godoc: atomically update filter file

R=rsc
CC=golang-dev
https://golang.org/cl/2206041

src/cmd/godoc/godoc.go

@@ -170,10 +170,8 @@ func updateFilterFile() {
 	})
 
 	// update filter file
-	// TODO(gri) should write a tmp file and atomically rename
-	err := ioutil.WriteFile(*filter, buf.Bytes(), 0666)
-	if err != nil {
-		log.Stderrf("ioutil.Writefile(%s): %s", *filter, err)
+	if err := writeFileAtomically(*filter, buf.Bytes()); err != nil {
+		log.Stderrf("writeFileAtomically(%s): %s", *filter, err)
 		filterDelay.backoff(24 * 60) // back off exponentially, but try at least once a day
 	} else {
 		filterDelay.set(*filterMin) // revert to regular filter update schedule

src/cmd/godoc/utils.go

@@ -7,6 +7,8 @@
 package main
 
 import (
+	"io"
+	"io/ioutil"
 	"os"
 	pathutil "path"
 	"sort"
@@ -85,3 +87,23 @@ func canonicalizePaths(list []string, filter func(path string) bool) []string {
 
 	return list[0:i]
 }
+
+
+// writeFileAtomically writes data to a temporary file and then
+// atomically renames that file to the file named by filename.
+//
+func writeFileAtomically(filename string, data []byte) os.Error {
+	f, err := ioutil.TempFile(cwd, filename)
+	if err != nil {
+		return err
+	}
+	n, err := f.Write(data)
+	f.Close()
+	if err != nil {
+		return err
+	}
+	if n < len(data) {
+		return io.ErrShortWrite
+	}
+	return os.Rename(f.Name(), filename)
+}