Terraform: Try to use existing files on partially unpacked workspace

cli/internal/terraform/loader.go

@@ -7,8 +7,10 @@ SPDX-License-Identifier: AGPL-3.0-only
 package terraform
 
 import (
+	"bytes"
 	"embed"
 	"errors"
+	"fmt"
 	"io/fs"
 	"path"
 	"path/filepath"
@@ -41,7 +43,24 @@ func prepareWorkspace(fileHandler file.Handler, provider cloudprovider.Provider,
 			return err
 		}
 		fileName := strings.Replace(filepath.Join(workingDir, path), rootDir+"/", "", 1)
-		return fileHandler.Write(fileName, content, file.OptMkdirAll)
+		if err := fileHandler.Write(fileName, content, file.OptMkdirAll); errors.Is(err, afero.ErrFileExists) {
+			// If a file already exists, check if it is identical. If yes, continue and don't write anything to disk.
+			// If no, don't overwrite it and instead throw an error. The affected file could be from a different version,
+			// provider, corrupted or manually modified in general.
+			existingFileContent, err := fileHandler.Read(fileName)
+			if err != nil {
+				return err
+			}
+
+			if !bytes.Equal(content, existingFileContent) {
+				return fmt.Errorf("trying to overwrite existing Terraform file with different version")
+			}
+			return nil
+		} else if err != nil {
+			return err
+		}
+
+		return nil
 	})
 }
 

cli/internal/terraform/loader_test.go

@@ -23,6 +23,7 @@ func TestLoader(t *testing.T) {
 	testCases := map[string]struct {
 		provider            cloudprovider.Provider
 		fileList            []string
+		testAlreadyUnpacked bool
 	}{
 		"aws": {
 			provider: cloudprovider.AWS,
@@ -51,6 +52,16 @@ func TestLoader(t *testing.T) {
 				"modules",
 			},
 		},
+		"continue on (partially) unpacked": {
+			provider: cloudprovider.AWS,
+			fileList: []string{
+				"main.tf",
+				"variables.tf",
+				"outputs.tf",
+				"modules",
+			},
+			testAlreadyUnpacked: true,
+		},
 	}
 
 	for name, tc := range testCases {
@@ -62,9 +73,16 @@ func TestLoader(t *testing.T) {
 
 			err := prepareWorkspace(file, tc.provider, constants.TerraformWorkingDir)
 			require.NoError(err)
-
 			checkFiles(t, file, func(err error) { assert.NoError(err) }, tc.fileList)
 
+			if tc.testAlreadyUnpacked {
+				// Let's try the same again and check if we don't get a "file already exists" error.
+				require.NoError(file.Remove(filepath.Join(constants.TerraformWorkingDir, "variables.tf")))
+				err := prepareWorkspace(file, tc.provider, constants.TerraformWorkingDir)
+				assert.NoError(err)
+				checkFiles(t, file, func(err error) { assert.NoError(err) }, tc.fileList)
+			}
+
 			err = cleanUpWorkspace(file, constants.TerraformWorkingDir)
 			require.NoError(err)