cmd/cgo: does not replace references to paths in GOROOT/ with $GOROOT

[orivej]

Normally Go compiles its own sources such that debug info pathnames look like $GOROOT/src/package/path.go rather than /absolute/path/to/go/src/package/path.go. However, this does not happen when compiling sources with cgo. In particular src/plugin/plugin_dlopen.go produces this temporary file plugin_dlopen.cgo1.go:

…
//line /tmp/nds-install-go_1_9/share/go/src/plugin/plugin_dlopen.go:43
func lastIndexByte(s string, c byte) int {
…

and after unpacking the resulting plugin.a (ar x plugin.a) and disassemly (go tool objdump _go_.o) it can be seen that pure Go sources are refered through $GOROOT, unlike cgo sources:

…
TEXT %22%22.Open(SB) gofile..$GOROOT/src/plugin/plugin.go
…
TEXT %22%22.lastIndexByte(SB) gofile../tmp/nds-install-go_1_9/share/go/src/plugin/plugin_dlopen.go
…

I would like the second line to be:

TEXT %22%22.lastIndexByte(SB) gofile..$GOROOT/src/plugin/plugin_dlopen.go

Here is a possible fix that replaces absolute path names in *.cgo1.go:

diff --git a/src/go/printer/printer.go b/src/go/printer/printer.go
index dbb4bbd90c..16a0b84fa5 100644
--- a/src/go/printer/printer.go
+++ b/src/go/printer/printer.go
@@ -11,6 +11,7 @@ import (
 	"go/token"
 	"io"
 	"os"
+	"runtime"
 	"strconv"
 	"strings"
 	"text/tabwriter"
@@ -207,7 +208,8 @@ func (p *printer) lineFor(pos token.Pos) int {
 func (p *printer) writeLineDirective(pos token.Position) {
 	if pos.IsValid() && (p.out.Line != pos.Line || p.out.Filename != pos.Filename) {
 		p.output = append(p.output, tabwriter.Escape) // protect '\n' in //line from tabwriter interpretation
-		p.output = append(p.output, fmt.Sprintf("//line %s:%d\n", pos.Filename, pos.Line)...)
+		filename := strings.Replace(pos.Filename, runtime.GOROOT(), "$GOROOT", -1)
+		p.output = append(p.output, fmt.Sprintf("//line %s:%d\n", filename, pos.Line)...)
 		p.output = append(p.output, tabwriter.Escape)
 		// p.out must match the //line directive
 		p.out.Filename = pos.Filename

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

go1.9

Does this issue reproduce with the latest release?

yes

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

NixOS Linux amd64

What did you do?

I tried to build journalbeat that uses the plugin package with Go 1.9. It failed Nix check that executables should not refer to Go root via an absolute pathname.

[odeke-em]

Thank you for the report @orivej and for the proposed fix.

So we can't yet review patches on Github, but we do on Gerrit. If you are interested in submitting one, please take a look at https://golang.org/doc/contribute.html just in case you haven't yet.

Otherwise I'll also tag the cgo crew @ianlancetaylor @hirochachacha who might be interested in this issue.

[ianlancetaylor]

Changing go/printer is not going to be the right approach. go/printer is a general purpose package and it shouldn't be in the business of modifying file names. The place to change this would be cmd/cgo or cmd/go.

Also I think @crawshaw has been looking at something in this area.

[orivej]

Somehow Go 1.8 was not affected: even though its cgo files were the same, object files referred to $GOROOT/src/plugin/plugin_dlopen.go. This bug was introduced in 4808fc4.

CC @griesemer

[ianlancetaylor]

Since it worked in 1.8, tentatively setting milestone to 1.9.1. We'll see how invasive the fix is.

[crawshaw]

I experimented with a fix for this as part of #21373. It's easy enough not to use absolute paths, but I assume @griesemer introduced them for a reason, so I'll leave this bug to him.