cmd/syncthing: Apply file permissions and modtimes when decrypting (#8412)

Directories are not represented in the encrypted storagage so they get
to keep default permissions...

cmd/syncthing/decrypt/decrypt.go

@@ -129,6 +129,9 @@ func (c *CLI) getFolderID() (string, error) {
 // process handles the file named path in srcFs, decrypting it into dstFs
 // unless dstFs is nil.
 func (c *CLI) process(srcFs fs.Filesystem, dstFs fs.Filesystem, path string) error {
+	// Which filemode bits to preserve
+	const retainBits = fs.ModePerm | fs.ModeSetgid | fs.ModeSetuid | fs.ModeSticky
+
 	if c.Verbose {
 		log.Printf("Processing %q", path)
 	}
@@ -168,6 +171,9 @@ func (c *CLI) process(srcFs fs.Filesystem, dstFs fs.Filesystem, path string) err
 			return fmt.Errorf("%s: %w", plainFi.Name, err)
 		}
 		defer plainFd.Close() // also closed explicitly in the return
+		if err := dstFs.Chmod(plainFi.Name, fs.FileMode(plainFi.Permissions&uint32(retainBits))); err != nil {
+			return fmt.Errorf("%s: %w", plainFi.Name, err)
+		}
 	}
 
 	if err := c.decryptFile(encFi, &plainFi, encFd, plainFd); err != nil {
@@ -184,7 +190,12 @@ func (c *CLI) process(srcFs fs.Filesystem, dstFs fs.Filesystem, path string) err
 	}
 
 	if plainFd != nil {
-		return plainFd.Close()
+		if err := plainFd.Close(); err != nil {
+			return fmt.Errorf("%s: %w", plainFi.Name, err)
+		}
+		if err := dstFs.Chtimes(plainFi.Name, plainFi.ModTime(), plainFi.ModTime()); err != nil {
+			return fmt.Errorf("%s: %w", plainFi.Name, err)
+		}
 	}
 	return nil
 }