hcl2template: add text(encode|decode)base64 funcs

Compared to Terraform, Packer was lacking a capability to encode/decode
strings to/from base64-encoded text encoded with another encoding.

This could be problematic in some cases, mainly when working with
Windows, as most of the OS uses UTF-16LE as its standard encoding for
many operations.

Therefore, we take a page from Terraform here, and add those functions
to what Packer supports in an HCL2 context.

hcl2template/functions.go

@@ -4,6 +4,8 @@
 package hcl2template
 
 import (
+	"bytes"
+	"encoding/base64"
 	"fmt"
 
 	"github.com/hashicorp/go-cty-funcs/cidr"
@@ -19,6 +21,7 @@ import (
 	"github.com/zclconf/go-cty/cty"
 	"github.com/zclconf/go-cty/cty/function"
 	"github.com/zclconf/go-cty/cty/function/stdlib"
+	"golang.org/x/text/encoding/ianaindex"
 )
 
 // Functions returns the set of functions that should be used to when
@@ -102,6 +105,8 @@ func Functions(basedir string) map[string]function.Function {
 		"split":              stdlib.SplitFunc,
 		"strrev":             stdlib.ReverseFunc,
 		"substr":             stdlib.SubstrFunc,
+		"textdecodebase64":   TextDecodeBase64Func,
+		"textencodebase64":   TextEncodeBase64Func,
 		"timestamp":          pkrfunction.TimestampFunc,
 		"timeadd":            stdlib.TimeAddFunc,
 		"title":              stdlib.TitleFunc,
@@ -130,6 +135,98 @@ func Functions(basedir string) map[string]function.Function {
 	return funcs
 }
 
+// TextEncodeBase64Func constructs a function that encodes a string to a target encoding and then to a base64 sequence.
+var TextEncodeBase64Func = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name: "string",
+			Type: cty.String,
+		},
+		{
+			Name: "encoding",
+			Type: cty.String,
+		},
+	},
+	Description:  "Encodes the input string (UTF-8) to the destination encoding. The output is base64 to account for cty limiting strings to NFC normalised UTF-8 strings.",
+	Type:         function.StaticReturnType(cty.String),
+	RefineResult: func(rb *cty.RefinementBuilder) *cty.RefinementBuilder { return rb.NotNull() },
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		encoding, err := ianaindex.IANA.Encoding(args[1].AsString())
+		if err != nil || encoding == nil {
+			return cty.UnknownVal(cty.String), function.NewArgErrorf(1, "%q is not a supported IANA encoding name or alias", args[1].AsString())
+		}
+
+		encName, err := ianaindex.IANA.Name(encoding)
+		if err != nil { // would be weird, since we just read this encoding out
+			encName = args[1].AsString()
+		}
+
+		encoder := encoding.NewEncoder()
+		encodedInput, err := encoder.Bytes([]byte(args[0].AsString()))
+		if err != nil {
+			// The string representations of "err" disclose implementation
+			// details of the underlying library, and the main error we might
+			// like to return a special message for is unexported as
+			// golang.org/x/text/encoding/internal.RepertoireError, so this
+			// is just a generic error message for now.
+			//
+			// We also don't include the string itself in the message because
+			// it can typically be very large, contain newline characters,
+			// etc.
+			return cty.UnknownVal(cty.String), function.NewArgErrorf(0, "the given string contains characters that cannot be represented in %s", encName)
+		}
+
+		return cty.StringVal(base64.StdEncoding.EncodeToString(encodedInput)), nil
+	},
+})
+
+// TextDecodeBase64Func constructs a function that decodes a base64 sequence from the source encoding to UTF-8.
+var TextDecodeBase64Func = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name: "source",
+			Type: cty.String,
+		},
+		{
+			Name: "encoding",
+			Type: cty.String,
+		},
+	},
+	Type:         function.StaticReturnType(cty.String),
+	Description:  "Encodes the input base64 blob from an encoding to utf-8. The input is base64 to account for cty limiting strings to NFC normalised UTF-8 strings.",
+	RefineResult: func(rb *cty.RefinementBuilder) *cty.RefinementBuilder { return rb.NotNull() },
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		encoding, err := ianaindex.IANA.Encoding(args[1].AsString())
+		if err != nil || encoding == nil {
+			return cty.UnknownVal(cty.String), function.NewArgErrorf(1, "%q is not a supported IANA encoding name or alias", args[1].AsString())
+		}
+
+		encName, err := ianaindex.IANA.Name(encoding)
+		if err != nil { // would be weird, since we just read this encoding out
+			encName = args[1].AsString()
+		}
+
+		s := args[0].AsString()
+		sDec, err := base64.StdEncoding.DecodeString(s)
+		if err != nil {
+			switch err := err.(type) {
+			case base64.CorruptInputError:
+				return cty.UnknownVal(cty.String), function.NewArgErrorf(0, "the given value is has an invalid base64 symbol at offset %d", int(err))
+			default:
+				return cty.UnknownVal(cty.String), function.NewArgErrorf(0, "invalid source string: %w", err)
+			}
+		}
+
+		decoder := encoding.NewDecoder()
+		decoded, err := decoder.Bytes(sDec)
+		if err != nil || bytes.ContainsRune(decoded, '�') {
+			return cty.UnknownVal(cty.String), function.NewArgErrorf(0, "the given string contains symbols that are not defined for %s", encName)
+		}
+
+		return cty.StringVal(string(decoded)), nil
+	},
+})
+
 var unimplFunc = function.New(&function.Spec{
 	Type: func([]cty.Value) (cty.Type, error) {
 		return cty.DynamicPseudoType, fmt.Errorf("function not yet implemented")

