David Feuer authored Dec 24, 2020 and Marge Bot committed Feb 22, 2021

* `openFile` could sometimes leak file descriptors if it
  received an asynchronous exception (#19114, #19115). Fix this
  on POSIX.

* `openFile` and more importantly `openFileBlocking` could not
  be interrupted effectively during the `open` system call (#17912).
  Fix this on POSIX.

* Implement `readFile'` using `withFile` to ensure the file is closed promptly on exception.

* Avoid `bracket` in `withFile`, reducing the duration of masking.

Closes #19130.

Addresses #17912, #19114, and #19115 on POSIX systems, but not
on Windows.