Major Description
The Crunch AvroInputFormat class explicitly checks each input path for a '.avro' extension.
If none of the input paths have such an extension, the job continues and succeeds even though no map tasks get allocated, and none of the pipeline stages are executed.
( I notice this behaviour is taken from Avro's own input format. It does happen in standard Avro mapreduce too therefore, but only in the old mapred API - the new mapreduce API version will happily read files regardless of extension. I'm going to report this as a bug on Avro as well...)