Stack Output Files Read Race

terragrunt stack commands no longer crash on stacks with multiple units

Running terragrunt stack output (or any command that resolves concurrently parsing multiple configuration files) against a stack with several units could intermittently crash while the units were being parsed in parallel due to a race on internal bookkeeping of files read (used in the reading filter attribute).

Parallel unit parsing now coordinates safely when recording which source files were read, preventing crashes.