SSLSession.putValue / getValue / removeValue / getValueNames must be thread-safe. (#8648)

Motivation:

SSLSession.putValue / getValue / removeValue / getValueNames must be thread-safe as it may be called from multiple threads. This is also the case in the OpenJDK implementation.

Modifications:

Guard with synchronized (this) blocks to keep the memory overhead low as we do not expect to have these called frequently.

Result:

SSLSession implementation is thread-safe.

handler/src/main/java/io/netty/handler/ssl/ReferenceCountedOpenSslEngine.java

@@ -2016,12 +2016,16 @@ public class ReferenceCountedOpenSslEngine extends SSLEngine implements Referenc
             if (value == null) {
                 throw new NullPointerException("value");
             }
+            final Object old;
+            synchronized (this) {
                 Map<String, Object> values = this.values;
                 if (values == null) {
                     // Use size of 2 to keep the memory overhead small
                     values = this.values = new HashMap<String, Object>(2);
                 }
-            Object old = values.put(name, value);
+                old = values.put(name, value);
+            }
+
             if (value instanceof SSLSessionBindingListener) {
                 // Use newSSLSessionBindingEvent so we alway use the wrapper if needed.
                 ((SSLSessionBindingListener) value).valueBound(newSSLSessionBindingEvent(name));
@@ -2034,33 +2038,42 @@ public class ReferenceCountedOpenSslEngine extends SSLEngine implements Referenc
             if (name == null) {
                 throw new NullPointerException("name");
             }
+            synchronized (this) {
                 if (values == null) {
                     return null;
                 }
                 return values.get(name);
             }
+        }
 
         @Override
         public void removeValue(String name) {
             if (name == null) {
                 throw new NullPointerException("name");
             }
+
+            final Object old;
+            synchronized (this) {
                 Map<String, Object> values = this.values;
                 if (values == null) {
                     return;
                 }
-            Object old = values.remove(name);
+                old = values.remove(name);
+            }
+
             notifyUnbound(old, name);
         }
 
         @Override
         public String[] getValueNames() {
+            synchronized (this) {
                 Map<String, Object> values = this.values;
                 if (values == null || values.isEmpty()) {
                     return EmptyArrays.EMPTY_STRINGS;
                 }
                 return values.keySet().toArray(new String[0]);
             }
+        }
 
         private void notifyUnbound(Object value, String name) {
             if (value instanceof SSLSessionBindingListener) {