[llvm-dev] RFC: Reconsidering adding gmock to LLVM's unittest utilities

[Chandler Carruth via llvm-dev]

So far I've not heard any objections to the core of:

1) add the utility code
2) use it in the *clear* places where it makes a substantial improvement,
both matchers and mocks

I'd really like to hear if there are serious concerns here, but so far this
looks like pretty strong consensus. If possible I'd like to make progress
on landing the actual code Friday, so if you haven't given a shout yet,
please do. Of course, if new concerns come up, we can always revisit this.
It's just internal testing utilities, so it seems especially low-risk.


We still need to sort out several details of course:

a) I will put together some good LLVM-focused primitives (mostly around
matchers) in a common location. At the least this will give us a good
pattern to follow as new bits of common stuff come up. I'll get the initial
skeleton of this quickly and then everyone should be able to chip in with
the bits that they need. I'll send this out as a relatively small follow-up
patch that we can discuss in code review to get the location / pattern
right.

b) We will definitely want some guidelines around *how* and *when* to use
this stuff. I'll try and distill something more brief than my email and
incorporating some of the comments on this thread, and put it up for review
as an addition to the coding standards. This will take me a bit more time
but I'm happy to make sure this happens. This code review can then serve as
a place to discuss the somewhat mechanical bits that are still important
such as should we write `EXPECT_EQ(b, a)`, `EXPECT_THAT(a, Eq(b))`, or
(with some custom magic) `EXPECT(a, Eq(b))`.

c) It might be helpful to have an LLVM-focused explanatory guide to how
gtest+gmock work and how to use them effectively. I'm not the best at
writing this documentation, so if anyone else wants to take a stab at it,
honestly I'd appreciate that. Happy to review of course. But if no one else
feels like they can help with this, I can try to pull this together as
well. It will definitely take a bit though.

I don't think any of these really need to be blocking as it seems like the
example usages I posted weren't terribly controversail, and it'll be easy
to update based on any changes in suggested practice from (a) or (b).

Does that sound right? Anything I'm missing? Any concerns with this path
forward?

Also, thanks everyone! I know my writeup was a bit long, appreciate taking
the time.
-Chandler

On Thu, Jan 5, 2017 at 3:19 PM Matthias Braun <mbraun at apple.com> wrote:

