Fix PathPrefixedAdapter checksum

* Add the missing prefix path in the first case (instanceof ChecksumProvider)
* Remove unwanted double prefix in the second case (`prefixPath` method already called in `readStream`, itself called by `calculateChecksumFromStream`)

src/PathPrefixing/PathPrefixedAdapter.php

@@ -205,10 +205,10 @@ public function publicUrl(string $path, Config $config): string
     public function checksum(string $path, Config $config): string
     {
         if ($this->adapter instanceof ChecksumProvider) {
-            return $this->adapter->checksum($path, $config);
+            return $this->adapter->checksum($this->prefix->prefixPath($path), $config);
         }
 
-        return $this->calculateChecksumFromStream($this->prefix->prefixPath($path), $config);
+        return $this->calculateChecksumFromStream($path, $config);
     }
 
     public function temporaryUrl(string $path, DateTimeInterface $expiresAt, Config $config): string

src/PathPrefixing/PathPrefixedAdapterTest.php

@@ -2,6 +2,7 @@
 
 namespace League\Flysystem\PathPrefixing;
 
+use League\Flysystem\ChecksumProvider;
 use League\Flysystem\Config;
 use League\Flysystem\InMemory\InMemoryFilesystemAdapter;
 use League\Flysystem\UnableToGeneratePublicUrl;
@@ -95,6 +96,37 @@ public function publicUrl(string $path, Config $config): string
         self::assertEquals('memory://prefix/path.txt', $url);
     }
 
+    /**
+     * @test
+     */
+    public function calculate_checksum_using_decorated_adapter(): void
+    {
+        $adapter = new class() extends InMemoryFilesystemAdapter implements ChecksumProvider {
+            public function checksum(string $path, Config $config): string
+            {
+                return hash('md5', $this->read($path));
+            }
+        };
+
+        $prefixedAdapter = new PathPrefixedAdapter($adapter, 'prefix');
+        $prefixedAdapter->write('foo.txt', 'bla', new Config);
+
+        self::assertEquals('128ecf542a35ac5270a87dc740918404', $prefixedAdapter->checksum('foo.txt', new Config()));
+    }
+
+    /**
+     * @test
+     */
+    public function calculate_checksum_using_current_adapter(): void
+    {
+        $adapter = new InMemoryFilesystemAdapter();
+        $prefixedAdapter = new PathPrefixedAdapter($adapter, 'prefix');
+        $prefixedAdapter->write('foo.txt', 'bla', new Config);
+
+        self::assertEquals('128ecf542a35ac5270a87dc740918404', hash('md5', 'bla'));
+        self::assertEquals('128ecf542a35ac5270a87dc740918404', $prefixedAdapter->checksum('foo.txt', new Config()));
+    }
+
     /**
      * @test
      */