LibPthread: Move the pthread and semaphore implementation to LibC

This additionally adds some compatibility code to redirect linking
attempts for LibPthread to LibC instead.

Userland/Libraries/LibC/pthread_once.cpp

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2020, Sergey Bugaev <bugaevc@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Assertions.h>
+#include <AK/Atomic.h>
+#include <AK/Types.h>
+#include <pthread.h>
+#include <serenity.h>
+
+enum State : i32 {
+    INITIAL = PTHREAD_ONCE_INIT,
+    DONE,
+    PERFORMING_NO_WAITERS,
+    PERFORMING_WITH_WAITERS,
+};
+
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_once.html
+int pthread_once(pthread_once_t* self, void (*callback)(void))
+{
+    auto& state = reinterpret_cast<Atomic<State>&>(*self);
+
+    // See what the current state is, and at the same time grab the lock if we
+    // got here first. We need acquire ordering here because if we see
+    // State::DONE, everything we do after that should "happen after" everything
+    // the other thread has done before writing the State::DONE.
+    State state2 = State::INITIAL;
+    bool have_exchanged = state.compare_exchange_strong(
+        state2, State::PERFORMING_NO_WAITERS, AK::memory_order_acquire);
+
+    if (have_exchanged) {
+        // We observed State::INITIAL and we've changed it to
+        // State::PERFORMING_NO_WAITERS, so it's us who should perform the
+        // operation.
+        callback();
+        // Now, record that we're done.
+        state2 = state.exchange(State::DONE, AK::memory_order_release);
+        switch (state2) {
+        case State::INITIAL:
+        case State::DONE:
+            VERIFY_NOT_REACHED();
+        case State::PERFORMING_NO_WAITERS:
+            // The fast path: there's no contention, so we don't have to wake
+            // anyone.
+            break;
+        case State::PERFORMING_WITH_WAITERS:
+            futex_wake(self, INT_MAX);
+            break;
+        }
+
+        return 0;
+    }
+
+    // We did not get there first. Let's see if we have to wait.
+    // state2 contains the observed state.
+    while (true) {
+        switch (state2) {
+        case State::INITIAL:
+            VERIFY_NOT_REACHED();
+        case State::DONE:
+            // Awesome, nothing to do then.
+            return 0;
+        case State::PERFORMING_NO_WAITERS:
+            // We're going to wait for it, but we have to record that we're
+            // waiting and the other thread should wake us up. We need acquire
+            // ordering here for the same reason as above.
+            have_exchanged = state.compare_exchange_strong(
+                state2, State::PERFORMING_WITH_WAITERS, AK::memory_order_acquire);
+            if (!have_exchanged) {
+                // Something has changed already, reevaluate without waiting.
+                continue;
+            }
+            state2 = State::PERFORMING_WITH_WAITERS;
+            [[fallthrough]];
+        case State::PERFORMING_WITH_WAITERS:
+            // Let's wait for it.
+            futex_wait(self, state2, nullptr, 0);
+            // We have been woken up, but that might have been due to a signal
+            // or something, so we have to reevaluate. We need acquire ordering
+            // here for the same reason as above. Hopefully we'll just see
+            // State::DONE this time, but who knows.
+            state2 = state.load(AK::memory_order_acquire);
+            continue;
+        }
+    }
+}