cmd/link: mark unexported methods for plugins

When plugin is used, we already mark all exported methods
reachable. However, when the plugin and the host program share
a common package, an unexported method could also be reachable
from both the plugin and the host via interfaces. We need to mark
them as well.

Fixes #51621.

Change-Id: I1a70d3f96b66b803f2d0ab14d00ed0df276ea500
Reviewed-on: https://go-review.googlesource.com/c/go/+/393365
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

misc/cgo/testplugin/plugin_test.go

@@ -283,6 +283,12 @@ func TestMethod2(t *testing.T) {
 	run(t, "./method2.exe")
 }
 
+func TestMethod3(t *testing.T) {
+	goCmd(t, "build", "-buildmode=plugin", "-o", "method3.so", "./method3/plugin.go")
+	goCmd(t, "build", "-o", "method3.exe", "./method3/main.go")
+	run(t, "./method3.exe")
+}
+
 func TestIssue44956(t *testing.T) {
 	goCmd(t, "build", "-buildmode=plugin", "-o", "issue44956p1.so", "./issue44956/plugin1.go")
 	goCmd(t, "build", "-buildmode=plugin", "-o", "issue44956p2.so", "./issue44956/plugin2.go")

misc/cgo/testplugin/testdata/method3/main.go

@@ -0,0 +1,32 @@
+// Copyright 2022 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.
+
+// An unexported method can be reachable from the plugin via interface
+// when a package is shared. So it need to be live.
+
+package main
+
+import (
+	"plugin"
+
+	"testplugin/method3/p"
+)
+
+var i p.I
+
+func main() {
+	pl, err := plugin.Open("method3.so")
+	if err != nil {
+		panic(err)
+	}
+
+	f, err := pl.Lookup("F")
+	if err != nil {
+		panic(err)
+	}
+
+	f.(func())()
+
+	i = p.T(123)
+}

misc/cgo/testplugin/testdata/method3/p/p.go

@@ -0,0 +1,17 @@
+// Copyright 2022 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 p
+
+type T int
+
+func (T) m() { println("m") }
+
+type I interface { m() }
+
+func F() {
+	i.m()
+}
+
+var i I = T(123)

misc/cgo/testplugin/testdata/method3/plugin.go

@@ -0,0 +1,11 @@
+// Copyright 2022 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 main
+
+import "testplugin/method3/p"
+
+func main() {}
+
+func F() { p.F() }

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

@@ -355,7 +355,7 @@ func deadcode(ctxt *Link) {
 		// in the last pass.
 		rem := d.markableMethods[:0]
 		for _, m := range d.markableMethods {
-			if (d.reflectSeen && m.isExported()) || d.ifaceMethod[m.m] || d.genericIfaceMethod[m.m.name] {
+			if (d.reflectSeen && (m.isExported() || d.dynlink)) || d.ifaceMethod[m.m] || d.genericIfaceMethod[m.m.name] {
 				d.markMethod(m)
 			} else {
 				rem = append(rem, m)