save: use renameio

I checked in strace that the save handler now results in a rename syscall,
and the specific syscall is renameat thanks to the *os.Root API:

[pid 596578] close(9</home/michael/tmp/garden/.link.md4944995753028614592>) = 0
[pid 596578] renameat(4</home/michael/tmp/garden>, ".link.md4944995753028614592", 4</home/michael/tmp/garden>, "link.md") = 0

Author: stapelberg

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/BurntSushi/toml v1.4.1-0.20240526193622-a339e1f7089c
 	github.com/fsnotify/fsnotify v1.8.0
 	github.com/google/go-cmp v0.6.0
+	github.com/google/renameio/v2 v2.0.2
 	github.com/yuin/goldmark v1.7.8
 	go.abhg.dev/goldmark/wikilink v0.5.0
 	golang.org/x/image v0.23.0

go.sum

@@ -8,6 +8,8 @@ github.com/fsnotify/fsnotify v1.8.0 h1:dAwr6QBTBZIkG8roQaJjGof0pp0EeF+tNV7YBP3F/
 github.com/fsnotify/fsnotify v1.8.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
 github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+github.com/google/renameio/v2 v2.0.2 h1:qKZs+tfn+arruZZhQ7TKC/ergJunuJicWS6gLDt/dGw=
+github.com/google/renameio/v2 v2.0.2/go.mod h1:OX+G6WHHpHq3NVj7cAOleLOwJfcQ1s3uUJQCrr78SWo=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=

internal/bull/save.go

@@ -3,9 +3,10 @@ package bull
 import (
 	"fmt"
 	"net/http"
-	"os"
 	"path/filepath"
 	"strings"
+
+	"github.com/google/renameio/v2"
 )
 
 func (b *bullServer) save(w http.ResponseWriter, r *http.Request) error {
@@ -26,9 +27,6 @@ func (b *bullServer) save(w http.ResponseWriter, r *http.Request) error {
 	pageName := pageFromURL(r)
 	possibilities := page2files(pageName)
 
-	// TODO(go1.25): use https://github.com/google/renameio/ to make writes
-	// safer once Go 1.25 ships os.Root.Rename.
-
 	var firstFn string
 	for _, fn := range possibilities {
 		_, err := b.content.Stat(fn)
@@ -45,15 +43,15 @@ func (b *bullServer) save(w http.ResponseWriter, r *http.Request) error {
 	if err := mkdirAll(b.content, filepath.Dir(firstFn), 0755); err != nil {
 		return err
 	}
-	f, err := b.content.OpenFile(firstFn, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0666)
+	pf, err := renameio.NewPendingFile(firstFn, renameio.WithRoot(b.content))
 	if err != nil {
 		return err
 	}
-	defer f.Close()
-	if _, err := f.Write([]byte(md)); err != nil {
+	defer pf.Cleanup()
+	if _, err := pf.Write([]byte(md)); err != nil {
 		return err
 	}
-	if err := f.Close(); err != nil {
+	if err := pf.CloseAtomicallyReplace(); err != nil {
 		return err
 	}