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...

test/util.go

@@ -9,7 +9,6 @@
 package integration
 
 import (
-	"crypto/md5"
 	cr "crypto/rand"
 	"errors"
 	"fmt"
@@ -27,6 +26,7 @@ import (
 	"unicode"
 
 	"github.com/syncthing/syncthing/lib/rc"
+	"github.com/syncthing/syncthing/lib/sha256"
 )
 
 func init() {
@@ -395,7 +395,7 @@ type fileInfo struct {
 	name string
 	mode os.FileMode
 	mod  int64
-	hash [16]byte
+	hash [sha256.Size]byte
 	size int64
 }
 
@@ -442,11 +442,7 @@ func startWalker(dir string, res chan<- fileInfo, abort <-chan struct{}) chan er
 			if err != nil {
 				return err
 			}
-			h := md5.New()
-			h.Write([]byte(tgt))
-			hash := h.Sum(nil)
-
-			copy(f.hash[:], hash)
+			f.hash = sha256.Sum256([]byte(tgt))
 		} else if info.IsDir() {
 			f = fileInfo{
 				name: rn,
@@ -463,7 +459,7 @@ func startWalker(dir string, res chan<- fileInfo, abort <-chan struct{}) chan er
 				mod:  info.ModTime().Unix(),
 				size: info.Size(),
 			}
-			sum, err := md5file(path)
+			sum, err := sha256file(path)
 			if err != nil {
 				return err
 			}
@@ -490,14 +486,14 @@ func startWalker(dir string, res chan<- fileInfo, abort <-chan struct{}) chan er
 	return errc
 }
 
-func md5file(fname string) (hash [16]byte, err error) {
+func sha256file(fname string) (hash [sha256.Size]byte, err error) {
 	f, err := os.Open(fname)
 	if err != nil {
 		return
 	}
 	defer f.Close()
 
-	h := md5.New()
+	h := sha256.New()
 	io.Copy(h, f)
 	hb := h.Sum(nil)
 	copy(hash[:], hb)