[Butane] Support for external templating systems

butane/base/util/file.go

@@ -15,6 +15,7 @@
 package util
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
@@ -37,17 +38,29 @@ func EnsurePathWithinFilesDir(path, filesDir string) error {
 	return nil
 }
 
-func ReadLocalFile(configPath, filesDir string) ([]byte, error) {
-	if filesDir == "" {
+func ReadLocalFile(configPath string, options common.TranslateOptions) ([]byte, error) {
+	if options.FilesDir == "" {
 		// a files dir isn't configured; refuse to read anything
 		return nil, common.ErrNoFilesDir
 	}
 	// calculate file path within FilesDir and check for path traversal
-	filePath := filepath.Join(filesDir, filepath.FromSlash(configPath))
-	if err := EnsurePathWithinFilesDir(filePath, filesDir); err != nil {
+	filePath := filepath.Join(options.FilesDir, filepath.FromSlash(configPath))
+	if err := EnsurePathWithinFilesDir(filePath, options.FilesDir); err != nil {
 		return nil, err
 	}
-	return os.ReadFile(filePath)
+	return ReadFile(filePath, options.LocalFileReader)
+}
+
+// ReadFile reads an already-validated path with the optional reader.
+func ReadFile(path string, reader func(string) ([]byte, error)) ([]byte, error) {
+	if reader == nil {
+		return os.ReadFile(path)
+	}
+	contents, err := reader(path)
+	if err != nil {
+		return nil, fmt.Errorf("reading %s: %w", path, err)
+	}
+	return contents, nil
 }
 
 // CheckForDecimalMode fails if the specified mode appears to have been

butane/base/v0_2/translate.go

@@ -118,7 +118,7 @@ func translateResource(from Resource, options common.TranslateOptions) (to types
 
 	if from.Local != nil {
 		c := path.New("yaml", "local")
-		contents, err := baseutil.ReadLocalFile(*from.Local, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.Local, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -265,7 +265,7 @@ func walkTree(yamlPath path.ContextPath, ts *translate.TranslationSet, r *report
 					ts.AddTranslation(yamlPath, path.New("json", "storage", "files"))
 				}
 			}
-			contents, err := os.ReadFile(srcPath)
+			contents, err := baseutil.ReadFile(srcPath, options.LocalFileReader)
 			if err != nil {
 				r.AddOnError(yamlPath, err)
 				return nil

butane/base/v0_3/translate.go

@@ -125,7 +125,7 @@ func translateResource(from Resource, options common.TranslateOptions) (to types
 
 	if from.Local != nil {
 		c := path.New("yaml", "local")
-		contents, err := baseutil.ReadLocalFile(*from.Local, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.Local, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -272,7 +272,7 @@ func walkTree(yamlPath path.ContextPath, ts *translate.TranslationSet, r *report
 					ts.AddTranslation(yamlPath, path.New("json", "storage", "files"))
 				}
 			}
-			contents, err := os.ReadFile(srcPath)
+			contents, err := baseutil.ReadFile(srcPath, options.LocalFileReader)
 			if err != nil {
 				r.AddOnError(yamlPath, err)
 				return nil

butane/base/v0_4/translate.go

@@ -140,7 +140,7 @@ func translateResource(from Resource, options common.TranslateOptions) (to types
 
 	if from.Local != nil {
 		c := path.New("yaml", "local")
-		contents, err := baseutil.ReadLocalFile(*from.Local, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.Local, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -287,7 +287,7 @@ func walkTree(yamlPath path.ContextPath, ts *translate.TranslationSet, r *report
 					ts.AddTranslation(yamlPath, path.New("json", "storage", "files"))
 				}
 			}
-			contents, err := os.ReadFile(srcPath)
+			contents, err := baseutil.ReadFile(srcPath, options.LocalFileReader)
 			if err != nil {
 				r.AddOnError(yamlPath, err)
 				return nil

butane/base/v0_5/translate.go

@@ -143,7 +143,7 @@ func translateResource(from Resource, options common.TranslateOptions) (to types
 
 	if from.Local != nil {
 		c := path.New("yaml", "local")
-		contents, err := baseutil.ReadLocalFile(*from.Local, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.Local, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -236,7 +236,7 @@ func translatePasswdUser(from PasswdUser, options common.TranslateOptions) (to t
 		}
 
