Avoid concurrent modification in mock log appender (elastic#41424)

It can be the case that while we are setting up expectations that also a
log message is appended. For example, if we are setting up these
expectations after a cluster has formed and messages start being sent
around the cluster. In this case, we would hit a concurrent modification
exception while we are mutating the expectations, and also while the
expectations are being iterated over as a message is appended. This
commit avoids this by using a copy-on-write array list which is safe for
concurrent modification and iteration. Note that another possible
approach here is to use synchronized, but that seems unnecessary since
we don't appear to rely on messages that are sent while we are setting
up expectations. Rather, we are setting up some expectations and some
situation that we think will cause those expectations to be met. Using
copy-on-write array list here is nice since we avoid bottlenecking these
tests on synchronizing these methods.

Author: jasontedor

test/framework/src/main/java/org/elasticsearch/test/MockLogAppender.java

@@ -24,8 +24,8 @@
 import org.apache.logging.log4j.core.filter.RegexFilter;
 import org.elasticsearch.common.regex.Regex;
 
-import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.regex.Pattern;
 
 import static org.hamcrest.CoreMatchers.equalTo;
@@ -42,7 +42,12 @@ public class MockLogAppender extends AbstractAppender {
 
     public MockLogAppender() throws IllegalAccessException {
         super("mock", RegexFilter.createFilter(".*(\n.*)*", new String[0], false, null, null), null);
-        expectations = new ArrayList<>();
+        /*
+         * We use a copy-on-write array list since log messages could be appended while we are setting up expectations. When that occurs,
+         * we would run into a concurrent modification exception from the iteration over the expectations in #append, concurrent with a
+         * modification from #addExpectation.
+         */
+        expectations = new CopyOnWriteArrayList<>();
     }
 
     public void addExpectation(LoggingExpectation expectation) {

test/framework/src/main/java/org/elasticsearch/transport/AbstractSimpleTransportTestCase.java

@@ -993,7 +993,6 @@ public void handleException(TransportException exp) {
     }
 
     @TestLogging(value = "org.elasticsearch.transport.TransportService.tracer:trace")
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/40586")
     public void testTracerLog() throws Exception {
         TransportRequestHandler<TransportRequest> handler = (request, channel, task) -> channel.sendResponse(new StringMessageResponse(""));
         TransportRequestHandler<StringMessageRequest> handlerWithError = (request, channel, task) -> {