Link fieldtype recursively augments linked entries causing infinite loop #14696

Bug description

A link fieldtype that points to an entry whose own link field points back triggers infinite recursion as soon as anything deep-augments the entry (live preview,Entry::toArray(), JSON serialization etc.).

Expected:
The augmentation should bottom out, like it does for the other relationship fieldtypes (entries, terms, users).

How to reproduce

1. Create a blueprint with a link field.
2. Add two entries A and B; A's link_field = entry::B, B's link_field = entry::A.
3. Any template that JSON-serializes the entry, e.g. {{ page | to_json }}.
4. Visit A in the browser:

Maximum call stack size … reached. Infinite recursion?

Logs

Environment

Environment
Application Name: Statamic
Laravel Version: 12.60.1
PHP Version: 8.4.19
Composer Version: 2.9.8
Environment: local
Debug Mode: ENABLED
Maintenance Mode: OFF
Timezone: UTC
Locale: en

Cache
Config: NOT CACHED
Events: NOT CACHED
Routes: NOT CACHED
Views: CACHED

Drivers
Broadcasting: log
Cache: file
Database: mysql
Logs: stack / single
Mail: smtp
Queue: sync
Session: file

Storage
public/storage: NOT LINKED

Statamic
Sites: 8 (English, Deutsch, Español, and 5 more)
Stache Watcher: Disabled
Static Caching: Disabled
Version: 5.73.22 PRO

Installation

Fresh statamic/statamic site via CLI

Additional details

It is also a V6 issue.

Possible "fix"

In src/Fieldtypes/Link/ArrayableLink.php:

public function toArray()
{
-    return is_object($this->value)
-        ? $this->value->toAugmentedArray()
-        : ['url' => $this->url()];
+    if (! is_object($this->value)) {
+        return ['url' => $this->url()];
+    }
+
+    if ($this->value instanceof \Statamic\Contracts\Data\Augmentable) {
+        return array_merge(
+            $this->value->toShallowAugmentedArray(),
+            ['url' => $this->url()]
+        );
+    }
+
+    return ['url' => $this->url()];
}