cmd/link: prefix syms with "_" on external darwin links

Fixes #33808

Change-Id: If1f30bc80004093ffdf9121768464dfb3a6e1f63
Reviewed-on: https://go-review.googlesource.com/c/go/+/194381
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/ld/issue33808_test.go

@@ -0,0 +1,53 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ld
+
+import (
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+const prog = `
+package main
+
+import "log"
+
+func main() {
+	log.Fatalf("HERE")
+}
+`
+
+func TestIssue33808(t *testing.T) {
+	if runtime.GOOS != "darwin" {
+		return
+	}
+	testenv.MustHaveGoBuild(t)
+
+	dir, err := ioutil.TempDir("", "TestIssue33808")
+	if err != nil {
+		t.Fatalf("could not create directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	f := gobuild(t, dir, prog, "-ldflags=-linkmode=external")
+	f.Close()
+
+	syms, err := f.Symbols()
+	if err != nil {
+		t.Fatalf("Error reading symbols: %v", err)
+	}
+
+	name := "log.Fatalf"
+	for _, sym := range syms {
+		if strings.Contains(sym.Name, name) {
+			return
+		}
+	}
+	t.Fatalf("Didn't find %v", name)
+}

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

@@ -869,6 +869,7 @@ func machosymtab(ctxt *Link) {
 		symtab.AddUint32(ctxt.Arch, uint32(symstr.Size))
 
 		export := machoShouldExport(ctxt, s)
+		isGoSymbol := strings.Contains(s.Extname(), ".")
 
 		// In normal buildmodes, only add _ to C symbols, as
 		// Go symbols have dot in the name.
@@ -877,8 +878,8 @@ func machosymtab(ctxt *Link) {
 		// symbols like crosscall2 are in pclntab and end up
 		// pointing at the host binary, breaking unwinding.
 		// See Issue #18190.
-		cexport := !strings.Contains(s.Extname(), ".") && (ctxt.BuildMode != BuildModePlugin || onlycsymbol(s))
+		cexport := !isGoSymbol && (ctxt.BuildMode != BuildModePlugin || onlycsymbol(s))
-		if cexport || export {
+		if cexport || export || isGoSymbol {
 			symstr.AddUint8('_')
 		}