Send extra request headers and throw PreconditionFailedException on 412

src/Id/Exceptions/ApiException.php

@@ -28,10 +28,16 @@ class ApiException extends ResourceException
     public static function fromResult(Result $result): static
     {
         $body = is_object($result->data) ? $result->data : null;
+        $status = $result->response()?->getStatusCode() ?? 0;
+
+        // a failed conditional write; subclasses (CollectionException) keep their own type
+        if ($status === 412 && static::class === self::class) {
+            return PreconditionFailedException::fromResult($result);
+        }
 
         return new static(
             rtrim((string) ($body->message ?? $result->error()), '.'),
-            $result->response()?->getStatusCode() ?? 0,
+            $status,
             $body->error ?? null,
             $result->exception,
         );

src/Id/Exceptions/PreconditionFailedException.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Anikeen\Id\Exceptions;
+
+/**
+ * A conditional write failed (412): the stored version wasn't the one sent with `If-Match`
+ * ("preference_version_mismatch"), or the key was already set and the write was create-only
+ * ("preference_exists"). Reload, merge and retry with currentVersion().
+ */
+class PreconditionFailedException extends ApiException
+{
+    /**
+     * The version stored now, null when the key isn't set (or Anikeen ID didn't send one).
+     */
+    public function currentVersion(): ?int
+    {
+        $version = $this->body()->current_version ?? null;
+
+        return is_numeric($version) ? (int) $version : null;
+    }
+}