website/content/docs/templates/hcl_templates/functions/encoding/textdecodebase64.mdx

@@ -0,0 +1,28 @@
+---
+page_title: testdecodebase64 - Functions - Configuration Language
+description: The testdecodebase64 function converts a base64 encoded string, whose underlying encoding is the one specified as argument, into a UTF-8 string.
+---
+
+# `textdecodebase64` Function
+
+Encodes the input string from a speicified encoding into UTF-8.
+The input is base64-encoded to account for HCL's string encoding limitations: they must be UTF-8, NFC-normalised.
+
+Packer uses the "standard" Base64 alphabet as defined in
+[RFC 4648 section 4](https://tools.ietf.org/html/rfc4648#section-4).
+
+The `encoding_name` argument must contain one of the encoding names or aliases recorded in
+[the IANA character encoding registry](https://www.iana.org/assignments/character-sets/character-sets.xhtml).
+
+## Examples
+
+```shell-session
+# Usage: textencodebase64(input_base64, encoding_name)
+> textdecodebase64("SABlAGwAbABvACAAVwBvAHIAbABkAA==", "UTF-16LE")
+Hello World
+```
+
+## Related Functions
+
+- [`base64encode`](/packer/docs/templates/hcl_templates/functions/encoding/base64encode) performs the opposite operation,
+  encoding the UTF-8 bytes for a string as Base64.

website/content/docs/templates/hcl_templates/functions/encoding/textencodebase64.mdx

@@ -0,0 +1,28 @@
+---
+page_title: testencodebase64 - Functions - Configuration Language
+description: The testencodebase64 function converts a UTF-8 NFC input string to a base64 blob that encodes the target encoding's rendering of the input string.
+---
+
+# `textencodebase64` Function
+
+Encodes the input string to the destination encoding.
+The output is base64-encoded to account for HCL's string encoding limitations: they must be UTF-8, NFC-normalised.
+
+Packer uses the "standard" Base64 alphabet as defined in
+[RFC 4648 section 4](https://tools.ietf.org/html/rfc4648#section-4).
+
+The `encoding_name` argument must contain one of the encoding names or aliases recorded in
+[the IANA character encoding registry](https://www.iana.org/assignments/character-sets/character-sets.xhtml).
+
+## Examples
+
+```shell-session
+# Usage: textencodebase64(input_string, encoding_name)
+> textencodebase64("Hello World", "UTF-16LE")
+SABlAGwAbABvACAAVwBvAHIAbABkAA==
+```
+
+## Related Functions
+
+- [`base64encode`](/packer/docs/templates/hcl_templates/functions/encoding/base64encode) performs the opposite operation,
+  encoding the UTF-8 bytes for a string as Base64.

website/data/docs-nav-data.json

@@ -443,6 +443,14 @@
                     "title": "urlencode",
                     "path": "templates/hcl_templates/functions/encoding/urlencode"
                   },
+                  {
+                    "title": "textencodebase64",
+                    "path": "templates/hcl_templates/functions/encoding/textencodebase64"
+                  },
+                  {
+                    "title": "textdecodebase64",
+                    "path": "templates/hcl_templates/functions/encoding/textdecodebase64"
+                  },
                   {
                     "title": "yamldecode",
                     "path": "templates/hcl_templates/functions/encoding/yamldecode"