Refactor file handing for self-update.

* Write new file payload to a temp file before touching the original
binary. Minimizes the possibility of failing mid-write and corrupting
the binary.
* On Windows, move the original binary out to a temp file rather than
removing it as the running binary is locked. Fixes issue #2248.

changelog/unreleased/issue-2248

@@ -0,0 +1,8 @@
+Bugfix: Support self-update on Windows
+
+Restic self-update would fail in situations where the operating system
+locks running binaries, including Windows. The new behavior works around
+this by renaming the running file and swapping the updated file in place.
+
+https://github.com/restic/restic/issues/2248
+https://github.com/restic/restic/pull/3675

internal/selfupdate/download.go

@@ -10,6 +10,7 @@ import (
 	"encoding/hex"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -40,14 +41,6 @@ func findHash(buf []byte, filename string) (hash []byte, err error) {
 }
 
 func extractToFile(buf []byte, filename, target string, printf func(string, ...interface{})) error {
-	var mode = os.FileMode(0755)
-
-	// get information about the target file
-	fi, err := os.Lstat(target)
-	if err == nil {
-		mode = fi.Mode()
-	}
-
 	var rd io.Reader = bytes.NewReader(buf)
 	switch filepath.Ext(filename) {
 	case ".bz2":
@@ -74,33 +67,44 @@ func extractToFile(buf []byte, filename, target string, printf func(string, ...i
 		rd = file
 	}
 
-	err = os.Remove(target)
-	if os.IsNotExist(err) {
-		err = nil
-	}
-	if err != nil {
-		return fmt.Errorf("unable to remove target file: %v", err)
-	}
-
-	dest, err := os.OpenFile(target, os.O_CREATE|os.O_EXCL|os.O_WRONLY, mode)
+	// Write everything to a temp file
+	dir := filepath.Dir(target)
+	new, err := ioutil.TempFile(dir, "restic")
 	if err != nil {
 		return err
 	}
 
-	n, err := io.Copy(dest, rd)
+	n, err := io.Copy(new, rd)
 	if err != nil {
-		_ = dest.Close()
-		_ = os.Remove(dest.Name())
+		_ = new.Close()
+		_ = os.Remove(new.Name())
+		return err
+	}
+	if err = new.Sync(); err != nil {
+		return err
+	}
+	if err = new.Close(); err != nil {
 		return err
 	}
 
-	err = dest.Close()
-	if err != nil {
+	mode := os.FileMode(0755)
+	// attempt to find the original mode
+	if fi, err := os.Lstat(target); err == nil {
+		mode = fi.Mode()
+	}
+
+	// Remove the original binary.
+	if err := removeResticBinary(dir, target); err != nil {
 		return err
 	}
 
-	printf("saved %d bytes in %v\n", n, dest.Name())
-	return nil
+	// Rename the temp file to the final location atomically.
+	if err := os.Rename(new.Name(), target); err != nil {
+		return err
+	}
+
+	printf("saved %d bytes in %v\n", n, target)
+	return os.Chmod(target, mode)
 }
 
 // DownloadLatestStableRelease downloads the latest stable released version of

internal/selfupdate/download_unix.go

@@ -0,0 +1,10 @@
+//go:build !windows
+// +build !windows
+
+package selfupdate
+
+// Remove the target binary.
+func removeResticBinary(dir, target string) error {
+	// removed on rename on this platform
+	return nil
+}

internal/selfupdate/download_windows.go

@@ -0,0 +1,23 @@
+//go:build windows
+// +build windows
+
+package selfupdate
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// Rename (rather than remove) the running version. The running binary will be locked
+// on Windows and cannot be removed while still executing.
+func removeResticBinary(dir, target string) error {
+	backup := filepath.Join(dir, filepath.Base(target)+".bak")
+	if _, err := os.Stat(backup); err == nil {
+		_ = os.Remove(backup)
+	}
+	if err := os.Rename(target, backup); err != nil {
+		return fmt.Errorf("unable to rename target file: %v", err)
+	}
+	return nil
+}