Ensure that MethodParameter.findParameterIndex() is thread-safe

Prior to this commit, parallel invocations of
MethodParameter.findParameterIndex() (invoked indirectly via
SynthesizingMethodParameter.forParameter() and
MethodParameter.forParameter()) could intermittently lead to an
IllegalArgumentException being thrown due to a race condition in the
internal implementation of the JDK's
java.lang.reflect.Executable.getParameters() method.

This commit addresses this issue by introducing a fallback for-loop
that iterates over the candidate parameters a second time using
equality checks instead of identity checks.

Issue: SPR-17534

spring-core/src/main/java/org/springframework/core/MethodParameter.java

@@ -733,11 +733,19 @@ public class MethodParameter {
 	protected static int findParameterIndex(Parameter parameter) {
 		Executable executable = parameter.getDeclaringExecutable();
 		Parameter[] allParams = executable.getParameters();
+		// Try first with identity checks for greater performance.
 		for (int i = 0; i < allParams.length; i++) {
 			if (parameter == allParams[i]) {
 				return i;
 			}
 		}
+		// Potentially try again with object equality checks in order to avoid race
+		// conditions while invoking java.lang.reflect.Executable.getParameters().
+		for (int i = 0; i < allParams.length; i++) {
+			if (parameter.equals(allParams[i])) {
+				return i;
+			}
+		}
 		throw new IllegalArgumentException("Given parameter [" + parameter +
 				"] does not match any parameter in the declaring executable");
 	}