result exactly.
+
+While we do not recommend manipulating large, raw binary data in the Packer
+language, this function can be used to compress reasonably sized text strings
+generated within the Packer language. For example, the result of this
+function can be used to create a compressed object in Amazon S3 as part of
+an S3 website.
+
+## Related Functions
+
+- [`base64decode`](/packer/docs/templates/hcl_templates/functions/encoding/base64decode) performs the opposite operation,
+  decoding Base64 data and interpreting it as a UTF-8 string.

website/data/docs-nav-data.json

@@ -427,6 +427,10 @@
                     "title": "base64encode",
                     "path": "templates/hcl_templates/functions/encoding/base64encode"
                   },
+                  {
+                    "title": "base64gzip",
+                    "path": "templates/hcl_templates/functions/encoding/base64gzip"
+                  },
                   {
                     "title": "csvdecode",
                     "path": "templates/hcl_templates/functions/encoding/csvdecode"