Compatibility with Cache Control Override: is max-age=0 really necessary for BigPipe's placeholders?

Even though this is a test can we setup a test to check for the cache age?

The patch in #8 makes big_pipe responses cacheable when using big_pipe_sessionless and cache_control_override but it does not correctly bubble the cache tags from elements rendered by big_pipe.

BigPipe::sendPlaceholders() calls sendChunk() with the chunk as a string.
This means that sendChunk() never gets the cache tags and max age from these chunks.

BigPipeSessionless::sendChunk() is able to use the chunk's cache parameters and apply them to the final cached page.
If core does not send these cache parameters, cache tags from elements rendered by big_pipe will not bubble to the cached response.
This means page cache can store a page even if it contains a block with max-age = 0 rendered by big_pipe; because its max-age will not bubble to the stored response.

The attached patch makes core pass the chunk cache tags to big_pipe_sessionless.

Looks like #18 needs review.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

Writing a functional test seems hard without big_pipe_sessionless because core itself never uses the cache metadata from chunks.
I added a unit test that checks if the chunks are cacheable.

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

Adding a couple of related issues, I'm not sure exactly how they would interact with this one yet.

The main focus of those issues would be to enable big pipe for session-less users (in core, probably as a new configuration setting) but by pre-empting big pipe for cached placeholders, avoiding it entirely on warm caches (and hence the response would be cacheable by reverse proxies/page cache as usual).

Have not reviewed but issue summary appears to be missing.

I updated the summary.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

The bot says the patch does not apply but I don't see any conflict on the MR.
I rebased to see if it works better.