Add library option to disable saving response bodies

[bluekeyes]

Proposal

Add an option for Attacker that does not save the response body into a buffer, but copies it directly to ioutil.Discard. I've implemented this in my fork: https://github.com/bluekeyes/vegeta/tree/bluekeyes/discard-body

Background

This is similar to #302, but slightly different. I'm using Vegeta as a library to test a download server, with file sizes ranging from 1kb to several GB. While the latest version of the CLI allows me to filter out the body from result files (or I could set the body to nil when using the library and processing results), reading large responses quickly uses up available memory during the test.

For the largest downloads in my test (0.25GB - 4GB), my test containers were running out of memory unpredictably based on the download rate (I tested with up to 8GB of available memory.) Since I don't use the response body for anything, discarding it keeps the memory usage low and allows the large tests to run with the same system resources as tests for smaller files.

Workarounds

Running tests on hosts with more memory or distributing the load across more machines can solve this problem, but is wasteful and may be hard to setup.

[tsenart]

Thanks for reporting this with such useful context. Out of curiosity, would test your servers with HEAD instead of GET solve the issue? Or do they not handle that method?

[bluekeyes]

I didn't consider using HEAD requests, but in this case I don't think it would work. I'm mainly interested in how the server and the reverse proxy handle the data transfer as we adjust things like buffer settings or file system configuration. For that, it's important to send the file content to the client.

[tsenart]

Released: https://github.com/tsenart/vegeta/releases/tag/cli/v11.1.0

[tsenart]

Feedback welcome :-)

[bluekeyes]

Thanks for the quick turnaround on this. I haven't tested it out yet, but looking at the implementation I'm a bit worried that when setting the max body size to 0, the LimitReader won't actually read any data from the server, which could lead to weird behavior like the client closing the connection before reading the full body. When I get a chance to try it in my application, I'll update this with the results.