Description of problem:

Following a fresh install of Fedora 28, KDE logout hangs indefinitely. Issue persists after a full package upgrade (except kernel*).

Version-Release number of selected component (if applicable): 0.17.0-2 and 0.17.0-3

How reproducible:

I am satisfied it is a timing issue, so mileage may vary.

Steps to Reproduce:
1. Install F28 KDE spin to disk
2. Update packages (dnf update -x kernel*)
3. Login to KDE, do stuff, logout.

Actual results:
Logout hangs indefinitely

Expected results:
Logout completed and SDDM greeter is restarted.

Additional info:

After logout, sddm-helper process is still running even though startkde script has terminated. It leaves a bunch of other session processes active too.

Same results after rebuilding sddm package from source.

After applying patch:
--- sddm-0.17.0-old/src/helper/HelperApp.cpp    2017-12-05 16:00:16.000000000 +0000
+++ sddm-0.17.0-new/src/helper/HelperApp.cpp    2018-05-29 16:49:44.270509954 +0100
@@ -235,6 +235,7 @@
     }

     void HelperApp::sessionOpened(bool success) {
+       qInfo() << "HelperApp::sessionOpened: enter";
         Msg m = Msg::MSG_UNKNOWN;
         SafeDataStream str(m_socket);
         str << Msg::SESSION_STATUS << success;
@@ -244,6 +245,7 @@
         if (m != SESSION_STATUS) {
             qCritical() << "Received a wrong opcode instead of SESSION_STATUS:" << m;
         }
+       qInfo() << "HelperApp::sessionOpened: exit";
     }

     UserSession *HelperApp::session() {

Messages after logout has hung contain only "HelperApp::sessionOpened: enter" (i.e. not the exit message).

Applying additional patch:
--- sddm-0.17.0-old/src/auth/Auth.cpp   2017-12-05 16:00:16.000000000 +0000
+++ sddm-0.17.0-new/src/auth/Auth.cpp   2018-05-29 17:07:06.315543046 +0100
@@ -191,11 +191,13 @@
                 break;
             }
             case SESSION_STATUS: {
+               qInfo() << "Auth::Private::dataPending: rx SESSION_STATUS";
                 bool status;
                 str >> status;
                 Q_EMIT auth->session(status);
                 str.reset();
                 str << SESSION_STATUS;
+               qInfo() << "Auth::Private::dataPending: tx SESSION_STATUS";
                 str.send();
                 break;
             }

Messages still only show "HelperApp::sessionOpened: enter".

Applying patch:
--- sddm-0.17.0-old/src/auth/Auth.cpp   2017-12-05 16:00:16.000000000 +0000
+++ sddm-0.17.0-new/src/auth/Auth.cpp   2018-05-29 17:16:18.476962930 +0100
@@ -163,6 +163,7 @@
                 break;
             }
             case INFO: {
+               qInfo() << "Auth::Private::dataPending: rx INFO";
                 QString message;
                 Info type = INFO_NONE;
                 str >> message >> type;
@@ -176,6 +177,7 @@
                 break;
             }
             case AUTHENTICATED: {
+               qInfo() << "Auth::Private::dataPending: rx AUTHENTICATED";
                 QString user;
                 str >> user;
                 if (!user.isEmpty()) {
@@ -183,6 +185,7 @@
                     Q_EMIT auth->authentication(user, true);
                     str.reset();
                     str << AUTHENTICATED << environment << cookie;
+                   qInfo() << "Auth::Private::dataPending: tx AUTHENTICATED";
                     str.send();
                 }
                 else {
@@ -191,11 +194,13 @@
                 break;
             }
             case SESSION_STATUS: {
+               qInfo() << "Auth::Private::dataPending: rx SESSION_STATUS";
                 bool status;
                 str >> status;
                 Q_EMIT auth->session(status);
                 str.reset();
                 str << SESSION_STATUS;
+               qInfo() << "Auth::Private::dataPending: tx SESSION_STATUS";
                 str.send();
                 break;
             }

Now logout succeeds, sddm-helper terminates and fresh greeter displayed. Log show:
May 29 17:18:33 localhost.localdomain sddm[12507]: Auth::Private::dataPending: rx INFO
May 29 17:18:33 localhost.localdomain sddm[12507]: Authentication information: "Last failed login: Tue May 29 16:53:13 BST 2018"
May 29 17:18:33 localhost.localdomain sddm[12507]: Auth::Private::dataPending: rx INFO
May 29 17:18:33 localhost.localdomain sddm[12507]: Authentication information: "There were 12 failed login attempts since the last successful login."
May 29 17:18:33 localhost.localdomain sddm-helper[12590]: Starting: "/etc/X11/xinit/Xsession \"/usr/bin/startkde\""
May 29 17:18:33 localhost.localdomain sddm-helper[12590]: HelperApp::sessionOpened: enter
May 29 17:18:33 localhost.localdomain sddm[12507]: Auth::Private::dataPending: rx SESSION_STATUS
May 29 17:18:33 localhost.localdomain sddm[12507]: Auth::Private::dataPending: tx SESSION_STATUS
May 29 17:18:33 localhost.localdomain sddm-helper[12590]: HelperApp::sessionOpened: exit

Restoring the original Auth.cpp and apply the following patch:
--- sddm-0.17.0-old/src/auth/Auth.cpp   2017-12-05 16:00:16.000000000 +0000
+++ sddm-0.17.0-new/src/auth/Auth.cpp   2018-05-29 17:24:37.965392094 +0100
@@ -191,6 +191,7 @@
                 break;
             }
             case SESSION_STATUS: {
+               usleep(1000);
                 bool status;
                 str >> status;
                 Q_EMIT auth->session(status);

Same result, logout works as above.

All indications point to timing issue, although it is way beyond me to figure where and how. I can only hope the big boys can get to the bottom of it.