Add transform TSIOBuffer watermark APIs.

[ghost]

This feature PR allows a LUA transform handler to set the upstream buffer watermark in units of bytes via the TSIOBufferWaterMarkGet/Set() APIs.

Setting the watermark to values of 256kb, 512kb, or even 1MB seems to dramatically improve the performance of a LUA transform handler when processing high-throughput response body.

[shukitchan]

[approvie ci]

[shukitchan]

[approve ci]

[shukitchan]

sorry. I miss this till now. will review over the weekend.

[shukitchan]

[approve ci freebsd]

[shukitchan]

Description and the code makes sense. Will be trying this out over the weekend before merging.

[ghost]

Thanks for taking a look. Let me know if you need anything else to accurately test this patch. The code submitted was used to improve the performance of a broadband speed testing script written in LUA. The upload portion of the script that uses a POST request transform handler to measure speed saw dramatic improvements in speed by setting the watermark to 256kb, 512kb, or 1MB.

Do you think I should add a maximum value limit for the watermark?

[shukitchan]

I think it is ok not to have a max limit at this point.

Can you give a simple example script in the doc section to show how to use this this function?

[shukitchan]

I think everything else looks good.

[bryancall]

Cherry picked to 8.1.0

[ezelkow1]

We probably need this cherry picked in to 8.0.x as well, as of now it only exists on 8.1