Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/999#discussion_r36587360
--- Diff:
flink-staging/flink-streaming/flink-streaming-core/src/main/java/org/apache/flink/streaming/api/datastream/DataStream.java
---
@@ -256,9 +256,11 @@ public ExecutionConfig getExecutionConfig() {
DataStream<OUT> returnStream = this.copy();
for (DataStream<OUT> stream : streams) {
- for (DataStream<OUT> ds : stream.unionedStreams) {
- validateUnion(ds.getId());
- returnStream.unionedStreams.add(ds.copy());
+ if (stream != null) {
--- End diff --
Right there will be an error, but the error is helpful because it indicates
a problem with the user program.
If the union is silently ignored, the program might behave differently from
what the user expects and produce invalid results.
I would argue, that a `union(null)` is never intended and should be be
brought to the user's attention. Also, this error might reveal more problems of
the user program.
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