# gemma-wrapper has incomplete files

Gemma wrapper caches files - but it can happen a cached file is incomplete and never updated again. The problem appears when GNU parallel is invoked and hits an error. The task here is to make gemma-wrapper transactional.

## Tags

* assigned: pjotrp, zachs

## Tasks

* [ ] parse parallel job log for failed tasks and remove the output files.
* [X] create a (global) lock file for gemma-wrapper

## Info

GNU parallel can fail, but does not tell how individual processes did. Need to check if it can return a thread (number). If not we have the option of checking the GEMMA status file and/or see if the output file is complete (by counting number of lines).

The 'obvious' fix would be to create an error handler in GEMMA itself that would clean up output files on error exit. E.g. using

=> https://www.cplusplus.com/reference/exception/set_terminate/

The problem is that it is NOT a catch all. If there is a hardware fault - a hanging CPU core, for example, which we see - or a problem in a library, such as openblas, there is no guarantee that the terminate handler will be called. Another complication is that a terminate handler needs to be aware of the files being output - i.e., we need to carry the state down somehow. I think we can probably address these issues as much is handled in the GEMMA PARAM class, but it is not worth the effort.

It turns out that GNU parallel can keep track of jobs in a job log - and even rerun the ones missing. The last we don't need because we are using a cache. But we can use the log file to remove any incomplete output files! To me this is the obvious solution because 'parallel' is monitoring outside the GEMMA process and is a hardened piece of software. On failure it simply designates runs that way and we can clean up any (partly) produced files followed by a safe rerun. The lock routine below ascertains no processes are creating the same output at the same time.

## Dealing with locks

There is another parallel issue (pun intended) where gemma-wrapper is invoked twice for the same job. This is quite possible when people get impatient waiting for a first job to finish.

One solution is to write a lock file using the inputs as a hash. The lock file can contain a PID and we can check if that is still alive. I should do the same for sheepdog locks(!)

=> https://github.com/genetics-statistics/gemma-wrapper/commit/e7e516ec5a6ffc5b398302fa204685a40e76e171 Added locking support