client: Allow AsyncContentListener.onContent to throw checked Exceptions

[mperktold]

Jetty version(s)
Jetty 12.x

Enhancement Description
AsyncContentListener implements ContentSourceListener.onContentSource and delegates to its own hook called onContent. Exceptions (or any Throwable really) thrown from onContent are catched and handled inside AsyncContentListener.onContentSource.

Now I have an implementation of AsyncContentListener.onContent that writes the content to a file, which can fail with an IOException. I would like to let these exceptions bubble up to onContentSource, which would handle it nicely. But this is not possible since IOException is a checked exception.

There are two workarounds:
One is to wrap the IOException in a RuntimeException and before rethrowing it.
The other is to completely handle the exception myself, basically copying the catch block from AsyncContentListener .onContentSource.

[mperktold]

The same would apply to ContentListener, which in implements AsyncContentListener.onContent and delegates to its own onContent hook.

Here, I see that the default implementation indeed catches the exception and just aborts the response. AsyncContentListener.onContentSource would also notify the content source about the failure, but the source isn't available in onContent, so that's not possible there. Is that a problem?

If so, that would be a strong argument for allowing these hooks to throw checked exceptions, such that the failure can be properly detected and forwarded to the source. Otherwise, I guess catching the exception and aborting the response manually could be acceptable as well.

[lorban]

I've created a PR to modify the ContentListener and AsyncContentListener contracts to allow throwing Exception as that would be safe and backward compatible: #11825

The fact that ContentListener does not fail the source would still make onContent be called for all remaining contents despite the fact that the response was aborted. While correct, I believe a more sane default would be to fail the original Content.Source so that the flow of content would stop, so the above PR changes that too.

Thanks for the suggestion!