[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

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/unittests/Support/ErrnoTest.cpp b/llvm/unittests/Support/ErrnoTest.cpp
index 67f834a938d..701ac960825 100644
--- a/llvm/unittests/Support/ErrnoTest.cpp
+++ b/llvm/unittests/Support/ErrnoTest.cpp
@@ -33,4 +33,7 @@ TEST(ErrnoTest, RetryAfterSignal) {
 
   std::unique_ptr<int> P(RetryAfterSignal(nullptr, [] { return new int(47); }));
   EXPECT_EQ(47, *P);
+
+  errno = EINTR;
+  EXPECT_EQ(-1, RetryAfterSignal(-1, [] { return -1; }));
 }