[Support] Clear errno before calling the function in RetryAfterSignal.
For certain APIs, the return value of the function does not distinguish
between failure (which populates errno) and other non-error conditions
(which do not set errno).
For example, `fgets` returns `NULL` both when an error has occurred, or
upon EOF. If `errno` is already `EINTR` for whatever reason, then
```
RetryAfterSignal(nullptr, fgets, ...);
```
on a stream that has reached EOF would infinite loop.
Fix this by setting `errno` to `0` before each attempt in
`RetryAfterSignal`.
Patch by Ricky Zhou!
Differential Revision: https://reviews.llvm.org/D48755
llvm-svn: 336479
2 files changed