This is caused by a syscall violation.
rdar://problem/49060359
Created attachment 366401 [details] Patch
Comment on attachment 366401 [details] Patch r=me
Comment on attachment 366401 [details] Patch Thanks for reviewing!
Comment on attachment 366401 [details] Patch Clearing flags on attachment: 366401 Committed r243699: <https://trac.webkit.org/changeset/243699>
All reviewed patches have been landed. Closing bug.