kernel: Store a thread's parent directly instead of its parent's PID

kernel/src/sys/exec.cpp

@@ -125,12 +125,12 @@ Result<u64> sys_fork(Registers* regs, SyscallArgs)
 
     thread->state = ThreadState::Runnable;
     thread->is_kernel = false;
-    thread->parent_id = current->id;
     thread->fp_data.save();
     thread->name = current->name;
     thread->auth = current->auth;
     thread->current_directory = current->current_directory;
     thread->current_directory_path = move(current_directory_path);
+    thread->parent = current;
 
     for (int i = 0; i < FD_MAX; i++) { thread->fd_table[i] = current->fd_table[i]; }
 

kernel/src/sys/exit.cpp

@@ -8,7 +8,7 @@ Result<u64> sys_exit(Registers*, SyscallArgs args)
     Thread* current = Scheduler::current();
 
     Scheduler::for_each_child(current, [](Thread* child) {
-        child->parent_id = 1;
+        child->parent = Scheduler::init_thread();
         return true;
     });
 

kernel/src/sys/id.cpp

@@ -9,7 +9,9 @@ Result<u64> sys_getpid(Registers*, SyscallArgs)
 
 Result<u64> sys_getppid(Registers*, SyscallArgs)
 {
-    return Scheduler::current()->parent_id;
+    auto* parent = Scheduler::current()->parent;
+
+    return parent ? parent->id : 0;
 }
 
 Result<u64> sys_getuid(Registers*, SyscallArgs)

kernel/src/sys/waitpid.cpp

@@ -17,7 +17,7 @@ Result<u64> sys_waitpid(Registers*, SyscallArgs args)
     {
         thread = TRY(Result<Thread*>::from_option(Scheduler::find_by_pid(pid), ESRCH));
 
-        if (thread->parent_id != current->id) return err(ECHILD);
+        if (thread->parent && thread->parent != current) return err(ECHILD);
 
         while (thread->state != ThreadState::Exited)
         {

kernel/src/thread/Scheduler.cpp

@@ -11,6 +11,7 @@
 
 static Thread g_idle;
 static Thread* g_current = nullptr;
+static Thread* g_init = nullptr;
 
 static const usize TICKS_PER_TIMESLICE = 20;
 
@@ -23,7 +24,7 @@ namespace Scheduler
         g_idle.set_ip((u64)CPU::idle_loop);
         g_idle.state = ThreadState::Idle;
         g_idle.is_kernel = true;
-        g_idle.parent_id = 0;
+        g_idle.parent = nullptr;
         g_idle.name = "[idle]";
 
         g_idle.ticks_left = 1;
@@ -52,6 +53,11 @@ namespace Scheduler
         return &g_idle;
     }
 
+    Thread* init_thread()
+    {
+        return g_init;
+    }
+
     Result<void> new_kernel_thread_impl(Thread* thread, const char* name)
     {
         // If anything fails, make sure to clean up.
@@ -66,8 +72,6 @@ namespace Scheduler
 
         thread->stack = thread_stack;
 
-        thread->parent_id = 0;
-
         thread->name = name;
 
         thread->is_kernel = true;
@@ -119,7 +123,6 @@ namespace Scheduler
         thread->is_kernel = false;
         thread->id = 1;
         thread->name = name;
-        thread->parent_id = 0;
         thread->auth = Credentials { .uid = 0, .euid = 0, .suid = 0, .gid = 0, .egid = 0, .sgid = 0 };
 
         Vector<String> args;
@@ -143,6 +146,7 @@ namespace Scheduler
                 thread->sp(), thread->kernel_stack.top());
 
         g_threads.append(thread);
+        g_init = thread;
 
         return thread;
     }

kernel/src/thread/Scheduler.h

@@ -8,6 +8,7 @@ namespace Scheduler
 
     Thread* current();
     Thread* idle();
+    Thread* init_thread();
 
     Result<void> new_kernel_thread(u64 address, const char* name);
     Result<void> new_kernel_thread(void (*func)(void), const char* name);
@@ -33,7 +34,7 @@ namespace Scheduler
     {
         for (Thread* current = thread; current; current = g_threads.next(current).value_or(nullptr))
         {
-            if (current->parent_id == thread->id)
+            if (current->parent == thread)
             {
                 bool should_continue = callback(current);
                 if (!should_continue) return;

kernel/src/thread/Thread.h

@@ -55,7 +55,6 @@ struct Thread : public LinkedListNode<Thread>
     Registers regs;
 
     u64 id;
-    u64 parent_id;
 
     Credentials auth;
 
@@ -90,6 +89,8 @@ struct Thread : public LinkedListNode<Thread>
     String current_directory_path = {};
     SharedPtr<VFS::Inode> current_directory = {};
 
+    Thread* parent { nullptr };
+
     PageDirectory* directory;
 
     bool is_idle()