Snappy 1.1.7.1 fails when decompressing empty serialized data

I use pyspark  like that

```

from pyspark.storagelevel import StorageLevel
df=spark.sql("select * from xzn.person")
df.persist(StorageLevel(False, True, False, False))
df.count()

```

table person is a simple table stored as orc files and some orc files is empty. When I run the query, it throw the error : 

```

19/03/22 21:46:31 INFO MemoryStore:54 - Block rdd_2_1 stored as values in memory (estimated size 0.0 B, free 1662.6 MB)
19/03/22 21:46:31 INFO FileScanRDD:54 - Reading File path: viewfs://name/xzn.db/person/part-00011, range: 0-49, partition values: [empty row]
19/03/22 21:46:31 INFO FileScanRDD:54 - Reading File path: viewfs://name/xzn.db/person/part-00011_copy_1, range: 0-49, partition values: [empty row]
19/03/22 21:46:31 INFO FileScanRDD:54 - Reading File path: viewfs://name/xzn.db/person/part-00012, range: 0-49, partition values: [empty row]
19/03/22 21:46:31 INFO FileScanRDD:54 - Reading File path: viewfs://name/xzn.db/person/part-00012_copy_1, range: 0-49, partition values: [empty row]
19/03/22 21:46:31 INFO FileScanRDD:54 - Reading File path: viewfs://name/xzn.db/person/part-00013, range: 0-49, partition values: [empty row]
19/03/22 21:46:31 ERROR Executor:91 - Exception in task 1.0 in stage 0.0 (TID 1)
org.xerial.snappy.SnappyIOException: [EMPTY_INPUT] Cannot decompress empty stream
at org.xerial.snappy.SnappyInputStream.readHeader(SnappyInputStream.java:94)
at org.xerial.snappy.SnappyInputStream.<init>(SnappyInputStream.java:59)
at org.apache.spark.io.SnappyCompressionCodec.compressedInputStream(CompressionCodec.scala:164)
at org.apache.spark.serializer.SerializerManager.wrapForCompression(SerializerManager.scala:163)
at org.apache.spark.serializer.SerializerManager.dataDeserializeStream(SerializerManager.scala:209)
at org.apache.spark.storage.BlockManager.getLocalValues(BlockManager.scala:596)
at org.apache.spark.storage.BlockManager.getOrElseUpdate(BlockManager.scala:886)
at org.apache.spark.rdd.RDD.getOrCompute(RDD.scala:335)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:286)
at org.apache.spark.rdd.MapPartitionsRDD.compute(MapPartitionsRDD.scala:52)
at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:324)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:288)
at org.apache.spark.rdd.MapPartitionsRDD.compute(MapPartitionsRDD.scala:52)
at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:324)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:288)
at org.apache.spark.rdd.MapPartitionsRDD.compute(MapPartitionsRDD.scala:52)
at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:324)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:288)
at org.apache.spark.rdd.MapPartitionsRDD.compute(MapPartitionsRDD.scala:52)
at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:324)
at org.apache.spark.rdd.RDD.iterator(RDD.scala:288)
at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:99)
at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:55)
at org.apache.spark.scheduler.Task.run(Task.scala:121)
at org.apache.spark.executor.Executor$TaskRunner$$anonfun$10.apply(Executor.scala:402)
at org.apache.spark.util.Utils$.tryWithSafeFinally(Utils.scala:1360)
at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:408)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
```

After I search it, I find that 1.1.7.x  snappy-java 's behavior is different from 1.1.2.x (that  spark 2.0.2 use this version). SnappyOutputStream in 1.1.2.x version always writes a snappy header whether or not to write a value,  but  SnappyOutputStream in 1.1.7.x don't generate header if u don't write value into it, so in spark 2.4 if RDD cache a empty value, memoryStore will not cache any bytes ( no snappy header ),  then it will throw the empty error. 

 

Maybe we can change SnappyOutputStream to fix it in 1.1.7.x snappy-java, there is my SnappyOutputStream method compressInput code 

```

protected void compressInput()
throws IOException
{
// generate header
if (!headerWritten)

{ outputCursor = writeHeader(); headerWritten = true; }

if (inputCursor <= 0)

{ return; // no need to dump }

// if (!headerWritten)

{ // outputCursor = writeHeader(); // headerWritten = true; // }

// Compress and dump the buffer content
if (!hasSufficientOutputBufferFor(inputCursor))

{ dumpOutput(); }

writeBlockPreemble();

int compressedSize = Snappy.compress(inputBuffer, 0, inputCursor, outputBuffer, outputCursor + 4);
// Write compressed data size
writeInt(outputBuffer, outputCursor, compressedSize);
outputCursor += 4 + compressedSize;
inputCursor = 0;
}

```