FakeStorageRpc: NPE on copying file

[domrde]

Environment details

1. FakeStorageRpc
2. OS type and version: macOS 11.6.5
3. Java version: 11
4. google-cloud-nio version(s): 0.123.27

Steps to reproduce

If you use LocalStorageHelper to create a storage, and create a file in it using

storage.create(BlobInfo.newBuilder(sourceBucket, sourcePath).build(), content);

It will be stored in FakeStorageRpc with null generation. If you try to copy some other file to overwrite the original one using

final var request = CopyRequest.newBuilder()
            .setSource(BlobId.of(sourceBucket, sourcePath))
            .setTarget(BlobId.of(destinationBucket, destinationPath))
            .build();

storage.copy(request)
            .getResult();

You'll get a NullPointerException in the following FakeStorageRpc's code

long generation = 1;
if (metadata.containsKey(destKey)) {
   generation = metadata.get(destKey).getGeneration() + 1;
}

since metadata.get(destKey).getGeneration() is null. Checking the generation for nullability before the increment, like it is done in write method, should solve the issue.

Stack trace

Caused by: java.lang.NullPointerException
	at com.google.cloud.storage.contrib.nio.testing.FakeStorageRpc.openRewrite(FakeStorageRpc.java:416)
	at com.google.cloud.storage.StorageImpl.lambda$copy$22(StorageImpl.java:511)
	at com.google.api.gax.retrying.DirectRetryingExecutor.submit(DirectRetryingExecutor.java:103)
	at com.google.cloud.RetryHelper.run(RetryHelper.java:76)
	at com.google.cloud.RetryHelper.runWithRetries(RetryHelper.java:50)
	at com.google.cloud.storage.Retrying.run(Retrying.java:51)