Fix ConcurrentModificationException on Redis / PT

```
master: #6068
```

core/cas-server-core-tickets-api/src/main/java/org/apereo/cas/ticket/TicketGrantingTicketImpl.java

@@ -22,6 +22,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 /**
  * Concrete implementation of a TicketGrantingTicket. A TicketGrantingTicket is
@@ -55,7 +56,7 @@ public class TicketGrantingTicketImpl extends AbstractTicket implements TicketGr
     /**
      * The services associated to this ticket.
      */
-    private Map<String, Service> services = new HashMap<>(0);
+    private Map<String, Service> services = new ConcurrentHashMap<>(0);
 
     /**
      * The {@link TicketGrantingTicket} this is associated with.

support/cas-server-support-redis-ticket-registry/src/test/java/org/apereo/cas/ticket/registry/RedisServerTicketRegistryTests.java

@@ -1,16 +1,25 @@
 package org.apereo.cas.ticket.registry;
 
+import org.apereo.cas.CasProtocolConstants;
 import org.apereo.cas.authentication.CoreAuthenticationTestUtils;
+import org.apereo.cas.authentication.principal.Service;
+import org.apereo.cas.authentication.principal.WebApplicationServiceFactory;
 import org.apereo.cas.config.RedisCoreConfiguration;
 import org.apereo.cas.config.RedisTicketRegistryConfiguration;
 import org.apereo.cas.redis.core.CasRedisTemplate;
 import org.apereo.cas.services.RegisteredServiceTestUtils;
+import org.apereo.cas.ticket.ProxyGrantingTicketImpl;
 import org.apereo.cas.ticket.ServiceTicket;
 import org.apereo.cas.ticket.Ticket;
 import org.apereo.cas.ticket.TicketGrantingTicket;
 import org.apereo.cas.ticket.TicketGrantingTicketImpl;
 import org.apereo.cas.ticket.expiration.HardTimeoutExpirationPolicy;
 import org.apereo.cas.ticket.expiration.NeverExpiresExpirationPolicy;
+import org.apereo.cas.ticket.proxy.ProxyGrantingTicket;
+import org.apereo.cas.ticket.proxy.ProxyTicket;
+import org.apereo.cas.ticket.tracking.TicketTrackingPolicy;
+import org.apereo.cas.util.ProxyGrantingTicketIdGenerator;
+import org.apereo.cas.util.ProxyTicketIdGenerator;
 import org.apereo.cas.util.ServiceTicketIdGenerator;
 import org.apereo.cas.util.TicketGrantingTicketIdGenerator;
 import org.apereo.cas.util.junit.EnabledIfListeningOnPort;
@@ -29,6 +38,7 @@
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.test.context.TestPropertySource;
 
 import java.util.ArrayList;
@@ -345,7 +355,7 @@ private void addTicketAndWait(final String principalId) {
             "cas.ticket.registry.redis.host=localhost",
             "cas.ticket.registry.redis.port=6379"
     })
-    class ConcurrentTests {
+    class ConcurrentAddTicketGrantingTicketTests {
         @Autowired
         @Qualifier(TicketRegistry.BEAN_NAME)
         private TicketRegistry ticketRegistry;
@@ -356,7 +366,7 @@ void verifyConcurrentAddTicket() throws Throwable {
             val testHasFailed = new AtomicBoolean();
             val threads = new ArrayList<Thread>();
             for (var i = 1; i <= 3; i++) {
-                val runnable = new RunnableAddTicket(ticketRegistry, principalId, 100);
+                val runnable = new RunnableAddTicketGrantingTicket(ticketRegistry, principalId, 100);
                 val thread = new Thread(runnable);
                 thread.setName("Thread-" + i);
                 thread.setUncaughtExceptionHandler((t, e) -> {
@@ -379,7 +389,7 @@ void verifyConcurrentAddTicket() throws Throwable {
         }
 
