Rule for detecting useless os.Exit in TestMain

[alexandear]

Is your feature request related to a problem? Please describe.

Implement a rule that suggests removing os.Exit in TestMain for test files.

Before:

// simple case
func TestMain(m *testing.M) {
	os.Exit(m.Run())
}

// real case
func TestMain(m *testing.M) {
	code = m.Run()
	cleanup()
	os.Exit(code)
}

After:

// simple case
func TestMain(m *testing.M) {
	m.Run()
}

// real case
func TestMain(m *testing.M) {
	defer cleanup()
	m.Run()
}

Describe the solution you'd like

A new rule redundant-test-main-os-exit that suggests removing os.Exit.

Describe alternatives you've considered

Search for os.Exit manually and refactor TestMain functions.

Additional context

As of Go 1.15, we do not need to call os.Exit(m.Run()) explicitly, as value returned by m.Run() is stored into unexported field of m and go test executable is smart enough to automatically call os.Exit(retValue) when TestMain returns. See golang/go#34129 and https://pkg.go.dev/testing#hdr-Main

These PRs performs removing os.Exit from TestMain in a wild:

• tests: calling os.Exit in TestMain is not required go-delve/delve#3856
• test: remove redundant os.Exit in TestMain goreleaser/goreleaser#5257
• downloader: simplify tests lima-vm/lima#2869

[alexandear]

Connected issues: #290

[ccoVeille]

As of Go 1.15, we do not need to call os.Exit(m.Run()) explicitly, as value returned by m.Run() is stored into unexported field of m and go test executable is smart enough to automatically call os.Exit(retValue) when TestMain returns.

TIL