keep file permissions and try to keep user/group on supported systems (fixes #1467)

fs/config.go

@@ -652,7 +652,18 @@ func SaveConfig() {
 		log.Fatalf("Failed to close config file: %v", err)
 	}
 
-	err = os.Chmod(f.Name(), 0600)
+	var fileMode os.FileMode = 0600
+	info, err := os.Stat(ConfigPath)
+	if err != nil {
+		Debugf(nil, "Using default permissions for config file: %v", fileMode)
+	} else if info.Mode() != fileMode {
+		Debugf(nil, "Keeping previous permissions for config file: %v", info.Mode())
+		fileMode = info.Mode()
+	}
+
+	attemptCopyGroup(ConfigPath, f.Name())
+
+	err = os.Chmod(f.Name(), fileMode)
 	if err != nil {
 		Errorf(nil, "Failed to set permissions on config file: %v", err)
 	}

fs/config_other.go

@@ -0,0 +1,10 @@
+// Read, write and edit the config file
+// Non-unix specific functions.
+
+// +build !darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd,!solaris
+
+package fs
+
+// attemptCopyGroups tries to keep the group the same, which only makes sense
+// for system with user-group-world permission model.
+func attemptCopyGroup(fromPath, toPath string) {}

fs/config_unix.go

@@ -0,0 +1,35 @@
+// Read, write and edit the config file
+// Unix specific functions.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package fs
+
+import (
+	"os"
+	"os/user"
+	"strconv"
+	"syscall"
+)
+
+// attemptCopyGroups tries to keep the group the same. User will be the one
+// who is currently running this process.
+func attemptCopyGroup(fromPath, toPath string) {
+	info, err := os.Stat(fromPath)
+	if err != nil || info.Sys() == nil {
+		return
+	}
+	if stat, ok := info.Sys().(*syscall.Stat_t); ok {
+		uid := int(stat.Uid)
+		// prefer self over previous owner of file, because it has a higher chance
+		// of success
+		if user, err := user.Current(); err == nil {
+			if tmpUID, err := strconv.Atoi(user.Uid); err == nil {
+				uid = tmpUID
+			}
+		}
+		if err = os.Chown(toPath, uid, int(stat.Gid)); err != nil {
+			Debugf(nil, "Failed to keep previous owner of config file: %v", err)
+		}
+	}
+}