[release-branch.go1.17] 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 #51736.
Updates #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>
(cherry picked from commit 91631bc7e0131367eb051b581cf34573399ac592)
Reviewed-on: https://go-review.googlesource.com/c/go/+/397484

1 files changed, 32 insertions, 0 deletions

diff --git a/misc/cgo/testplugin/testdata/method3/main.go b/misc/cgo/testplugin/testdata/method3/main.go
new file mode 100644
index 0000000000..a3a51711cd
--- /dev/null
+++ b/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)
+}