cmd/cgo: record full source path to input .go files

Fixes #5122.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13395046

src/cmd/cgo/ast.go

@@ -13,6 +13,7 @@ import (
 	"go/scanner"
 	"go/token"
 	"os"
+	"path/filepath"
 	"strings"
 )
 
@@ -44,6 +45,13 @@ func sourceLine(n ast.Node) int {
 // a list of exported functions, and the actual AST, to be rewritten and
 // printed.
 func (f *File) ReadGo(name string) {
+	// Create absolute path for file, so that it will be used in error
+	// messages and recorded in debug line number information.
+	// This matches the rest of the toolchain. See golang.org/issue/5122.
+	if aname, err := filepath.Abs(name); err == nil {
+		name = aname
+	}
+
 	// Two different parses: once with comments, once without.
 	// The printer is not good enough at printing comments in the
 	// right place when we start editing the AST behind its back,

src/cmd/go/test.bash

@@ -463,6 +463,26 @@ import "C"
 rm -rf $d
 unset GOPATH
 
+TEST cgo shows full path names
+d=$(TMPDIR=/var/tmp mktemp -d -t testgoXXX)
+export GOPATH=$d
+mkdir -p $d/src/x/y/dirname
+echo '
+package foo
+import "C"
+func f() {
+' >$d/src/x/y/dirname/foo.go
+if ./testgo build x/y/dirname >$d/err 2>&1; then
+	echo build succeeded unexpectedly.
+	ok=false
+elif ! grep x/y/dirname $d/err >/dev/null; then
+	echo error did not use full path.
+	cat $d/err
+	ok=false
+fi
+rm -rf $d
+unset GOPATH
+
 # clean up
 if $started; then stop; fi
 rm -rf testdata/bin testdata/bin1