MappingMediaTypeFileExtensionResolver ConcurrentModificationException

[ZikFat]

I'm seeing this issue running Spring 5.0.9 but the code involved doesn't look notably different in the master branch.

The MappingMediaTypeFileExtensionResolver can throw a ConcurrentModificationException when encountering a new media type. Here is the series of events in an example I'm seeing:

1. A request comes into the server at a URL like /something.tar.gz
2. The .gz suffix isn't initially mapped to any media type, but AbstractMappingContentNegotiationStrategy's handleNoMatch method is able to figure out that application/x-gzip is a reasonable content type for the "gz" suffix.
3. MappingMediaTypeFileExtensionResolver.addMapping is called to permanently add the mapping between application/x-gzip and the "gz" content type. This method is vulnerable to a ConcurrentModificationException because the fileExtensions map it's trying to update is not thread-safe.

Example stack:

Exception caught during HTTP request. Returning status 500
java.util.ConcurrentModificationException: null
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1134) ~[na:na]
	at org.springframework.util.LinkedMultiValueMap.add(LinkedMultiValueMap.java:87) ~[spring-core-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.accept.MappingMediaTypeFileExtensionResolver.addMapping(MappingMediaTypeFileExtensionResolver.java:80) ~[spring-web-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.accept.AbstractMappingContentNegotiationStrategy.resolveMediaTypeKey(AbstractMappingContentNegotiationStrategy.java:121) ~[spring-web-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.accept.AbstractMappingContentNegotiationStrategy.resolveMediaTypes(AbstractMappingContentNegotiationStrategy.java:102) ~[spring-web-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.accept.ContentNegotiationManager.resolveMediaTypes(ContentNegotiationManager.java:124) ~[spring-web-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.mvc.condition.ProducesRequestCondition.getAcceptedMediaTypes(ProducesRequestCondition.java:262) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.mvc.condition.ProducesRequestCondition.getMatchingCondition(ProducesRequestCondition.java:199) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.mvc.method.RequestMappingInfo.getMatchingCondition(RequestMappingInfo.java:222) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.mvc.method.RequestMappingInfoHandlerMapping.getMatchingMapping(RequestMappingInfoHandlerMapping.java:93) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.mvc.method.RequestMappingInfoHandlerMapping.getMatchingMapping(RequestMappingInfoHandlerMapping.java:57) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.handler.AbstractHandlerMethodMapping.addMatchingMappings(AbstractHandlerMethodMapping.java:384) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.handler.AbstractHandlerMethodMapping.lookupHandlerMethod(AbstractHandlerMethodMapping.java:352) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.handler.AbstractHandlerMethodMapping.getHandlerInternal(AbstractHandlerMethodMapping.java:318) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.handler.AbstractHandlerMethodMapping.getHandlerInternal(AbstractHandlerMethodMapping.java:62) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.handler.AbstractHandlerMapping.getHandler(AbstractHandlerMapping.java:350) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.getHandler(DispatcherServlet.java:1188) ~[spring-webmvc-5.0.9.RELEASE.jar:5.0.9.RELEASE]