From 702ed8ecc19edfd53a45b11df66c67e940e8637b Mon Sep 17 00:00:00 2001
From: Hazem Krimi <me@hazemkrimi.tech>
Date: Fri, 9 May 2025 06:40:32 +0100
Subject: [PATCH] fix(config): deep copy configuration defaults (fixes #9916)
 (#10101)

### Purpose

Setting default configuration was not working properly since the
defaults struct is not deeply copied.

### Testing

Try running commands to change default configuration and either inspect
`config.xml` or `/rest/config` result to see the applied changed.
Example:
```
./syncthing cli config defaults folder versioning params set keep 5
```
---
 lib/config/config.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/config/config.go b/lib/config/config.go
index e3d9f11c9..73c48ecd0 100644
--- a/lib/config/config.go
+++ b/lib/config/config.go
@@ -235,6 +235,10 @@ func ReadJSON(r io.Reader, myID protocol.DeviceID) (Configuration, error) {
 func (cfg Configuration) Copy() Configuration {
 	newCfg := cfg
 
+	// Deep copy Defaults
+	newCfg.Defaults.Folder = cfg.Defaults.Folder.Copy()
+	newCfg.Defaults.Device = cfg.Defaults.Device.Copy()
+
 	// Deep copy FolderConfigurations
 	newCfg.Folders = make([]FolderConfiguration, len(cfg.Folders))
 	for i := range newCfg.Folders {