Add tasks sequence support

[Ramshell]

This pr is related to this issue.

As I said In slack, I know there is an easy workaround of defining only one task in the TaskSet and defining all the behavior there. The problem with the workaround is that it is a bit hacky, since you have to do self.wait() whenever it's needed instead of letting the framework handle it.

If accepted this pr will

Introduce a new class: TaskSequence which accepts a new decorator @seq_task(int) to define the order of the tasks.

For example:

class BrowseDocumentationSequence(TaskSquence):
    def on_start(self):
        self.urls_on_current_page = self.toc_urls

    # assume all users arrive at the index page
    @seq_task(1)
    def index_page(self):
        r = self.client.get("/")
        pq = PyQuery(r.content)
        link_elements = pq(".toctree-wrapper a.internal")
        self.toc_urls = [
            l.attrib["href"] for l in link_elements
        ]

    @seq_task(2)
    @task(50)
    def load_page(self, url=None):
        url = random.choice(self.toc_urls)
        r = self.client.get(url)
        pq = PyQuery(r.content)
        link_elements = pq("a.internal")
        self.urls_on_current_page = [
            l.attrib["href"] for l in link_elements
        ]

    @seq_task(3)
    @task(30)
    def load_sub_page(self):
        url = random.choice(self.urls_on_current_page)
        r = self.client.get(url)

In the example above, index_page/1 will be executed first, then load_page/1 fifty times, and then load_sub_page/1 thirty times.

@seq_task(int) doesn't have a default parameter because I found that if I set the same order value to every task, the final order ends up being kind of random.

[codecov]

Codecov Report

Merging #827 into master will increase coverage by 0.44%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #827      +/-   ##
==========================================
+ Coverage    66.1%   66.55%   +0.44%     
==========================================
  Files          14       14              
  Lines        1422     1438      +16     
  Branches      224      226       +2     
==========================================
+ Hits          940      957      +17     
+ Misses        431      430       -1     
  Partials       51       51

Impacted Files	Coverage Δ
locust/core.py	84.25% <100%> (+1.75%)	⬆️

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[cgoldberg]

can you also include an update to the docs in your PR if we decide to merge it?

[Ramshell]

@cgoldberg Yeah, Is it okay if I add and example in quick start and an example with explanation in writing a locust file?

[Ramshell]

Well, finally I decided not to change the quick start because it's supposed to be short. Instead I've updated the api.rst and writing-locustfile.rst files. If there is anything else needed, just tell me :)

[ityoung]

That's a real good work! I try to do that but met some trouble, thanks for your work!

[cgoldberg]

I'm +1 on merging this. Anyone else?
/cc @locustio/committers

[ityoung]

I think you should add TaskSequence and seq_task to file locust/__init__.py, so that we can use from locust import seq_task.

[Ramshell]

Adding TaskSequence and seq_task to file locust/init.py sounds like something useful. @cgoldberg what do you say?

[cgoldberg]

agreed.

[aldenpeterson-wf]

This LGTM

[Ramshell]

If there is anything else you think I could add, just tell me! :)

[cgoldberg]

👍

[cgoldberg]

let's leave this for another day or 2 to see if any other maintainers comment, then I'll merge it.

[mbeacom]

LGTM 🚢

[cgoldberg]

looks like consensus for this PR is a 👍 ... merging now.

[cgoldberg]

thanks very much for the contribution!

[Ramshell]

My pleasure!