Clear errno before calling the function in RetryAfterSignal.

[ricky]

Summary

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.

[chandlerc]

Thanks, this is obviously correct. Do you have commit access or do you need someone to land this for you?

[ricky]

I don't have commit access, so would be great if you could land it - thanks!

Ricky

llvm/include/llvm/Support/Errno.h

 
 /// Like the no-argument version above, but uses \p errnum instead of errno.
 std::string StrError(int errnum);
 
 template <typename FailT, typename Fun, typename... Args>
 inline auto RetryAfterSignal(const FailT &Fail, const Fun &F,
                              const Args &... As) -> decltype(F(As...)) {
   decltype(F(As...)) Res;
-  do
+  do {
+    errno = 0;
     Res = F(As...);
-  while (Res == Fail && errno == EINTR);
+  } while (Res == Fail && errno == EINTR);
   return Res;
 }
 
 }  // namespace sys
 }  // namespace llvm
 
 #endif  // LLVM_SYSTEM_ERRNO_H

llvm/unittests/Support/ErrnoTest.cpp

EXPECT_EQ(1, RetryAfterSignal(-1, [&calls] {
               return calls == 1 ? -1 : 1;
             }));
   EXPECT_EQ(2u, calls);
 
   EXPECT_EQ(1, RetryAfterSignal(-1, [](int x) { return x; }, 1));
 
   std::unique_ptr<int> P(RetryAfterSignal(nullptr, [] { return new int(47); }));
   EXPECT_EQ(47, *P);
+
+  errno = EINTR;
+  EXPECT_EQ(-1, RetryAfterSignal(-1, [] { return -1; }));
 }