MGS3023: pipe cycle

A magus run whose stdin comes from another magus waits until that upstream stage is done with the projects the run needs (see Piping one magus into another). The chain of writers is followed through other tools too (cat, tee, jq). When it leads back to the run itself past another magus stage, every stage in the loop would wait on the one before it. None of them could ever start, so the run is refused before it takes any lock:

[MGS3023] this run's standard input is written, through magus pid 40118, by this run
  itself; each stage would wait on the one before it, so it is refused instead. Break
  the loop so one stage's input does not depend on its own output.

A plain shell pipe cannot form this loop. It takes a construct that wires a process's output back into its own input, such as bash's coproc, or a program that connects two pipes by hand.

Resolution

• Break the loop. Write the upstream's output to a file, then run the downstream stage with the file as its input.

See also

• MGS3007: a nested run that needs a project its own ancestor holds.
• Concurrency: how one magus waits on another it reads from.