Fix controller hang when submission exits early

[simonlindholm]

Having the u_to_c_w fd inherit to the controller means that when the submission exits, the controller will never read EOF, because it still itself has the write end of the pipe open. Easy fix is just to make that not inherit: nothing actually needs the inheritance, in fact it's rather a bit of a security issue that each spawned submission get access to extra fd's.

Small additional related fixes:

• if the interactive keeper hits walltime limit (claude pointed out some obscure circumstances in which this can happen, e.g. if the submission causes lots of isolate warning spam which clogs an stderr pipe which is never read from), we currently crash and leave the child un-waited. Better to generate a proper error.
• the change to make c_to_u_r not inherit means that now if the submission exits, controller writing will cause SIGPIPE unless the controller ignores that signal. We don't particularly want that to happen (it makes it impossible to detect which process exited first, which we really ought to do to give proper verdicts); better to leave the pipe unclosed.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.89%. Comparing base (6a28840) to head (e0af547).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cms/grading/tasktypes/Interactive.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1695   +/-   ##
=======================================
  Coverage   53.89%   53.89%           
=======================================
  Files         341      341           
  Lines       27977    27978    +1     
=======================================
+ Hits        15077    15078    +1     
  Misses      12900    12900           

Flag	Coverage Δ
functionaltests	0.00% <0.00%> (ø)
unittests	53.89% <0.00%> (+<0.01%)	⬆️

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[prandla]

i thought "manager gets sigpipe" is an issue that can happen anyways and that properly written managers always need to signal(SIGPIPE, SIG_IGN) anyways? (at least, looking at managers from old communication tasks, most of them do that.)

i admit this isn't very well documented, but imo leaving file descriptors open in two processes is not the right way to solve this.

i suppose this is at least not a resource leak, given that interactive_keeper should also exit shortly after the solution.