Kernel: Tidy up Process::try_create_user_process()

This function is currently only ever used to create the init process
(SystemServer). It had a few idiosyncratic things about it that this
patch cleans up:

- Errors were returned in an int& out-param.
- It had a path for non-0 process PIDs which was never taken.

Kernel/Process.cpp

@@ -143,32 +143,26 @@ void Process::register_new(Process& process)
     });
 }
 
-RefPtr<Process> Process::create_user_process(RefPtr<Thread>& first_thread, const String& path, UserID uid, GroupID gid, ProcessID parent_pid, int& error, Vector<String>&& arguments, Vector<String>&& environment, TTY* tty)
+KResultOr<NonnullRefPtr<Process>> Process::try_create_user_process(RefPtr<Thread>& first_thread, String const& path, UserID uid, GroupID gid, Vector<String> arguments, Vector<String> environment, TTY* tty)
 {
     auto parts = path.split('/');
     if (arguments.is_empty()) {
         arguments.append(parts.last());
     }
 
-    RefPtr<Custody> cwd;
-    if (auto parent = Process::from_pid(parent_pid))
-        cwd = parent->m_cwd;
-    if (!cwd)
-        cwd = VirtualFileSystem::the().root_custody();
-
-    auto process = Process::try_create(first_thread, parts.take_last(), uid, gid, parent_pid, false, move(cwd), nullptr, tty);
+    auto process = Process::try_create(first_thread, parts.take_last(), uid, gid, ProcessID(0), false, VirtualFileSystem::the().root_custody(), nullptr, tty);
     if (!process || !first_thread)
-        return {};
+        return ENOMEM;
+
     if (!process->m_fds.try_resize(process->m_fds.max_open())) {
         first_thread = nullptr;
-        return {};
+        return ENOMEM;
     }
     auto& device_to_use_as_tty = tty ? (CharacterDevice&)*tty : NullDevice::the();
     auto description_or_error = device_to_use_as_tty.open(O_RDWR);
-    if (description_or_error.is_error()) {
-        error = description_or_error.error().error();
-        return {};
-    }
+    if (description_or_error.is_error())
+        return description_or_error.error();
+
     auto& description = description_or_error.value();
 
     auto setup_description = [&process, &description](int fd) {
@@ -179,20 +173,18 @@ RefPtr<Process> Process::create_user_process(RefPtr<Thread>& first_thread, const
     setup_description(1);
     setup_description(2);
 
-    error = process->exec(path, move(arguments), move(environment)).error();
-    if (error != 0) {
-        dbgln("Failed to exec {}: {}", path, error);
+    if (auto result = process->exec(path, move(arguments), move(environment)); result.is_error()) {
+        dbgln("Failed to exec {}: {}", path, result);
         first_thread = nullptr;
-        return {};
+        return result;
     }
 
     register_new(*process);
-    error = 0;
 
     // NOTE: All user processes have a leaked ref on them. It's balanced by Thread::WaitBlockerSet::finalize().
-    (void)process.leak_ref();
+    process->ref();
 
-    return process;
+    return process.release_nonnull();
 }
 
 RefPtr<Process> Process::create_kernel_process(RefPtr<Thread>& first_thread, String&& name, void (*entry)(void*), void* entry_data, u32 affinity, RegisterProcess do_register)

Kernel/Process.h

@@ -179,7 +179,7 @@ public:
     }
 
     static RefPtr<Process> create_kernel_process(RefPtr<Thread>& first_thread, String&& name, void (*entry)(void*), void* entry_data = nullptr, u32 affinity = THREAD_AFFINITY_DEFAULT, RegisterProcess do_register = RegisterProcess::Yes);
-    static RefPtr<Process> create_user_process(RefPtr<Thread>& first_thread, const String& path, UserID, GroupID, ProcessID ppid, int& error, Vector<String>&& arguments = Vector<String>(), Vector<String>&& environment = Vector<String>(), TTY* = nullptr);
+    static KResultOr<NonnullRefPtr<Process>> try_create_user_process(RefPtr<Thread>& first_thread, String const& path, UserID, GroupID, Vector<String> arguments, Vector<String> environment, TTY*);
     static void register_new(Process&);
 
     bool unref() const;

Kernel/init.cpp

@@ -336,18 +336,17 @@ void init_stage2(void*)
     // NOTE: Everything in the .ksyms section becomes inaccessible after this point.
     MM.unmap_ksyms_after_init();
 
-    int error;
-
     // FIXME: It would be nicer to set the mode from userspace.
     // FIXME: It would be smarter to not hardcode that the first tty is the only graphical one
     ConsoleManagement::the().first_tty()->set_graphical(GraphicsManagement::the().framebuffer_devices_exist());
     RefPtr<Thread> thread;
     auto userspace_init = kernel_command_line().userspace_init();
     auto init_args = kernel_command_line().userspace_init_args();
-    Process::create_user_process(thread, userspace_init, UserID(0), GroupID(0), ProcessID(0), error, move(init_args), {}, tty0);
-    if (error != 0) {
-        PANIC("init_stage2: Error spawning SystemServer: {}", error);
-    }
+
+    auto init_or_error = Process::try_create_user_process(thread, userspace_init, UserID(0), GroupID(0), move(init_args), {}, tty0);
+    if (init_or_error.is_error())
+        PANIC("init_stage2: Error spawning init process: {}", init_or_error.error());
+
     thread->set_priority(THREAD_PRIORITY_HIGH);
 
     if (boot_profiling) {