Bst open too many files which causes `OSError`s for the next calls
## Summary
[//]: # (Summarize the bug encountered concisely)
While trying to build [gnome-build-meta][1] today, I hit the following bug. `bst` seems like it have opened too many File descriptors and the rest syscalls failed with ` OSError: [Errno 24] Too many open files`
## Steps to reproduce
[//]: # (How one can reproduce the issue - this is very important)
Not really sure how to reproduce it, It only happened once thus far. I had a recent checkout and build artifacts of the project, did a `git pull` and tried building again. Most of the fetches should have pulled down little if anything at all.
## What is the current bug behavior?
[//]: # (What actually happens)
For each pull task the following happened:
An unhandled exception is thrown that keeps. Upon `control-C` neither `quit` nor `terminate` options seem to be able to exit.
## What is the expected correct behavior?
[//]: # (What you should see instead)
Buildstream never gets to have too many files open at which point the `syscalls` start to fail.
## Relevant logs and/or screenshots
[//]: # (Paste any relevant logs - please use code blocks ``` to format console output, logs, and code as it's hard to read otherwise. You can also add a Snippet and link it here. Check the markdown giude on Gitlab for further tips)
I couldn't copy more than a task due to a weird bug with my terminal, but the following trace was identical for the rest of the too.
```python
Unknown exception in SIGCHLD handler
Traceback (most recent call last):
File "/usr/lib64/python3.6/asyncio/unix_events.py", line 819, in _sig_chld
File "/usr/lib64/python3.6/asyncio/unix_events.py", line 885, in _do_waitpid_all
File "/usr/lib64/python3.6/asyncio/unix_events.py", line 919, in _do_waitpid
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/jobs/job.py", line 557, in _parent_child_completed
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/scheduler.py", line 238, in job_completed
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/scheduler.py", line 288, in _schedule_queue_jobs
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/queues/queue.py", line 174, in enqueue
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/queues/queue.py", line 174, in <listcomp>
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/queues/queue.py", line 171, in create_job
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/jobs/elementjob.py", line 69, in __init__
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/jobs/job.py", line 112, in __init__
[--:--:--][????????][ pull:bootstrap-junction.bst:coreutils.bst] START freedesktop-sdk-bootstrap/coreutils/????????-pull.4976.log
[--:--:--][????????][ pull:bootstrap-junction.bst:coreutils.bst] INFO Pulling ???????? <- https://cache.gitlab.gnome.org:11001
[--:--:--][????????][ pull:bootstrap-junction.bst:coreutils.bst] INFO Pulling ???????? <- https://testcache.codethink.co.uk:11001
[--:--:--][????????][ pull:bootstrap-junction.bst:coreutils.bst] INFO Pulling ???????? <- https://cache.gitlab.gnome.org:11001
[--:--:--][????????][ pull:bootstrap-junction.bst:coreutils.bst] INFO Pulling ???????? <- https://testcache.codethink.co.uk:11001
[00:00:01][????????][ pull:bootstrap-junction.bst:coreutils.bst] SUCCESS freedesktop-sdk-bootstrap/coreutils/????????-pull.4976.log
[--:--:--][????????][ main:bootstrap-junction.bst:coreutils.bst] BUG Unhandled exception in post processing
Traceback (most recent call last):
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/queues/queue.py", line 310, in _job_done
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/queues/pullqueue.py", line 65, in done
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/scheduler.py", line 332, in _check_cache_size_real
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/jobs/cachesizejob.py", line 25, in __init__
File "/usr/lib/python3.6/site-packages/buildstream/_scheduler/jobs/job.py", line 112, in __init__
File "/usr/lib64/python3.6/multiprocessing/context.py", line 102, in Queue
File "/usr/lib64/python3.6/multiprocessing/queues.py", line 42, in __init__
File "/usr/lib64/python3.6/multiprocessing/context.py", line 67, in Lock
File "/usr/lib64/python3.6/multiprocessing/synchronize.py", line 163, in __init__
File "/usr/lib64/python3.6/multiprocessing/synchronize.py", line 60, in __init__
OSError: [Errno 24] Too many open files
```
## Possible fixes
[//]: # (If you can, link to the line of code that might be responsible for the problem)
## Other relevant information
[//]: # (Add the BuildStream version. Substitute below the "x" by "1", "2" or the right version. Add more than one version if necessary)
* BuildStream version affected: `1.1.6` /milestone %BuildStream_v1.1
[1]: https://gitlab.gnome.org/GNOME/gnome-build-meta
----
issue
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