Incorrect use of positional read api in HFileBlock

Considering that read() is not guaranteed to read all bytes,
I'm interested to understand this particular piece of code and why is partial read treated as an error :

https://github.com/apache/hbase/blob/master/hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlock.java#L1446-L1450

Particularly, if hbase were to use a different filesystem, say WebhdfsFileSystem, this would not work, please also see https://issues.apache.org/jira/browse/HDFS-8943 for discussion around this.