static int getLeasedDecompressorsCount(@Nullable CompressionCodec codec) {
    if (decompressorCounts == null) {
      throw new IllegalStateException("initLeaseCounting() not called to set up lease counting");
    }
    return (codec == null) ? 0 : getLeaseCount(decompressorCounts, codec.getDecompressorType());
  }
Returns the number of leased {@link Decompressor}s for this {@link CompressionCodec}. Copied
from hadoop-common without significant modification.
hbase-common/src/main/java/org/apache/hadoop/hbase/io/compress/CodecPool.java