cmd/cover: cover mode "atomic" generates invalid code for files without a newline at EOF

[nilium]

What version of Go are you using (go version)?

$ go version
go version go1.20 linux/amd64

Does this issue reproduce with the latest release?

Yes.

• It does not reproduce under Go 1.19.5.
• Also does not reproduce with GOEXPERIMENT=nocoverageredesign and suggests the issue is possibly a regression related to cmd/cover: extend coverage testing to include applications #51430 -- thanks to @deltamualpha for discovering this.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/ncower/.cache/go-build"
GOENV="/home/ncower/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/ncower/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/ncower/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/ncower/p/break/go.mod"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build76322923=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Ran go test -cover -covermode=atomic on a package with a file that had a newline missing at EOF.

All code referred to is also under https://github.com/nilium/go-issue-test/tree/38c0047d4219026ecccbb2f35f96162fef5478a7. Because it involves the trailing newline at EOF on sources, there isn't a good way to reproduce it in the playground as far as I can see.

A more or less empty file can be used to reproduce this provided the empty.go file does not have a newline at EOF:

empty.go

package noeol

some_test.go (doesn't matter if this file has a newline at EOF to reproduce)

package noeol

import "testing"

func TestNothing(t *testing.T) {
}

What did you expect to see?

For test output:

$ go test -cover -covermode=atomic ./noeol
ok      github.com/nilium/go-issue-test/noeol   0.001s  coverage: [no statements]

What did you see instead?

$ go test -cover -covermode=atomic ./noeol
# github.com/nilium/go-issue-test/noeol [github.com/nilium/go-issue-test/noeol.test]
noeol/empty.go:2:51: syntax error: unexpected var after top level declaration
FAIL    github.com/nilium/go-issue-test/noeol [build failed]
FAIL

with the generated code for mode=atomic resembling the following:

//line noeol/empty.go:1:1
// empty.go
package noeol; import _cover_atomic_ "sync/atomic"var _ = _cover_atomic_.LoadUint32

var CoverageVariableNameP uint32
// etc.

[deltamualpha]

@thanm will probably be interested in this?

[thanm]

Thanks for the report. I'll take a look. NB as in many other cases, you can help reduce the frequency of this sort of surprise by running your code through "gofmt".

[gopherbot]

Change https://go.dev/cl/466115 mentions this issue: cmd/cover: fix buglet in -covermode=atomic fixup

[dr2chase]

Not sure if this is backport-worthy.

[nilium]

Not sure if this is backport-worthy.

Since it only seems to affect Go 1.20 out of the box where the coverage redesign is in use by default, probably not.