 		for keyFileIndex, sshKeyFile := range from.SSHAuthorizedKeysLocal {
-			sshKeys, err := baseutil.ReadLocalFile(sshKeyFile, options.FilesDir)
+			sshKeys, err := baseutil.ReadLocalFile(sshKeyFile, options)
 			if err != nil {
 				r.AddOnError(c.Append(keyFileIndex), err)
 				continue
@@ -265,7 +265,7 @@ func translateUnit(from Unit, options common.TranslateOptions) (to types.Unit, t
 
 	if util.NotEmpty(from.ContentsLocal) {
 		c := path.New("yaml", "contents_local")
-		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -284,7 +284,7 @@ func translateDropin(from Dropin, options common.TranslateOptions) (to types.Dro
 
 	if util.NotEmpty(from.ContentsLocal) {
 		c := path.New("yaml", "contents_local")
-		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -377,7 +377,7 @@ func walkTree(yamlPath path.ContextPath, ts *translate.TranslationSet, r *report
 					ts.AddTranslation(yamlPath, path.New("json", "storage", "files"))
 				}
 			}
-			contents, err := os.ReadFile(srcPath)
+			contents, err := baseutil.ReadFile(srcPath, options.LocalFileReader)
 			if err != nil {
 				r.AddOnError(yamlPath, err)
 				return nil

butane/base/v0_6/translate.go

@@ -143,7 +143,7 @@ func translateResource(from Resource, options common.TranslateOptions) (to types
 
 	if from.Local != nil {
 		c := path.New("yaml", "local")
-		contents, err := baseutil.ReadLocalFile(*from.Local, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.Local, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -236,7 +236,7 @@ func translatePasswdUser(from PasswdUser, options common.TranslateOptions) (to t
 		}
 
 		for keyFileIndex, sshKeyFile := range from.SSHAuthorizedKeysLocal {
-			sshKeys, err := baseutil.ReadLocalFile(sshKeyFile, options.FilesDir)
+			sshKeys, err := baseutil.ReadLocalFile(sshKeyFile, options)
 			if err != nil {
 				r.AddOnError(c.Append(keyFileIndex), err)
 				continue
@@ -265,7 +265,7 @@ func translateUnit(from Unit, options common.TranslateOptions) (to types.Unit, t
 
 	if util.NotEmpty(from.ContentsLocal) {
 		c := path.New("yaml", "contents_local")
-		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -284,7 +284,7 @@ func translateDropin(from Dropin, options common.TranslateOptions) (to types.Dro
 
 	if util.NotEmpty(from.ContentsLocal) {
 		c := path.New("yaml", "contents_local")
-		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -377,7 +377,7 @@ func walkTree(yamlPath path.ContextPath, ts *translate.TranslationSet, r *report
 					ts.AddTranslation(yamlPath, path.New("json", "storage", "files"))
 				}
 			}
-			contents, err := os.ReadFile(srcPath)
+			contents, err := baseutil.ReadFile(srcPath, options.LocalFileReader)
 			if err != nil {
 				r.AddOnError(yamlPath, err)
 				return nil

butane/base/v0_7/translate.go

@@ -143,7 +143,7 @@ func translateResource(from Resource, options common.TranslateOptions) (to types
 
 	if from.Local != nil {
 		c := path.New("yaml", "local")
-		contents, err := baseutil.ReadLocalFile(*from.Local, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.Local, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -237,7 +237,7 @@ func translatePasswdUser(from PasswdUser, options common.TranslateOptions) (to t
 		}
 
 		for keyFileIndex, sshKeyFile := range from.SSHAuthorizedKeysLocal {
-			sshKeys, err := baseutil.ReadLocalFile(sshKeyFile, options.FilesDir)
+			sshKeys, err := baseutil.ReadLocalFile(sshKeyFile, options)
 			if err != nil {
 				r.AddOnError(c.Append(keyFileIndex), err)
 				continue
@@ -266,7 +266,7 @@ func translateUnit(from Unit, options common.TranslateOptions) (to types.Unit, t
 
 	if util.NotEmpty(from.ContentsLocal) {
 		c := path.New("yaml", "contents_local")
-		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -285,7 +285,7 @@ func translateDropin(from Dropin, options common.TranslateOptions) (to types.Dro
 
 	if util.NotEmpty(from.ContentsLocal) {
 		c := path.New("yaml", "contents_local")
-		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -416,7 +416,7 @@ func walkTree(yamlPath path.ContextPath, ts *translate.TranslationSet, r *report
 					ts.AddTranslation(yamlPath, path.New("json", "storage", "files"))
 				}
 			}
-			contents, err := os.ReadFile(srcPath)
+			contents, err := baseutil.ReadFile(srcPath, options.LocalFileReader)
 			if err != nil {
 				r.AddOnError(yamlPath, err)
 				return nil

