Fix crash of scylla-sstableloader when files are deleted during import

[pgoron]

When importing a large table, md files can be removed before being processed, resulting in a NoSuchFileException. BulkLoader.openFile() is supposed to returrn null when facing IOException to skip processing of the file and avoid throwing unhandled exception.

    public static SSTableReader openFile(Pair<Descriptor, Set<Component>> p, CFMetaData cfm) {
        try {
            // To conserve memory, open SSTableReaders without bloom
            // filters and discard
            // the index summary after calculating the file sections to
            // stream and the estimated
            // number of keys for each endpoint. See CASSANDRA-5555 for
            // details.
            return openForBatch(p.left, p.right, cfm);
        } catch (Exception e) {
            logger.warn("Skipping file {}, error opening it: {}", p.left.baseFilename(), e.getMessage());
        }
        return null;
    }

Unfortunately org.apache.cassandra.io.util.ChannelProxy.openChannel() is wrapping IOException in RuntimeException making exception handler in BulkLoader.openFile() useless.

    public static FileChannel openChannel(File file)
    {
        try
        {
            return FileChannel.open(file.toPath(), StandardOpenOption.READ);
        }
        catch (IOException e)
        {
            throw new RuntimeException(e);
        }
    }

Below an example of uncaught exception interrupting import:

java.lang.RuntimeException: java.nio.file.NoSuchFileException: /var/opt/cassandra/data/biggraphite/datapoints_360p_3600s_aggr-6c402f30e40311e7b12e356deff79235/md-8887-big-Index.db
        at org.apache.cassandra.io.util.ChannelProxy.openChannel(ChannelProxy.java:55)
        at org.apache.cassandra.io.util.ChannelProxy.<init>(ChannelProxy.java:66)
        at org.apache.cassandra.io.util.RandomAccessReader.open(RandomAccessReader.java:315)
        at org.apache.cassandra.io.sstable.format.SSTableReader.buildSummary(SSTableReader.java:840)
        at org.apache.cassandra.io.sstable.format.SSTableReader.openForBatch(SSTableReader.java:451)
        at com.scylladb.tools.BulkLoader.openFile(BulkLoader.java:1520)
        at com.scylladb.tools.BulkLoader.process(BulkLoader.java:1565)
        at com.scylladb.tools.BulkLoader.lambda$main$1(BulkLoader.java:1367)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        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)
Caused by: java.nio.file.NoSuchFileException: /var/opt/cassandra/data/biggraphite/datapoints_360p_3600s_aggr-6c402f30e40311e7b12e356deff79235/md-8887-big-Index.db
        at sun.nio.fs.UnixException.translateToIOException(UnixException.java:86)
        at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
        at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
        at sun.nio.fs.UnixFileSystemProvider.newFileChannel(UnixFileSystemProvider.java:177)
        at java.nio.channels.FileChannel.open(FileChannel.java:287)
        at java.nio.channels.FileChannel.open(FileChannel.java:335)
        at org.apache.cassandra.io.util.ChannelProxy.openChannel(ChannelProxy.java:51)
        ... 12 more