         @RequiredArgsConstructor
-        private static final class RunnableAddTicket implements Runnable {
+        private static final class RunnableAddTicketGrantingTicket implements Runnable {
             private final TicketRegistry ticketRegistry;
             private final String principalId;
             private final int max;
@@ -397,4 +407,83 @@ public void run() {
             }
         }
     }
+
+    @Nested
+    @SpringBootTest(
+            classes = {
+                    RedisCoreConfiguration.class,
+                    RedisTicketRegistryConfiguration.class,
+                    BaseTicketRegistryTests.SharedTestConfiguration.class
+            }, properties = {
+            "cas.ticket.tgt.core.only-track-most-recent-session=false",
+            "cas.ticket.registry.redis.host=localhost",
+            "cas.ticket.registry.redis.port=6379"
+    })
+    class ConcurrentAddProxyTicketTests {
+        @Autowired
+        @Qualifier(TicketRegistry.BEAN_NAME)
+        private TicketRegistry ticketRegistry;
+
+        @Autowired
+        @Qualifier(org.apereo.cas.ticket.tracking.TicketTrackingPolicy.BEAN_NAME_SERVICE_TICKET_TRACKING)
+        private TicketTrackingPolicy serviceTicketSessionTrackingPolicy;
+
+        @Test
+        void verifyConcurrentAddTicket() throws Throwable {
+            val principalId = UUID.randomUUID().toString();
+            val authentication = CoreAuthenticationTestUtils.getAuthentication(principalId);
+            val tgtGenerator = new ProxyGrantingTicketIdGenerator(10, StringUtils.EMPTY);
+            val pgt = new ProxyGrantingTicketImpl(tgtGenerator.getNewTicketId(TicketGrantingTicket.PREFIX),
+                    authentication, NeverExpiresExpirationPolicy.INSTANCE);
+            ticketRegistry.addTicket(pgt);
+
+            val request = new MockHttpServletRequest();
+            request.setParameter(CasProtocolConstants.PARAMETER_SERVICE, "http://foo.com");
+            val service = new WebApplicationServiceFactory().createService(request);
+
+            val testHasFailed = new AtomicBoolean();
+            val threads = new ArrayList<Thread>();
+            for (var i = 1; i <= 3; i++) {
+                val runnable = new RunnableAddProxyTicket(ticketRegistry, pgt, service, serviceTicketSessionTrackingPolicy, 100);
+                val thread = new Thread(runnable);
+                thread.setName("Thread-" + i);
+                thread.setUncaughtExceptionHandler((t, e) -> {
+                    LOGGER.error(e.getMessage(), e);
+                    testHasFailed.set(true);
+                });
+                threads.add(thread);
+                thread.start();
+            }
+            for (val thread : threads) {
+                try {
+                    thread.join();
+                } catch (final Throwable e) {
+                    fail(e);
+                }
+            }
+            if (testHasFailed.get()) {
+                fail("Test failed");
+            }
+        }
+
+        @RequiredArgsConstructor
+        private static final class RunnableAddProxyTicket implements Runnable {
+            private final TicketRegistry ticketRegistry;
+            private final ProxyGrantingTicket proxyGrantingTicket;
+            private final Service service;
+            private final TicketTrackingPolicy serviceTicketSessionTrackingPolicy;
+            private final int max;
+
+            @Override
+            @SneakyThrows
+            public void run() {
+                val ptGenerator = new ProxyTicketIdGenerator(10, StringUtils.EMPTY);
+                for (int i = 0; i < max; i++) {
+                    val proxyTicket = proxyGrantingTicket.grantProxyTicket(ptGenerator.getNewTicketId(ProxyTicket.PREFIX),
+                            service, new HardTimeoutExpirationPolicy(20), serviceTicketSessionTrackingPolicy);
+                    ticketRegistry.addTicket(proxyTicket);
+                }
+            }
+        }
+    }
 }