Improve synchronization on the R2 RequestContext local attributes.

It is possible to cause a `ConcurrentModificationException` in the
`RequestContext` because we synchronize it by using
`Collections.synchronizedMap()`, which only synchronizes individual
reads/writes. When making a `RequestContext` copy we must iterate over
the collection, allowing other threads to interrupt it by modifying the
map during the operation.

This fix synchronizes the whole copy operation as well. The default
mutex used for synchronizing in a `SynchronizedMap` is `this`, so
synchronizing on the map itself guards against modification.

CHANGELOG.md

@@ -14,6 +14,9 @@ and what APIs have changed, if applicable.
 
 ## [Unreleased]
 
+## [29.42.2] - 2023-05-11
+- Fix synchronization on `RequestContext` to prevent `ConcurrentModificationException`.
+
 ## [29.42.1] - 2023-05-11
 - Add support for returning location of schema elements from the PDL schema encoder.
 
@@ -5465,7 +5468,8 @@ patch operations can re-use these classes for generating patch messages.
 
 ## [0.14.1]
 
-[Unreleased]: https://github.com/linkedin/rest.li/compare/v29.42.1...master
+[Unreleased]: https://github.com/linkedin/rest.li/compare/v29.42.2...master
+[29.42.2]: https://github.com/linkedin/rest.li/compare/v29.42.1...v29.42.2
 [29.42.1]: https://github.com/linkedin/rest.li/compare/v29.42.0...v29.42.1
 [29.42.0]: https://github.com/linkedin/rest.li/compare/v29.41.12...v29.42.0
 [29.41.12]: https://github.com/linkedin/rest.li/compare/v29.41.11...v29.41.12

gradle.properties

@@ -1,4 +1,4 @@
-version=29.42.1
+version=29.42.2
 group=com.linkedin.pegasus
 org.gradle.configureondemand=true
 org.gradle.parallel=true

r2-core/src/main/java/com/linkedin/r2/message/RequestContext.java

@@ -54,12 +54,14 @@ public RequestContext()
    */
   public RequestContext(RequestContext other)
   {
-    _localAttrs = Collections.synchronizedMap(new HashMap<>(other._localAttrs));
+    synchronized (other._localAttrs) {
+      _localAttrs = Collections.synchronizedMap(new HashMap<>(other._localAttrs));
+    }
   }
 
   private RequestContext(Map<String, Object> localAttrs)
   {
-    _localAttrs = localAttrs;
+    _localAttrs = Collections.synchronizedMap(localAttrs);
   }
 
   /**
@@ -117,7 +119,7 @@ public RequestContext clone()
   public boolean equals(Object o)
   {
     return (o instanceof RequestContext) &&
-            ((RequestContext)o)._localAttrs.equals(this._localAttrs);
+        ((RequestContext)o)._localAttrs.equals(this._localAttrs);
   }
 
   @Override