## Summary Today it happened to me that I could not use BuildStream due to a stack trace which happens at the beginning of the session. After applying an `fcntl` workaround which clears the file descriptor flags on stderr/stdio, BuildStream started working again. After *removing* the workaround, BuildStream no longer produces the unreproducible error. ## Steps to reproduce [//]: # (How one can reproduce the issue - this is very important) None. ## What is the current bug behavior? [//]: # (What actually happens) Spurious stack traces which rarely happen. ## What is the expected correct behavior? [//]: # (What you should see instead) No spurious stack traces ## 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) Here is a sample of the log file: ``` cached 15d0d536e33e473b6cdc71fce2c8d935cefa2480aa53e1c13e7d5878ff8fe6e8 freedesktop-sdk.bst:base/libassuan.bst cached 63c47a9a99233e6c20f720202a952cee77d278edd4578c5807493a35d05aa19b freedesktop-sdk.bst:base/libksba.bst cached e8b2afa8d69c88aeb6e148af8290c80eea616c11b6ef10bb3a85a6770d24bf07 freedesktop-sdk.bst:base/npth.bst cached 0277075af66ca787c632cfeb8633dcdbe8fb20875a8cdf55159b56205b8ec38d freedesktop-sdk.bst:base/gnupg.bst cached 7043a1feb89f7c416525cce0b315dbaccb2d61de92fca1994a545c5d952cb1b1 freedesktop-sdk.bst:base/pcre2.bst cached bf4f16eebdd80ff8b70a4fdd4e10f8a922db73e6f6b69a7c663c6638f932a527 freedesktop-sdk.bst:base/git.bst cached 61a91a356e8af39cd6f448cda9b621859b56293c414c9de309423e9040f8ca3d[--:--:--][][] BUG [Errno 11] write could not complete without blocking Traceback (most recent call last): File "/home/tristan/.local/bin/bst", line 8, in <module> sys.exit(cli()) File "/home/tristan/.local/lib/python3.5/site-packages/click/core.py", line 764, in __call__ return self.main(*args, **kwargs) File "/codethink/GNOME/buildstream/buildstream/_frontend/cli.py", line 162, in override_main standalone_mode=standalone_mode, **extra) File "/home/tristan/.local/lib/python3.5/site-packages/click/core.py", line 717, in main rv = self.invoke(ctx) File "/home/tristan/.local/lib/python3.5/site-packages/click/core.py", line 1137, in invoke return _process_result(sub_ctx.command.invoke(sub_ctx)) File "/home/tristan/.local/lib/python3.5/site-packages/click/core.py", line 956, in invoke return ctx.invoke(self.callback, **ctx.params) File "/home/tristan/.local/lib/python3.5/site-packages/click/core.py", line 555, in invoke return callback(*args, **kwargs) File "/home/tristan/.local/lib/python3.5/site-packages/click/decorators.py", line 27, in new_func return f(get_current_context().obj, *args, **kwargs) File "/codethink/GNOME/buildstream/buildstream/_frontend/cli.py", line 330, in build build_all=all_) File "/codethink/GNOME/buildstream/buildstream/_stream.py", line 213, in build self._run() File "/codethink/GNOME/buildstream/buildstream/_stream.py", line 1006, in _run self._session_start_callback() File "/codethink/GNOME/buildstream/buildstream/_frontend/app.py", line 637, in session_start_cb styling=self.colors) File "/codethink/GNOME/buildstream/buildstream/_frontend/widget.py", line 498, in print_heading click.echo(text, color=styling, nl=False, err=True) File "/home/tristan/.local/lib/python3.5/site-packages/click/utils.py", line 260, in echo file.write(message) BlockingIOError: [Errno 11] write could not complete without blocking ``` ## Possible fixes [//]: # (If you can, link to the line of code that might be responsible for the problem) Clear file descriptor flags unconditionally at startup, can't say right now that this workaround is completely safe. The idea for the workaround was taken from a [similar issue](https://github.com/travis-ci/travis-ci/issues/8920#issuecomment-352767521)