fix: check generation for nullability before incrementing it (#888)

Author: domrde

google-cloud-nio/src/main/java/com/google/cloud/storage/contrib/nio/testing/FakeStorageRpc.java

@@ -413,7 +413,10 @@ public RewriteResponse openRewrite(RewriteRequest rewriteRequest) throws Storage
     // if this is a new file, set generation to 1, else increment the existing generation
     long generation = 1;
     if (metadata.containsKey(destKey)) {
-      generation = metadata.get(destKey).getGeneration() + 1;
+      Long storedGeneration = metadata.get(destKey).getGeneration();
+      if (null != storedGeneration) {
+        generation = storedGeneration + 1;
+      }
     }
 
     checkGeneration(destKey, generationMatch);

google-cloud-nio/src/test/java/com/google/cloud/storage/contrib/nio/testing/LocalStorageHelperTest.java

@@ -156,4 +156,28 @@ public void testStorageOptionIsSerializable_customOptions() throws Exception {
       assertThat(ois.readObject()).isEqualTo(storageOptions);
     }
   }
+
+  @Test
+  public void testCopyOperationOverwritesExistingFile() {
+    String bucket = "bucket";
+    String original = "original";
+    String replacement = "replacement";
+    byte[] originalContent = "original content".getBytes();
+    byte[] replacementContent = "replacement content".getBytes();
+
+    localStorageService.create(BlobInfo.newBuilder(bucket, original).build(), originalContent);
+    localStorageService.create(
+        BlobInfo.newBuilder(bucket, replacement).build(), replacementContent);
+
+    final Storage.CopyRequest request =
+        Storage.CopyRequest.newBuilder()
+            .setSource(BlobId.of(bucket, replacement))
+            .setTarget(BlobId.of(bucket, original))
+            .build();
+
+    localStorageService.copy(request).getResult();
+
+    assertThat(localStorageService.readAllBytes(BlobId.of(bucket, original)))
+        .isEqualTo(replacementContent);
+  }
 }