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proposal: cmd/go: run vet loopclosure during go test #30176

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neild opened this issue Feb 11, 2019 · 4 comments

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commented Feb 11, 2019

Go test runs a high-confidence set of go vet checks when building a test binary. I propose adding the "loopclosure" check to that set.

The loopclosure check looks for references to loop variables from within nested functions. For example, it will catch the bug in the following code, which will usually (depending on the whim of the scheduler) print 222 rather than 012.:

for _, x := range []int{0, 1, 2} {
  go func() {
    fmt.Print(x)
  }()
}

We've been running this analyzer as part of all Go builds inside Google for a while now (not just tests). It catches a common coding mistake (FAQ entry https://golang.org/doc/faq#closures_and_goroutines). While it's theoretically possible that valid code could depend on the behavior it tests for, I've never seen any such code in practice. We should turn it on by default.

@gopherbot gopherbot added this to the Proposal milestone Feb 11, 2019

@gopherbot gopherbot added the Proposal label Feb 11, 2019

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commented Feb 12, 2019

Earlier open issue and discussion about adding a vet check #16520

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commented Feb 12, 2019

Earlier open issue and discussion about adding a vet check #16520

Note that #16520 is about improving the vet check. This proposal is to add the already existing check to the default set run by go test.

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commented Feb 13, 2019

Can someone characterize the false positives for loopclosure?
The usual rule for turning on a vet check during go test is that it
have basically no false positives, or the false positives are only
in code that you couldn't defend not rewriting anyway.

The docs in loopclosure.go say:

This analyzer checks for references to loop variables from within a
function literal inside the loop body. It checks only instances where
the function literal is called in a defer or go statement that is the
last statement in the loop body, as otherwise we would need whole
program analysis.

For example:

        for i, v := range s {
                go func() {
                        println(i, v) // not what you might expect
                }()
        }

See: https://golang.org/doc/go_faq.html#closures_and_goroutines

In this case it seems like the reason it's not on by default is it doesn't catch very much. It does catch this one example. Maybe that's enough?

Damien, you said that we've been running it inside Google and also claimed that it catches a common mistake, but it looks like it will flag very few of the actual mistakes I've seen, because of the restriction about looking only at the final statement of the loop. Do you know if this has caught real mistakes inside Google, and if so how many?

@rsc rsc changed the title proposal: add loopclosure to vet tests run by "go test" proposal: cmd/go: run vet loopclosure during go test Feb 13, 2019

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commented Mar 28, 2019

Timed out in state WaitingForInfo. Closing.

(I am just a bot, though. Please speak up if this is a mistake or you have the requested information.)

@gopherbot gopherbot closed this Mar 28, 2019

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