migrations: Use atomic file operations for subgid and subuid files

Signed-off-by: Evan Maddock <[email protected]>

core/migration.go

@@ -15,14 +15,16 @@
 package core
 
 import (
+	"errors"
 	"fmt"
-	"github.com/BurntSushi/toml"
-	"github.com/DataDrake/waterlog"
 	"io/ioutil"
 	"os"
 	gouser "os/user"
 	"path/filepath"
 	"strconv"
+
+	"github.com/BurntSushi/toml"
+	"github.com/DataDrake/waterlog"
 )
 
 // Migration contains the information about a migration, including where it is on the system and its requested system modifications
@@ -170,27 +172,33 @@ func (m *Migration) addSubIds(context *Context, task *AddSubIds) {
 	m.createSubGidFile(context, task, users)
 }
 
-func createFileIfNotExists(path string) (created bool) {
-	if _, err := os.Stat(path); err == nil {
-		waterlog.Debugf("\t%s already exists, skipping", path)
-
-		return false
-	}
+func createFileIfNotExists(path string) (bool, error) {
+	fh, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0644)
 
-	fh, err := os.Create(path)
 	if err != nil {
-		waterlog.Warnf("\tUnable to create %s due to error: %s\n", path, err)
+		if errors.Is(err, os.ErrExist) {
+			waterlog.Debugf("\t%s already exists, skipping", path)
+			// The file already existing isn't an error mode for us,
+			// so don't propagate the error.
+			return false, nil
+		}
 
-		return false
+		return false, err
 	}
 
 	fh.Close()
 
-	return true
+	return true, nil
 }
 
 func (m *Migration) createSubUidFile(context *Context, task *AddSubIds, users []User) {
-	if !createFileIfNotExists("/etc/subuid") {
+	created, err := createFileIfNotExists("/etc/subuid")
+
+	if !created {
+		if err != nil {
+			waterlog.Warnf("\tUnable to create /etc/subuid due to error: %s\n", err)
+		}
+
 		return
 	}
 
@@ -205,7 +213,13 @@ func (m *Migration) createSubUidFile(context *Context, task *AddSubIds, users []
 }
 
 func (m *Migration) createSubGidFile(context *Context, task *AddSubIds, users []User) {
-	if !createFileIfNotExists("/etc/subgid") {
+	created, err := createFileIfNotExists("/etc/subgid")
+
+	if !created {
+		if err != nil {
+			waterlog.Warnf("\tUnable to create /etc/subgid due to error: %s\n", err)
+		}
+
 		return
 	}