Spark SQL queries (though this seems to be a Spark Core issue - I'm just using queries in the REPL to surface this, so I mention Spark SQL) hang indefinitely under certain (not totally understood) circumstances.
This is resolved by setting spark.shuffle.blockTransferService=nio, which seems to point to netty as the issue. Netty was set as the default for the block transport layer in 1.2.0, which is when this issue started. Setting the service to nio allows queries to complete normally.
I do not see this problem when running queries over smaller (~20 5MB files) datasets. When I increase the scope to include more data (several hundred ~5MB files), the queries will get through several steps but eventuall hang indefinitely.
Here's the email chain regarding this issue, including stack traces:
For context, here's the announcement regarding the block transfer service change: http://mail-archives.apache.org/mod_mbox/spark-dev/201411.mbox/<CABPQxssL04Q+rBLtP-D8W+z3aTn+g-Um6GMdGDnh-hZcvDemail@example.com>