isaac_seed assert

[jesse99]

Noticed the following when running my app: Assertion failed: (ret == 0), function isaac_seed, file /Users/jessejones/Source/Third_Party/rust/src/rt/rust_util.h, line 153. Doesn't seem especially reproducible.

Looking at the header:

• The assert is firing when trying to close a /dev/urandom file descriptor. I can see checking for errors on a descriptor being written to (the close will often flush buffered data), but failing because close failed for reads seems a lot iffier.
• All the error checking in isaac_seed (including Windows) is using asserts which seems questionable: usually those should be reserved to catch errors programmers make, not arbitrary errors the runtime will throw up (especially if those errors may be ifdef'ed out).
• It'd be great if the Unix code used perror.

This was on a Mac. I note that man 2 close lists three reasons why close may fail including the perennial favorite "execution was interrupted by a signal".

[jesse99]

It's definitely intermittent, but bad enough that I wound up replacing the assert with an fwrite. Interestingly once I finally reproed it perror said that the problem was "Bad file descriptor".

[bblum]

Odd. Definitely related: #2870 and its fix 60d682b.

[brson]

I do think we should be doing this check for the closed file descriptor (it caught this error!), but for now we can turn it off as a bandaid.