## What changes were proposed in this pull request?
`ReceiverSupervisorImpl` on executor side reports block's meta back to `ReceiverTracker` on driver side. In current code, `askWithRetry` is used. However, for `AddBlock`, `ReceiverTracker` is not idempotent, which may result in messages are processed multiple times.
**To reproduce**:
1. Check if it is the first time receiving `AddBlock` in `ReceiverTracker`, if so sleep long enough(say 200 seconds), thus the first RPC call will be timeout in `askWithRetry`, then `AddBlock` will be resent.
2. Rebuild Spark and run following job:
```
def streamProcessing(): Unit = {
val conf = new SparkConf()
.setAppName("StreamingTest")
.setMaster(masterUrl)
val ssc = new StreamingContext(conf, Seconds(200))
val stream = ssc.socketTextStream("localhost", 1234)
stream.print()
ssc.start()
ssc.awaitTermination()
}
```
**To fix**:
It makes sense to provide a blocking version `ask` in RpcEndpointRef, as mentioned in SPARK-18113 (https://github.com/apache/spark/pull/16503#event-927953218). Because Netty RPC layer will not drop messages. `askWithRetry` is a leftover from akka days. It imposes restrictions on the caller(e.g. idempotency) and other things that people generally don't pay that much attention to when using it.
## How was this patch tested?
Test manually. The scenario described above doesn't happen with this patch.
Author: jinxing <jinxing@meituan.com>
Closes#16690 from jinxing64/SPARK-19347.
c5fcb7f68b