dominikh
staticcheck: SA4006 false-negative when a defer is used
Formalities
$ staticcheck --version staticcheck (devel, v0.0.0-20200121224137-2774002210e8) $ staticcheck --debug.version staticcheck (devel, v0.0.0-20200121224137-2774002210e8) Compiled with Go version: go1.13.7 Main module: honnef.co/go/[email protected] (sum: h1:LDZNqNckGrHSM2hO0hPWJKIqdodYH0Tov7n86ikeGJE=) Dependencies: github.com/BurntSushi/[email protected] (sum: h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=) golang.org/x/[email protected] (sum: h1:/iIZNFGxc/a7C3yWjGcnboV+Tkc7mxr+p6fDztwoxuM=) $ go version go version go1.13.7 linux/amd64 $ go env GO111MODULE="" GOARCH="amd64" GOBIN="" GOCACHE="/home/ainar/.cache/go-build" GOENV="/home/ainar/.config/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GONOPROXY="" GONOSUMDB="" GOOS="linux" GOPATH="/home/ainar/go" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/home/ainar/go/go1.13" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/home/ainar/go/go1.13/pkg/tool/linux_amd64" GCCGO="gccgo" AR="ar" CC="gcc" CXX="g++" CGO_ENABLED="1" GOMOD="" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config" GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build783553461=/tmp/go-build -gno-record-gcc-switches"
The code itself: https://play.golang.org/p/97Vr6raQX-i.
On line 21, an error check is missing, but because of the following
defer, staticcheck doesn't complain:
$ staticcheck ./go.tmp.go $
I remember a similar bug, but I couldn't find the issue.
From an implementation point of view: the anonymous function refers to err, which means that the variable can't be lifted into registers; it'll be an allocation. The way we check for unread assignments is by checking whether the register gets read.
For allocations, this isn't nearly as trivial (nor solvable for all cases.) For example, err could be aliased and read via the alias, or g could panic, leading to the deferred function reading the value. We could handle some of these cases (e.g. handle allocations that can't be aliased), but for your concrete example, we still couldn't claim that err is unread, due to the possibility of g panicking and thus reading that value.
