No longer throw when false-negative result setting same value

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [[*next-version*]] - YYYY-MM-DD
+### Fixed
+- Setting same value as existing caused an exception due to false-negative result (#22).
 
 ## [0.2.0-alpha1] - 2023-08-30
 ### Removed

src/CachePool.php

@@ -350,14 +350,20 @@ protected function getTransientOriginal(string $key)
      * @param int    $ttl   The amount of seconds after which the transient will expire.
      *
      * @throws RangeException If key invalid.
+     * @throws UnexpectedValueException If could not write the value to WP Transients API.
      * @throws RuntimeException If problem setting.
      */
     protected function setTransient(string $key, $value, int $ttl): void
     {
         $this->validateTransientKey($key);
 
         if (!set_transient($key, $value, $ttl)) {
-            throw new RuntimeException(sprintf('set_transient() failed with key "%1$s" with TTL %2$ss', $key, $ttl));
+            $actualValue = $this->getTransient($key);
+            if ($actualValue !== $value) {
+                throw new UnexpectedValueException(
+                    sprintf('set_transient() failed with key "%1$s" with TTL %2$ss', $key, $ttl)
+                );
+            }
         }
     }
 

tests/functional/CachePoolTest.php

@@ -359,4 +359,35 @@ public function testHas()
             $this->assertFalse($subject->has($notThereKey));
         }
     }
+
+    /**
+     * Imitates a scenario where `set_transient()` erroneously returns `false` if set value is same as existing.
+     */
+    public function testWritingSameValueSucceeds()
+    {
+        {
+            $poolName = uniqid('pool');
+            $defaultValue = uniqid('default');
+            $wpdb = $this->createWpdb();
+            $key = uniqid('key');
+            $value = uniqid('value');
+            $subject = $this->createInstance($wpdb, $poolName, $defaultValue);
+        }
+
+        {
+            Functions\expect('get_transient')
+                ->andReturn($value);
+            Functions\expect('set_transient')
+                ->andReturn();
+        }
+
+        {
+            // If no exception - success
+            $subject->set($key, $value, rand(1, 99999));
+
+            // Problem setting a different value results in an exception
+            $this->expectException(CacheException::class);
+            $subject->set($key, uniqid('other-value'), rand(1, 99999));
+        }
+    }
 }