LibThreading: Add a thread state enum

This will later be used by Thread to keep track of its state more simply
and obviously.

Userland/Libraries/LibThreading/Thread.h

@@ -18,6 +18,28 @@ namespace Threading {
 
 AK_TYPEDEF_DISTINCT_ORDERED_ID(intptr_t, ThreadError);
 
+// States of userspace threads are simplified over actual kernel states (and possibly POSIX states).
+// There are only a couple of well-defined transitions between these states, and any attempt to call a function in a state where this is not allowed will crash the program.
+enum class ThreadState : u8 {
+    // Thread has been constructed but not started.
+    // Transitions to Running via start().
+    Startable,
+    // Thread has been started, might be running, and can be joined.
+    // Note that join() (valid to call in this state) only changes the thread state after the thread has exited, so it only ever transitions from Exited to Joined.
+    // Transitions to Detached via detach(), transitions to Exited when the thread finishes its action function.
+    Running,
+    // Thread has not been detached and exited, and has to still be joined.
+    // Transitions to Joined via join().
+    Exited,
+    // Thread has been started but also detached, meaning it cannot be joined.
+    // Transitions to DetachedExited when the thread finishes its action function.
+    Detached,
+    // Thread has exited but was detached, meaning it cannot be joined.
+    DetachedExited,
+    // Thread has exited and been joined.
+    Joined,
+};
+
 class Thread final : public Core::Object {
     C_OBJECT(Thread);
 
@@ -71,3 +93,34 @@ struct AK::Formatter<Threading::Thread> : AK::Formatter<FormatString> {
         return Formatter<FormatString>::format(builder, "Thread \"{}\"({})"sv, thread.thread_name(), thread.tid());
     }
 };
+
+template<>
+struct AK::Formatter<Threading::ThreadState> : AK::Formatter<FormatString> {
+    ErrorOr<void> format(FormatBuilder& builder, Threading::ThreadState state)
+    {
+        String name = "";
+        switch (state) {
+        case Threading::ThreadState::Detached:
+            name = "Detached";
+            break;
+        case Threading::ThreadState::DetachedExited:
+            name = "DetachedExited";
+            break;
+        case Threading::ThreadState::Exited:
+            name = "Exited";
+            break;
+        case Threading::ThreadState::Joined:
+            name = "Joined";
+            break;
+        case Threading::ThreadState::Running:
+            name = "Running";
+            break;
+        case Threading::ThreadState::Startable:
+            name = "Startable";
+            break;
+        default:
+            VERIFY_NOT_REACHED();
+        }
+        return Formatter<FormatString>::format(builder, "{}"sv, name);
+    }
+};