feat: add ability to append to existing files with extrafiles

This PR introduces "op" to the extra files options. This allows for a
user to specify "append" as the op, which will create a copy of the file
specified, add the extra data provided, and bind mount over the existing
file. Will close #1467

Signed-off-by: Spencer Smith <robertspencersmith@gmail.com>

internal/app/machined/internal/phase/config/extra_files.go

@@ -5,11 +5,14 @@
 package config
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/hashicorp/go-multierror"
+	"golang.org/x/sys/unix"
 
 	"github.com/talos-systems/talos/internal/app/machined/internal/phase"
 	"github.com/talos-systems/talos/internal/pkg/runtime"
@@ -32,13 +35,48 @@ func (task *ExtraFiles) runtime(r runtime.Runtime) (err error) {
 	var result *multierror.Error
 
 	for _, f := range r.Config().Machine().Files() {
-		p := filepath.Join("/var", f.Path)
+		// Slurp existing file if append is our op and add contents to it
+		if f.Op == "append" {
+			var existingFileContents []byte
+
+			existingFileContents, err = ioutil.ReadFile(f.Path)
+			if err != nil {
+				result = multierror.Append(result, err)
+				continue
+			}
+
+			f.Contents = string(existingFileContents) + "\n" + f.Contents
+		}
+
+		// Determine if supplied path is in /var or not.
+		// If not, we'll write it to /var anyways and bind mount below
+		p := f.Path
+		inVar := true
+		explodedPath := strings.Split(
+			strings.TrimLeft(f.Path, "/"),
+			string(os.PathSeparator),
+		)
+
+		if explodedPath[0] != "var" {
+			p = filepath.Join("/var", f.Path)
+			inVar = false
+		}
+
 		if err = os.MkdirAll(filepath.Dir(p), os.ModeDir); err != nil {
 			result = multierror.Append(result, err)
+			continue
 		}
 
 		if err = ioutil.WriteFile(p, []byte(f.Contents), f.Permissions); err != nil {
 			result = multierror.Append(result, err)
+			continue
+		}
+
+		// File path was not /var/... so we assume a bind mount is wanted
+		if !inVar {
+			if err = unix.Mount(p, f.Path, "", unix.MS_BIND|unix.MS_RDONLY, ""); err != nil {
+				result = multierror.Append(result, fmt.Errorf("failed to create bind mount for %s: %w", p, err))
+			}
 		}
 	}
 

pkg/config/machine/machine.go

@@ -47,6 +47,7 @@ type File struct {
 	Contents    string      `yaml:"contents"`
 	Permissions os.FileMode `yaml:"permissions"`
 	Path        string      `yaml:"path"`
+	Op          string      `yaml:"op"`
 }
 
 // Security defines the requirements for a config that pertains to security