[libc] Implement pwait2 using pwait

[mikhailramalho]

This patch implements pwait2 using pwait. The implementation is an approximation of pwait2, since pwait only only supports timeouts in milliseconds, not nanoseconds, as required by pwait2.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Mikhail R. Gadelha (mikhailramalho)

Changes

This patch implements pwait2 using pwait. The implementation is an approximation of pwait2, since pwait only only supports timeouts in milliseconds, not nanoseconds, as required by pwait2.

---

Full diff: https://github.com/llvm/llvm-project/pull/99781.diff

1 Files Affected:

• (modified) libc/src/sys/epoll/linux/epoll_pwait2.cpp (+12)

diff --git a/libc/src/sys/epoll/linux/epoll_pwait2.cpp b/libc/src/sys/epoll/linux/epoll_pwait2.cpp
index 14b419399fe9b..4123157d29fff 100644
--- a/libc/src/sys/epoll/linux/epoll_pwait2.cpp
+++ b/libc/src/sys/epoll/linux/epoll_pwait2.cpp
@@ -25,10 +25,22 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(int, epoll_pwait2,
                    (int epfd, struct epoll_event *events, int maxevents,
                     const struct timespec *timeout, const sigset_t *sigmask)) {
+#ifdef SYS_epoll_pwait2
   int ret = LIBC_NAMESPACE::syscall_impl<int>(
       SYS_epoll_pwait2, epfd, reinterpret_cast<long>(events), maxevents,
       reinterpret_cast<long>(timeout), reinterpret_cast<long>(sigmask),
       NSIG / 8);
+#elif defined(SYS_epoll_pwait)
+  // Convert nanoseconds to milliseconds, rounding up if there are remaining
+  // nanoseconds
+  long timeout_ms = static_cast<long>(timeout->tv_sec * 1000 +
+                                      (timeout->tv_nsec + 999999) / 1000000);
+  int ret = LIBC_NAMESPACE::syscall_impl<int>(
+      SYS_epoll_pwait, epfd, reinterpret_cast<long>(events), maxevents,
+      timeout_ms, reinterpret_cast<long>(sigmask), NSIG / 8);
+#else
+#error "epoll_pwait and epoll_pwait2 syscalls not available."
+#endif
 
   // A negative return value indicates an error with the magnitude of the
   // value being the error code.

[michaelrj-google]

I don't think this is a good idea. I would rather just not provide epoll_pwait2 on platforms that don't support it.