Simplify and comment TempFile implementation for windows

changelog/unreleased/issue-3465

@@ -0,0 +1,10 @@
+Enhancement: Improve handling of temporary files on Windows
+
+In some cases restic failed to delete temporary files causing the current
+command to fail. This has been fixed by ensuring that Windows automatically
+deletes the file. In addition, temporary files are only written to disk if
+necessary to reduce disk writes.
+
+https://github.com/restic/restic/issues/3465
+https://github.com/restic/restic/issues/1551
+https://github.com/restic/restic/pull/3610

internal/fs/file_windows.go

@@ -1,41 +1,14 @@
 package fs
 
 import (
+	"math/rand"
 	"os"
 	"path/filepath"
-	"strings"
-	"syscall"
 	"strconv"
-	"sync"
+	"strings"
 	"time"
-)
 
-// Random number state.
-// We generate random temporary file names so that there's a good
-// chance the file doesn't exist yet - keeps the number of tries in
-// TempFile to a minimum.
-var rand uint32
-var randmu sync.Mutex
-
-func reseed() uint32 {
-	return uint32(time.Now().UnixNano() + int64(os.Getpid()))
-}
-
-func nextRandom() string {
-	randmu.Lock()
-	r := rand
-	if r == 0 {
-		r = reseed()
-	}
-	r = r*1664525 + 1013904223 // constants from Numerical Recipes
-	rand = r
-	randmu.Unlock()
-	return strconv.Itoa(int(1e9 + r%1e9))[1:]
-}
-
-const (
-	FILE_ATTRIBUTE_TEMPORARY  = 0x00000100
-	FILE_FLAG_DELETE_ON_CLOSE = 0x04000000
+	"golang.org/x/sys/windows"
 )
 
 // fixpath returns an absolute path on windows, so restic can open long file
@@ -61,25 +34,41 @@ func fixpath(name string) string {
 	return name
 }
 
-// TempFile creates a temporary file.
+// TempFile creates a temporary file which is marked as delete-on-close
 func TempFile(dir, prefix string) (f *os.File, err error) {
+	// slightly modified implementation of ioutil.TempFile(dir, prefix) to allow us to add
+	// the FILE_ATTRIBUTE_TEMPORARY | FILE_FLAG_DELETE_ON_CLOSE flags.
+	// These provide two large benefits:
+	// FILE_ATTRIBUTE_TEMPORARY tells Windows to keep the file in memory only if possible
+	// which reduces the amount of unnecessary disk writes.
+	// FILE_FLAG_DELETE_ON_CLOSE instructs Windows to automatically delete the file once
+	// all file descriptors are closed.
+
 	if dir == "" {
 		dir = os.TempDir()
 	}
 
-	access := uint32(syscall.GENERIC_READ | syscall.GENERIC_WRITE)
-	creation := uint32(syscall.CREATE_NEW)
-	flags := uint32(FILE_ATTRIBUTE_TEMPORARY | FILE_FLAG_DELETE_ON_CLOSE)
-	
-	for i := 0; i < 10000; i++ {
-		path := filepath.Join(dir, prefix+nextRandom())
+	access := uint32(windows.GENERIC_READ | windows.GENERIC_WRITE)
+	creation := uint32(windows.CREATE_NEW)
+	share := uint32(0) // prevent other processes from accessing the file
+	flags := uint32(windows.FILE_ATTRIBUTE_TEMPORARY | windows.FILE_FLAG_DELETE_ON_CLOSE)
 
-		h, err := syscall.CreateFile(syscall.StringToUTF16Ptr(path), access, 0, nil, creation, flags, 0)
-		if err == nil {
-			return os.NewFile(uintptr(h), path), nil
+	rnd := rand.New(rand.NewSource(time.Now().UnixNano()))
+	for i := 0; i < 10000; i++ {
+		randSuffix := strconv.Itoa(int(1e9 + rnd.Intn(1e9)%1e9))[1:]
+		path := filepath.Join(dir, prefix+randSuffix)
+
+		ptr, err := windows.UTF16PtrFromString(path)
+		if err != nil {
+			return nil, err
 		}
+		h, err := windows.CreateFile(ptr, access, share, nil, creation, flags, 0)
+		if os.IsExist(err) {
+			continue
+		}
+		return os.NewFile(uintptr(h), path), err
 	}
-	
+
 	// Proper error handling is still to do
 	return nil, os.ErrExist
 }