> - Providing some universal helpers for various situations that you want to
> EXPECT() on sounds great.
> - I can see how the "Mocks" stuff can help in the pass manager case. There
> is some cost learning yet another library just to test a feature, so we
> should keep pushing for simple/well known solutions (mostly thinking of
> "helper-command | FileCheck") by default and at least require people to
> write long justifications like this when they want to use gmock :)
>
> - Matthias
>
> On Jan 4, 2017, at 6:11 AM, Chandler Carruth via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> A long time ago I suggested that we might want to add gmock to compliment
> the facilities provided by gtest in LLVM's unittests. It didn't go over
> well:
>
> 1) There was concern over the benefit vs. the cost
> 2) Also concern about what the facilities would look like in practice and
> whether they would actually help
> 3) At the time, I didn't have good, large examples of what these things
> might look like or why they might be attractive
> 4) I didn't provide any real explanation of what gmock *did* and so it was
> vague and unclear.
>
> Since then, a lot has changed. We have more heavy use of unit testing in
> the project with more developers finding benefit from it. And I think I
> have compelling examples.
>
> ## Matchers
>
> To start off, it is important to understand that there are two components
> to what gmock offers. The first has very little to do with "mocks". It is
> actually a matcher language and system for writing test predicates:
>
>   EXPECT_EQ(expected, actual);
>   EXPECT_NE(something, something);
>
> Become instead:
>
>   EXPECT_THAT(actual, Eq(expected));
>   EXPECT_THAT(actual, Ne(not-expected));
>
> This pattern moves the *matcher* out of the *macro*, giving it a proper
> C++ API. With that, we get two huge benefits: extensibility and
> composability. You can easily write a matcher that summarizes concisely the
> expectation for custom data types. And you can compose these matchers in
> powerful ways. I'll give one example here:
>
>   EXPECT_THAT(MyDenseMap, UnorderedElementsAre(Eq(key1, value1), Eq(key2,
> value2), Eq(key3, value3)));
>
> Here I'm composing equality matchers inside a matcher that can handle
> *unordered* container element-wise comparison for generic, arbitrary
> containers. With a small patch, I've even extended it to support arbitrary
> iterator ranges! Combine this with custom matchers for the elements, and it
> becomes a very expressive an declarative way to write expectations in tests.
>
> I wanted to give a realistic and compelling example so I rewrote an entire
> test: https://reviews.llvm.org/D28290 Note that I moved *every* EXPECT to
> the new syntax so this is essentially worst-case. It also involves a
> non-trivial custom matcher. Despite this, the code is shorter, easier to
> read and easier to maintain. It has fewer unnecessary orderings enforced.
> And it is much easier to extend. Also, the error messages when it fails are
> substantially improved because these composed matchers have logic to
> carefully explain *why* they failed to match.
>
> I hope folks find this compelling. I think this alone is worth carrying
> the gmock code in tree -- it is just used by tests and not substantially
> larger than gtest. Even if we decide we want nothing to do with mocks, I
> would very much like to have the matchers.
>
>
> ## Mocks
>
> So, now let's consider mocks. First off, what are mocks? I'll give a
> fairly casual definition here: they are test objects which implement some
> API and allow the test to explicitly set expectations on how that API is
> used and how it in turn should behave. For a more detailed vocabulary see
> [1] and for a more lengthy discussion see [2].
>
> As came up in the original discussion, LLVM relatively infrequently has a
> need to test API interactions in this way. Usually we're in the business of
> translating things from format A to B (instructions, metadata, whatever)
> and can write down one format and write checks against the other format for
> tests. This is a wonderful world to live in with tests. I never want LLVM
> to *decrease* how much we leverage this.
>
> But we *do* have API interactions that we need to test. We have plugin
> APIs, and hookable interfaces, ranging from Clang frontend actions to JIT
> listeners. We also have *generic* code in ADT that is all about API
> interactions. Most generic code in fact is -- we want it to work for *any*
> T that behaves in a certain way, so we need to give it interesting Ts to
> test it.
>
> My immediate example is the pass manager. We plug in a bunch of passes to
> it, and expect it to run them in a precise way over specific bits of IR.
> When testing this, it is extremely cumbersome to write a test pass which
> does this in interesting and yet controllable and comprehensible ways.
> Let's look at a concrete example:
>
>
> https://github.com/llvm-project/llvm-project/blob/master/llvm/unittests/IR/PassManagerTest.cpp#L481-L509
>
> Here we have over 20 lines of code spent testing that the correct set of
> things happened the correct number of times. I had to write a long comment
> just to explain what these numbers mean. And I still never understand
> whether a change in the numbers really means a good or bad thing.
>
> Now, we *have* detailed logging based tests use FileCheck which is the
> primary way to avoid this in LLVM. But it isn't enough. In these tests we
> want to carefully *permute* the behavior of very specific runs of
> individual passes. A simple example of this can be seen here where we have
> somewhat magical state in a pass to flip-flop its behavior:
>
> https://github.com/llvm-project/llvm-project/blob/master/llvm/unittests/IR/PassManagerTest.cpp#L138-L139
>
> And it gets more complicated if you want statefulness like triggering on
> the *3rd* run of the pass.
>
> But this is exactly the kinds of scenarios that I needed to write tests
> for in order to get the code to be correct. I have consistently found and
> been able to fix bugs throughout the pass manager by writing careful
> unittests.
>
> Mocks with GoogleMock are, IMO, a *tool to create interesting and
> debuggable test objects*. These objects can then be fed into an API to
> exercise it in ways that are hard or impossible to control from a command
> line in sufficient granularity and precision. While doing this is never fun
> and should be avoided where possible, when we need to do this I think it
> provides a powerful tool for the job.
>
> Here is how it works at the highest level:
> 1) Create a class with a MOCK_METHOD*(...) API. This API is then hookable
> by gmock.
> 2) Use some APIs to register default behaviors for the APIs.
> 3) Setup the *minimal* amount of expected API interactions for a given
> test. IE, for this test to pass, X has to happen and in response to that my
> code needs to do Y.
> 4) Feed this class, or a wrapper around it if you need a copyable object,
> into the system you are testing and run it.
>
> If the expected interactions don't occur, you get a trace of which ones
> failed and why. These traces are somewhat verbose and hard to read, but
> they actually have the information needed to debug the system which saves
> you from building infrastructure to extract that over and over again.
>
> But a concrete example will likely work better. I've used gmock to build
> the unit tests for a major revision of the LoopPassManager in the new pass
> manager. This is a substantial redesign that now handles inserting new
> loops, deleting loops, and invalidating analyses. The tests for it are,
> IMO, dramatically more readable than the test linked above. And they are
> substantially more thorough and precise:
>
> https://reviews.llvm.org/D28292
>
> I hope this is compelling for folks. Just writing and debugging this one
> test was extremely compelling for me. I ended up with much better coverage
> and precision than I would have using any other technique without a
> tremendous amount of plumbing essentially re-inventing a framework to build
> test pass objects that work exactly the way these mock pass handles do.
>
> That said, all is not perfect. For instance, gmock suffers from being
> designed in  C++98 world. It has relatively poor support for move and value
> semantics, which resulted in my using a wrapper around the mock interfaces
> in the above patch to let a pimpl idiom provide the value semantics I
> wanted. However, that idiom works well, and this didn't substantially
> impede my use of the infrastructure.
>
> Also, I remain very sympathetic to the idea that this kind of testing
> apparatus should be relatively rarely needed. We shouldn't be writing new
> complex unit tests for APIs every week. But even a few use cases such as to
> test ADTs and generic tools like the pass manager seem to justify the cost
> to me, and I'm happy to help draw up fairly restrictive guidance around
> mocks for the coding standards.
>
>
> Thanks, and sorry for the long email, but I wanted to try and lay out the
> issues in a way folks could understand, and the examples, while hopefully
> useful, are quite large and complex.
>
> Please don't hesitate to ask questions if stuff isn't clear.
> -Chandler
>
> [1]: https://en.wikipedia.org/wiki/Test_double
> [2]: http://martinfowler.com/articles/mocksArentStubs.html
>
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