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

Zachary Turner via llvm-dev llvm-dev at lists.llvm.org
Wed Jan 4 09:49:43 PST 2017

TL;DR - I want this.

On Wed, Jan 4, 2017 at 6:11 AM Chandler Carruth via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> ## 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.

+1, these look amazing.  Often times I find myself writing many EXPECT
statements to test a single logical condition.  When you want to do this
for many different inputs / outputs of an API it turns into a long list of
expect statements that the person reading the test can't easily grok and
see how they're related.  Here's an example from the formatv tests that I

  Replacements = formatv_object_base::parseFormatString("{0,-3}");
  ASSERT_EQ(1u, Replacements.size());
  EXPECT_EQ(ReplacementType::Format, Replacements[0].Type);
  EXPECT_EQ(0u, Replacements[0].Index);
  EXPECT_EQ(3u, Replacements[0].Align);
  EXPECT_EQ(AlignStyle::Left, Replacements[0].Where);
  EXPECT_EQ("", Replacements[0].Options);

It would be nice if I could write:

EXPECT_THAT(Replacements, ReplacementsAre(Rep(Format, 0, 3, Left, "")));

This isn't a huge win here, but if you have a longer format string where
there's multiple replacements, you end up 5 lines per replacement, which
starts to become very unwieldy and hard to follow.  Now multiply that by
the number of different edge cases you want to test, and you end up losing
test coverage because you have to balance maintainability of the test's
code with test coverage, and adding 100 lines to test one API hurts
readability more than it helps test coverage.

Another thing.  Often times I find myself writing a function to test a
complex condition, like this:

EXPECT_TRUE(Value, ComplexTest(Value));

But then you lose the error message ability to see why the complex test
failed.  You say this is handled by the matcher infrastructure although I
don't see an example, but I'll take your word for it.  If so, these
matchers seem like an across the board win and I hope to be able to use
them in-tree soon.

> ## 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.
You're forgetting about that troublesome LLVM subproject that nobody wants
to think about which does things completely differently: LLDB.  ;-)  LLDB
*very frequently* has a need to test API interactions in this way, and is
*very infrequently* in the business of translating things from format A to
format B.

> 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.

In LLDB, I think this will end up being the most useful kind of unit test.
There is so little test coverage right now precisely because certain things
in an interactive application are hard/impossible to test with a garbage-in
garbage-out model.

Consider me on board.
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