Add Scheduler wireframe.

Right now does nothing but has containing classes for process and thread
information.

zion/scheduler/scheduler.cpp

@@ -0,0 +1,84 @@
+#include "scheduler/scheduler.h"
+
+#include "debug/debug.h"
+
+namespace sched {
+namespace {
+
+// Simple linked list class with the intent of eventually replacing this with a
+// map.
+class ProcList {
+ public:
+  ProcList() {}
+
+  // Takes ownership.
+  void InsertProcess(Process* proc) {
+    if (front_ == nullptr) {
+      front_ = new ProcEntry{
+          .proc = proc,
+          .next = nullptr,
+      };
+      return;
+    }
+
+    ProcEntry* back = front_;
+    while (back->next != nullptr) {
+      back = back->next;
+    }
+
+    back->next = new ProcEntry{
+        .proc = proc,
+        .next = nullptr,
+    };
+  }
+
+ private:
+  struct ProcEntry {
+    Process* proc;
+    ProcEntry* next;
+  };
+
+  ProcEntry* front_ = nullptr;
+};
+
+class Scheduler {
+ public:
+  Scheduler() {
+    Process* root = Process::RootProcess();
+    current_thread_ = root->GetThread(0);
+    proc_list_.InsertProcess(Process::RootProcess());
+  }
+  void Enable() { enabled_ = true; }
+
+  Process& CurrentProcess() { return current_thread_->process(); }
+  Thread& CurrentThread() { return *current_thread_; }
+
+  void Yield() {}
+
+ private:
+  bool enabled_ = false;
+  ProcList proc_list_;
+
+  Thread* current_thread_;
+};
+
+static Scheduler* gScheduler = nullptr;
+
+Scheduler& GetScheduler() {
+  if (!gScheduler) {
+    panic("Scheduler not initialized");
+  }
+  return *gScheduler;
+}
+
+}  // namespace
+
+void InitScheduler() { gScheduler = new Scheduler(); }
+void EnableScheduler() { GetScheduler().Enable(); }
+
+void Yield() { GetScheduler().Yield(); }
+
+Process& CurrentProcess() { return GetScheduler().CurrentProcess(); }
+Thread& CurrentThread() { return GetScheduler().CurrentThread(); }
+
+}  // namespace sched