Kernel: Add support for the WSTOPPED flag to the waitpid() syscall.

This makes waitpid() return when a child process is stopped via a signal.
Use this in Shell to catch stopped children and return control to the
command line. :^)

Fixes #298.

Kernel/Process.cpp

@@ -1393,8 +1393,12 @@ int Process::reap(Process& process)
 pid_t Process::sys$waitpid(pid_t waitee, int* wstatus, int options)
 {
     dbgprintf("sys$waitpid(%d, %p, %d)\n", waitee, wstatus, options);
-    // FIXME: Respect options
-    (void)options;
+
+    if (!options) {
+        // FIXME: This can't be right.. can it? Figure out how this should actually work.
+        options = WEXITED;
+    }
+
     if (wstatus)
         if (!validate_write_typed(wstatus))
             return -EFAULT;
@@ -1409,6 +1413,7 @@ pid_t Process::sys$waitpid(pid_t waitee, int* wstatus, int options)
     }
 
     if (options & WNOHANG) {
+        // FIXME: Figure out what WNOHANG should do with stopped children.
         if (waitee == -1) {
             pid_t reaped_pid = 0;
             InterruptDisabler disabler;
@@ -1417,7 +1422,7 @@ pid_t Process::sys$waitpid(pid_t waitee, int* wstatus, int options)
                     reaped_pid = process.pid();
                     exit_status = reap(process);
                 }
-                return true;
+                return IterationDecision::Continue;
             });
             return reaped_pid;
         } else {
@@ -1435,17 +1440,22 @@ pid_t Process::sys$waitpid(pid_t waitee, int* wstatus, int options)
     }
 
     current->m_waitee_pid = waitee;
+    current->m_wait_options = options;
     current->block(Thread::State::BlockedWait);
     if (current->m_was_interrupted_while_blocked)
         return -EINTR;
-    Process* waitee_process;
-    {
-        InterruptDisabler disabler;
-        // NOTE: If waitee was -1, m_waitee will have been filled in by the scheduler.
-        waitee_process = Process::from_pid(current->m_waitee_pid);
-    }
+
+    InterruptDisabler disabler;
+
+    // NOTE: If waitee was -1, m_waitee_pid will have been filled in by the scheduler.
+    Process* waitee_process = Process::from_pid(current->m_waitee_pid);
     ASSERT(waitee_process);
-    exit_status = reap(*waitee_process);
+    if (waitee_process->is_dead()) {
+        exit_status = reap(*waitee_process);
+    } else {
+        ASSERT(waitee_process->main_thread().state() == Thread::State::Stopped);
+        exit_status = 0x7f;
+    }
     return current->m_waitee_pid;
 }
 

Kernel/Process.h

@@ -411,7 +411,7 @@ inline void Process::for_each_child(Callback callback)
     for (auto* process = g_processes->head(); process;) {
         auto* next_process = process->next();
         if (process->ppid() == my_pid) {
-            if (!callback(*process))
+            if (callback(*process) == IterationDecision::Break)
                 break;
         }
         process = next_process;

Kernel/Scheduler.cpp

@@ -83,14 +83,21 @@ bool Scheduler::pick_next()
 
         if (thread.state() == Thread::BlockedWait) {
             process.for_each_child([&](Process& child) {
-                if (!child.is_dead())
-                    return true;
-                if (thread.waitee_pid() == -1 || thread.waitee_pid() == child.pid()) {
-                    thread.m_waitee_pid = child.pid();
-                    thread.unblock();
-                    return false;
-                }
-                return true;
+                if (thread.waitee_pid() != -1 && thread.waitee_pid() != child.pid())
+                    return IterationDecision::Continue;
+
+                bool child_exited = child.is_dead();
+                bool child_stopped = child.main_thread().state() == Thread::State::Stopped;
+
+                bool wait_finished = ((thread.m_wait_options & WEXITED) && child_exited)
+                    || ((thread.m_wait_options & WSTOPPED) && child_stopped);
+
+                if (!wait_finished)
+                    return IterationDecision::Continue;
+
+                thread.m_waitee_pid = child.pid();
+                thread.unblock();
+                return IterationDecision::Break;
             });
             return IterationDecision::Continue;
         }

Kernel/Thread.h

@@ -182,6 +182,7 @@ private:
     RefPtr<Region> m_kernel_stack_region;
     RefPtr<Region> m_kernel_stack_for_signal_handler_region;
     pid_t m_waitee_pid { -1 };
+    int m_wait_options { 0 };
     RefPtr<FileDescription> m_blocked_description;
     timeval m_select_timeout;
     SignalActionData m_signal_action_data[32];

Kernel/UnixTypes.h

@@ -3,6 +3,10 @@
 #include <AK/Types.h>
 
 #define WNOHANG 1
+#define WUNTRACED 2
+#define WSTOPPED WUNTRACED
+#define WEXITED 4
+#define WCONTINUED 8
 
 #define R_OK 4
 #define W_OK 2

Shell/main.cpp

@@ -488,7 +488,7 @@ static int run_command(const String& cmd)
     for (int i = 0; i < children.size(); ++i) {
         auto& child = children[i];
         do {
-            int rc = waitpid(child.pid, &wstatus, 0);
+            int rc = waitpid(child.pid, &wstatus, WEXITED | WSTOPPED);
             if (rc < 0 && errno != EINTR) {
                 if (errno != ECHILD)
                     perror("waitpid");
@@ -499,6 +499,8 @@ static int run_command(const String& cmd)
                     printf("Shell: %s(%d) exited with status %d\n", child.name.characters(), child.pid, WEXITSTATUS(wstatus));
                 if (i == 0)
                     return_value = WEXITSTATUS(wstatus);
+            } else if (WIFSTOPPED(wstatus)) {
+                printf("Shell: %s(%d) stopped.\n", child.name.characters(), child.pid);
             } else {
                 if (WIFSIGNALED(wstatus)) {
                     printf("Shell: %s(%d) exited due to signal '%s'\n", child.name.characters(), child.pid, strsignal(WTERMSIG(wstatus)));