butane/base/v0_8_exp/translate.go

@@ -148,7 +148,7 @@ func translateResource(from Resource, options common.TranslateOptions) (to types
 
 	if from.Local != nil {
 		c := path.New("yaml", "local")
-		contents, err := baseutil.ReadLocalFile(*from.Local, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.Local, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -242,7 +242,7 @@ func translatePasswdUser(from PasswdUser, options common.TranslateOptions) (to t
 		}
 
 		for keyFileIndex, sshKeyFile := range from.SSHAuthorizedKeysLocal {
-			sshKeys, err := baseutil.ReadLocalFile(sshKeyFile, options.FilesDir)
+			sshKeys, err := baseutil.ReadLocalFile(sshKeyFile, options)
 			if err != nil {
 				r.AddOnError(c.Append(keyFileIndex), err)
 				continue
@@ -271,7 +271,7 @@ func translateUnit(from Unit, options common.TranslateOptions) (to types.Unit, t
 
 	if util.NotEmpty(from.ContentsLocal) {
 		c := path.New("yaml", "contents_local")
-		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -290,7 +290,7 @@ func translateDropin(from Dropin, options common.TranslateOptions) (to types.Dro
 
 	if util.NotEmpty(from.ContentsLocal) {
 		c := path.New("yaml", "contents_local")
-		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options.FilesDir)
+		contents, err := baseutil.ReadLocalFile(*from.ContentsLocal, options)
 		if err != nil {
 			r.AddOnError(c, err)
 			return
@@ -340,7 +340,7 @@ func isTemplateInstance(name string) (bool, string) {
 func readLocalOrInlineContents(contentsLocal, contentsInline *string, ctxPath path.ContextPath, options common.TranslateOptions) (content []byte, contentPath path.ContextPath, err error) {
 	if util.NotEmpty(contentsLocal) {
 		contentPath = ctxPath.Append("contents_local")
-		localContents, err := baseutil.ReadLocalFile(*contentsLocal, options.FilesDir)
+		localContents, err := baseutil.ReadLocalFile(*contentsLocal, options)
 		if err != nil {
 			return content, contentPath, err
 		}
@@ -587,7 +587,7 @@ func walkTree(yamlPath path.ContextPath, ts *translate.TranslationSet, r *report
 					ts.AddTranslation(yamlPath, path.New("json", "storage", "files"))
 				}
 			}
-			contents, err := os.ReadFile(srcPath)
+			contents, err := baseutil.ReadFile(srcPath, options.LocalFileReader)
 			if err != nil {
 				r.AddOnError(yamlPath, err)
 				return nil

butane/config/common/common.go

@@ -15,9 +15,10 @@
 package common
 
 type TranslateOptions struct {
-	FilesDir                  string // allow embedding local files relative to this directory
-	NoResourceAutoCompression bool   // skip automatic compression of inline/local resources
-	DebugPrintTranslations    bool   // report translations to stderr
+	FilesDir                  string                       // allow embedding local files relative to this directory
+	LocalFileReader           func(string) ([]byte, error) // read local files; nil uses os.ReadFile
+	NoResourceAutoCompression bool                         // skip automatic compression of inline/local resources
+	DebugPrintTranslations    bool                         // report translations to stderr
 }
 
 type TranslateBytesOptions struct {

butane/internal/file_processor.go

@@ -0,0 +1,58 @@
+// Copyright 2026 Red Hat, Inc
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+func matchesExtension(path string, extensions []string) bool {
+	for _, ext := range extensions {
+		if ext == "*" || strings.HasSuffix(filepath.Base(path), "."+ext) {
+			return true
+		}
+	}
+	return false
+}
+
+func processFile(reader io.Reader, processor string) ([]byte, error) {
+	if processor == "" {
+		return io.ReadAll(reader)
+	}
+
+	cmd := exec.Command(processor)
+	cmd.Stdin = reader
+	cmd.Stderr = os.Stderr
+	output, err := cmd.Output()
+	if err != nil {
+		return nil, fmt.Errorf("file processor %q failed: %w", processor, err)
+	}
+	return output, nil
+}
+
+func processLocalFile(path, processor string) ([]byte, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	output, err := processFile(file, processor)
+	return output, errors.Join(err, file.Close())
+}