signal.c: preserve trap(:CHLD, "IGNORE") behavior with SIGCHLD

We need to preserve "IGNORE" behavior from Ruby 2.5 and earlier.
We can't rely on SA_NOCLDWAIT any more, since we always need
system() and MJIT to work; so we fake that behavior using
dedicated reaper (currently in timer-thread).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63879 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

internal.h

@@ -1683,6 +1683,7 @@ struct rb_execarg {
     unsigned uid_given : 1;
     unsigned gid_given : 1;
     unsigned exception : 1;
+    unsigned nocldwait_prev : 1;
     rb_pid_t pgroup_pgid; /* asis(-1), new pgroup(0), specified pgroup (0<V). */
     VALUE rlimit_limits; /* Qfalse or [[rtype, softlim, hardlim], ...] */
     mode_t umask_mask;

process.c

@@ -935,6 +935,7 @@ waitpid_notify(struct waitpid_state *w, rb_pid_t ret)
 #  define waitpid_sys(pid,status,options) do_waitpid((pid),(status),(options))
 #endif
 
+extern volatile unsigned int ruby_nocldwait; /* signal.c */
 /* called by timer thread */
 static void
 waitpid_each(struct list_head *head)
@@ -973,6 +974,11 @@ ruby_waitpid_all(rb_vm_t *vm)
     if (list_empty(&vm->waiting_pids)) {
         waitpid_each(&vm->waiting_grps);
     }
+    /* emulate SA_NOCLDWAIT */
+    if (list_empty(&vm->waiting_pids) && list_empty(&vm->waiting_grps)) {
+        while (ruby_nocldwait && do_waitpid(-1, 0, WNOHANG) > 0)
+            ; /* keep looping */
+    }
     rb_native_mutex_unlock(&vm->waitpid_lock);
 }
 
@@ -1153,10 +1159,18 @@ rb_waitpid(rb_pid_t pid, int *st, int flags)
     }
 
     if (st) *st = w.status;
-    if (w.ret > 0) {
-	rb_last_status_set(w.status, w.ret);
+    if (w.ret == -1) {
+        errno = w.errnum;
+    }
+    else if (w.ret > 0) {
+        if (ruby_nocldwait) {
+            w.ret = -1;
+            errno = ECHILD;
+        }
+        else {
+            rb_last_status_set(w.status, w.ret);
+        }
     }
-    if (w.ret == -1) errno = w.errnum;
     return w.ret;
 }
 
@@ -4306,16 +4320,22 @@ rb_f_system(int argc, VALUE *argv)
     struct rb_execarg *eargp;
 
     execarg_obj = rb_execarg_new(argc, argv, TRUE, TRUE);
+    TypedData_Get_Struct(execarg_obj, struct rb_execarg, &exec_arg_data_type, eargp);
+    eargp->nocldwait_prev = ruby_nocldwait;
+    ruby_nocldwait = 0;
     pid = rb_execarg_spawn(execarg_obj, NULL, 0);
 #if defined(HAVE_WORKING_FORK) || defined(HAVE_SPAWNV)
     if (pid > 0) {
         int ret, status;
         ret = rb_waitpid(pid, &status, 0);
-        if (ret == (rb_pid_t)-1)
+        if (ret == (rb_pid_t)-1) {
+            ruby_nocldwait = eargp->nocldwait_prev;
+            RB_GC_GUARD(execarg_obj);
             rb_sys_fail("Another thread waited the process started by system().");
+        }
     }
 #endif
-    TypedData_Get_Struct(execarg_obj, struct rb_execarg, &exec_arg_data_type, eargp);
+    ruby_nocldwait = eargp->nocldwait_prev;
     if (pid < 0) {
         if (eargp->exception) {
             int err = errno;

signal.c

@@ -531,6 +531,7 @@ static struct {
     rb_atomic_t cnt[RUBY_NSIG];
     rb_atomic_t size;
 } signal_buff;
+volatile unsigned int ruby_nocldwait;
 
 #ifdef __dietlibc__
 #define sighandler_t sh_t
@@ -614,12 +615,20 @@ ruby_signal(int signum, sighandler_t handler)
 #endif
 
     switch (signum) {
-#ifdef SA_NOCLDWAIT
       case SIGCHLD:
-	if (handler == SIG_IGN)
-	    sigact.sa_flags |= SA_NOCLDWAIT;
+	if (handler == SIG_IGN) {
+	    ruby_nocldwait = 1;
+	    if (sigact.sa_flags & SA_SIGINFO) {
+		sigact.sa_sigaction = (ruby_sigaction_t*)sighandler;
+	    }
+	    else {
+		sigact.sa_handler = sighandler;
+	    }
+	}
+	else {
+	    ruby_nocldwait = 0;
+	}
 	break;
-#endif
 #if defined(SA_ONSTACK) && defined(USE_SIGALTSTACK)
       case SIGSEGV:
 #ifdef SIGBUS
@@ -1183,9 +1192,6 @@ trap_handler(VALUE *cmd, int sig)
     VALUE command;
 
     if (NIL_P(*cmd)) {
-	if (sig == RUBY_SIGCHLD) {
-	    goto sig_dfl;
-	}
 	func = SIG_IGN;
     }
     else {
@@ -1216,9 +1222,6 @@ trap_handler(VALUE *cmd, int sig)
 	      case 7:
 		if (memcmp(cptr, "SIG_IGN", 7) == 0) {
 sig_ign:
-                    if (sig == RUBY_SIGCHLD) {
-                        goto sig_dfl;
-                    }
                     func = SIG_IGN;
                     *cmd = Qtrue;
 		}

test/ruby/test_signal.rb

@@ -326,4 +326,47 @@ class TestSignal < Test::Unit::TestCase
       end
     end;
   end
+
+  def test_sigchld_ignore
+    skip 'no SIGCHLD' unless Signal.list['CHLD']
+    old = trap(:CHLD, 'IGNORE')
+    cmd = [ EnvUtil.rubybin, '--disable=gems', '-e' ]
+    assert(system(*cmd, 'exit!(0)'), 'no ECHILD')
+    t0 = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+    IO.pipe do |r, w|
+      pid = spawn(*cmd, "STDIN.read", in: r)
+      nb = Process.wait(pid, Process::WNOHANG)
+      th = Thread.new(Thread.current) do |parent|
+        Thread.pass until parent.stop? # wait for parent to Process.wait
+        w.close
+      end
+      assert_raise(Errno::ECHILD) { Process.wait(pid) }
+      assert_nil nb
+    end
+
+    IO.pipe do |r, w|
+      pids = 3.times.map { spawn(*cmd, 'exit!', out: w) }
+      w.close
+      zombies = pids.dup
+      assert_nil r.read(1), 'children dead'
+
+      Timeout.timeout(3) do
+        zombies.delete_if do |pid|
+          begin
+            Process.kill(0, pid)
+            false
+          rescue Errno::ESRCH
+            true
+          end
+        end while zombies[0]
+      end
+      assert_predicate zombies, :empty?, 'zombies leftover'
+
+      pids.each do |pid|
+        assert_raise(Errno::ECHILD) { Process.waitpid(pid) }
+      end
+    end
+  ensure
+    trap(:CHLD, old)
+  end
 end