all: Remove crypto/md5 (#7493)

This is a mostly pointless change to make security scanners and static
analysis tools happy, as they all hate seeing md5. None of our md5 uses
were security relevant, but still. Only visible effect of this change is
that our temp file names for very long file names become slightly longer
than they were previously...

cmd/stwatchfile/main.go

@@ -7,31 +7,15 @@
 package main
 
 import (
-	"bytes"
-	"crypto/md5"
 	"flag"
 	"fmt"
 	"io"
 	"os"
 	"time"
+
+	"github.com/syncthing/syncthing/lib/sha256"
 )
 
-func getmd5(filePath string) ([]byte, error) {
-	var result []byte
-	file, err := os.Open(filePath)
-	if err != nil {
-		return result, err
-	}
-	defer file.Close()
-
-	hash := md5.New()
-	if _, err := io.Copy(hash, file); err != nil {
-		return result, err
-	}
-
-	return hash.Sum(result), nil
-}
-
 func main() {
 	period := flag.Duration("period", 200*time.Millisecond, "Sleep period between checks")
 	flag.Parse()
@@ -46,7 +30,7 @@ func main() {
 	exists := true
 	size := int64(0)
 	mtime := time.Time{}
-	hash := []byte{}
+	var hash [sha256.Size]byte
 
 	for {
 		time.Sleep(*period)
@@ -72,7 +56,7 @@ func main() {
 		if !exists {
 			size = 0
 			mtime = time.Time{}
-			hash = []byte{}
+			hash = [sha256.Size]byte{}
 			continue
 		}
 
@@ -83,12 +67,12 @@ func main() {
 		newSize := fi.Size()
 		newMtime := fi.ModTime()
 
-		newHash, err := getmd5(file)
+		newHash, err := sha256file(file)
 		if err != nil {
-			fmt.Println("getmd5:", err)
+			fmt.Println("sha256file:", err)
 		}
 
-		if newSize != size || newMtime != mtime || !bytes.Equal(newHash, hash) {
+		if newSize != size || newMtime != mtime || newHash != hash {
 			fmt.Println(file, "Size:", newSize, "Mtime:", newMtime, "Hash:", fmt.Sprintf("%x", newHash))
 			hash = newHash
 			size = newSize
@@ -96,3 +80,18 @@ func main() {
 		}
 	}
 }
+
+func sha256file(fname string) (hash [sha256.Size]byte, err error) {
+	f, err := os.Open(fname)
+	if err != nil {
+		return
+	}
+	defer f.Close()
+
+	h := sha256.New()
+	io.Copy(h, f)
+	hb := h.Sum(nil)
+	copy(hash[:], hb)
+
+	return
+}