process: sys_wait4: return ECHILD with no children

If the calling process has no children and no pending exit events to
handle, return ECHILD instead of waiting forever.

libkernel/src/error.rs

@@ -177,6 +177,9 @@ pub enum KernelError {
     #[error("No such process")]
     NoProcess,
 
+    #[error("No child process")]
+    NoChildProcess,
+
     #[error("Operation timed out")]
     TimedOut,
 

libkernel/src/error/syscall_error.rs

@@ -53,6 +53,7 @@ pub fn kern_err_to_syscall(err: KernelError) -> isize {
         KernelError::NotSupported => ENOSYS,
         KernelError::NoMemory => ENOMEM,
         KernelError::TimedOut => ETIMEDOUT,
+        KernelError::NoChildProcess => ECHILD,
         e => todo!("{e}"),
     }
 }

src/process/thread_group/wait.rs

@@ -170,7 +170,12 @@ pub async fn sys_wait4(
 
     let task = current_task_shared();
 
-    let (tgid, child_state) = if flags.contains(WaitFlags::WNOHANG) {
+    let child_proc_count = task.process.children.lock_save_irq().iter().count();
+
+    let (tgid, child_state) = if child_proc_count == 0 || flags.contains(WaitFlags::WNOHANG) {
+        // Special case for no children. See if there are any pending child
+        // notification events without sleeping. If there are no children and no
+        // pending events, return ECHILD.
         let mut ret = None;
         task.process.child_notifiers.inner.update(|s| {
             ret = do_wait(s, pid, flags);
@@ -178,8 +183,9 @@ pub async fn sys_wait4(
         });
 
         match ret {
-            None => return Ok(0),
             Some(ret) => ret,
+            None if child_proc_count == 0 => return Err(KernelError::NoChildProcess),
+            None => return Ok(0),
         }
     } else {
         task.process