cmd/go: go list starts the Package.Error.Err field with a newline

[perillo]

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

$ go version
go version go1.13.6 linux/amd64

Does this issue reproduce with the latest release?

Yes. I can reproduce it with go1.14beta1

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

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/home/manlio/.local/bin"
GOCACHE="/home/manlio/.cache/go-build"
GOENV="/home/manlio/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY="github.com/perillo"
GONOSUMDB="github.com/perillo"
GOOS="linux"
GOPATH="/home/manlio/.local/lib/go:/home/manlio/src/go"
GOPRIVATE="github.com/perillo"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/code/git/github.com/golang/go/src/go.mod"
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-build700153789=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.13.6 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.13.6
uname -sr: Linux 5.4.13-arch1-1
/usr/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.30.
gdb --version: GNU gdb (GDB) 8.3.1

What did you do?

go1.14beta1 list -e -json /tmp/empty.go

Where empty.go is an empty file.

What did you expect to see?

{
	"Dir": "/tmp",
	"ImportPath": "command-line-arguments",
	"Match": [
		"/tmp/empty.go"
	],
	"Incomplete": true,
	"Error": {
		"ImportStack": [
			"main"
		],
		"Pos": "",
		"Err": "/tmp/empty.go:1:1: expected 'package', found 'EOF'"
	}
}

What did you see instead?

{
	"Dir": "/tmp",
	"ImportPath": "command-line-arguments",
	"Match": [
		"/tmp/empty.go"
	],
	"Incomplete": true,
	"Error": {
		"ImportStack": [
			"main"
		],
		"Pos": "",
		"Err": "\n/tmp/empty.go:1:1: expected 'package', found 'EOF'"
	}
}

There is a '\n' at the begin of the Err string.

[mvdan]

This appears to be on purpose:

go/src/cmd/go/internal/base/base.go

Lines 182 to 193 in 69660ed

	// Prepare error with \n before each message.
	// When printed in something like context: %v
	// this will put the leading file positions each on
	// its own line. It will also show all the errors
	// instead of just the first, as err.Error does.
	var buf bytes.Buffer
	for _, e := range err {
	e.Pos.Filename = ShortPath(e.Pos.Filename)
	buf.WriteString("\n")
	buf.WriteString(e.Error())
	}
	return errors.New(buf.String())

I'm not sure if I agree with the logic applying to go list -e -json, though. The error is just a string field, so it doesn't make sense for it to start with a newline - especially when there's only one error.

/cc @bcmills @jayconrod @matloob

[matloob]

Just for my curiousity, is this breaking something or is it purely an aesthetic concern?

[perillo]

It is just a non expected behavior.

If i'm implementing a wrapper around go list -e -find -json, probably I would not think of trimming the error string.

[jayconrod]

Related to #36087. Another problem for tools wrapping go list -e -json is that the error position is not reported correctly.

CL 210938 (work in progress) would fix this for the common case where there's only one scan error.

[gopherbot]

Change https://golang.org/cl/210938 mentions this issue: cmd/go: report scan error position in 'go list -e'