cmd/link: mark DWARF function symbols as reachable

Otherwise we don't emit any required ELF relocations when doing an
external link, because elfrelocsect skips unreachable symbols.

Fixes #18745.

Change-Id: Ia3583c41bb6c5ebb7579abd26ed8689370311cd6
Reviewed-on: https://go-review.googlesource.com/35590
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/link/internal/ld/dwarf.go

@@ -1080,7 +1080,7 @@ func writelines(ctxt *Link, syms []*Symbol) ([]*Symbol, []*Symbol) {
 		epcs = s
 
 		dsym := ctxt.Syms.Lookup(dwarf.InfoPrefix+s.Name, int(s.Version))
-		dsym.Attr |= AttrHidden
+		dsym.Attr |= AttrHidden | AttrReachable
 		dsym.Type = obj.SDWARFINFO
 		for _, r := range dsym.R {
 			if r.Type == obj.R_DWARFREF && r.Sym.Size == 0 {

src/runtime/runtime-gdb_test.go

@@ -7,6 +7,7 @@ package runtime_test
 import (
 	"bytes"
 	"fmt"
+	"go/build"
 	"internal/testenv"
 	"io/ioutil"
 	"os"
@@ -67,7 +68,6 @@ func checkGdbPython(t *testing.T) {
 }
 
 const helloSource = `
-package main
 import "fmt"
 var gslice []string
 func main() {
@@ -85,9 +85,20 @@ func main() {
 `
 
 func TestGdbPython(t *testing.T) {
+	testGdbPython(t, false)
+}
+
+func TestGdbPythonCgo(t *testing.T) {
+	testGdbPython(t, true)
+}
+
+func testGdbPython(t *testing.T, cgo bool) {
 	if runtime.GOARCH == "mips64" {
 		testenv.SkipFlaky(t, 18173)
 	}
+	if cgo && !build.Default.CgoEnabled {
+		t.Skip("skipping because cgo is not enabled")
+	}
 
 	t.Parallel()
 	checkGdbEnvironment(t)
@@ -100,8 +111,15 @@ func TestGdbPython(t *testing.T) {
 	}
 	defer os.RemoveAll(dir)
 
+	var buf bytes.Buffer
+	buf.WriteString("package main\n")
+	if cgo {
+		buf.WriteString(`import "C"` + "\n")
+	}
+	buf.WriteString(helloSource)
+
 	src := filepath.Join(dir, "main.go")
-	err = ioutil.WriteFile(src, []byte(helloSource), 0644)
+	err = ioutil.WriteFile(src, buf.Bytes(), 0644)
 	if err != nil {
 		t.Fatalf("failed to create file: %v", err)
 	}