cmd/cover: spurious 0 line in coverprofile output

[ct1n]

Go version

go1.22.9 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/root/.cache/go-build'
GOENV='/root/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.22.9'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/build/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 -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1442515923=/tmp/go-build -gno-record-gcc-switches'

What did you do?

Running test coverage using a docker golang:1.22-alpine3.19 container. We run the following command:

go test -v -parallel 24 -p 24 -count 1 -covermode=atomic -coverprofile /coverage/coverage.out --timeout 30s ./...

What did you see happen?

The go test command sometimes generates either a line containing just the digit 0 at the end of coverage.out, or an empty line (though this seems rarer and I haven't checked that it's at the end of the file).

Trying to parse coverage.out with github.com/AlekSi/gocov-xml results in error: line "0" doesn't match expected format: couldn't find a before Count (from golang.org/x/tools.cover).

Other than the last line coverage.out seems valid: it starts with mode: atomic and contains about 15k report lines.

Not very easy to reproduce.

What did you expect to see?

A valid coverage.out profile that can be loaded by golang.org/x/tools/cover.

We can work around this pretty easily, but thought it might be useful in case others encounter this. Also it might be an easy fix.

[gabyhelp]

Related Issues

• cmd/cover: empty profile #45285 (closed)
• cmd/go: inconsistent coverage when using -coverprofile with packages without tests #70244
• cmd/cover: cover mode "atomic" generates invalid code for files without a newline at EOF #58370 (closed)
• cmd/go: go 1.22.0: go test throws errors when processing folders not listed in coverpkg argument #65653 (closed)
• cmd/cover: Compilation error when using -covermode=atomic on a file without a newline at the end #59081 (closed)
• cmd/go: app.test -test.coverprofile fails silently #30542 (closed)
• cmd/go: -cover stdout format is not consistent between failed \& pass #39070 (closed)
• cmd/go: test with coverage unexpect fail #43242 (closed)
• cmd/cover, cmd/go: go test reports wrong compiler error line numbers when using -coverprofile #6329 (closed)
• cmd/cover: inconsistent NumStmt: changed from 1 to 2 #45506 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

I think we need some form of reproducer

[ct1n]

Sorry for the noise. It seems this was because of our build process. Sometimes 2 builds ran at the same time and they weren't properly isolated, using the same output directory

[ianlancetaylor]

Thanks for following up.