[release-branch.go1.16] cmd/link: handle types as converted to interface when dynlink

When using plugins, a type (whose value) may be pass to a plugin
and get converted to interface there, or vice versa. We need to
treat the type as potentially converted to interface, and retain
its methods.

Updates #44586.
Fixes #44638.

Change-Id: I80dd35e68baedaa852a317543ccd78d94628d13b
Reviewed-on: https://go-review.googlesource.com/c/go/+/296709
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
(cherry picked from commit a655208c9ecd2fee4de6deff35a863b1c28a091c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/296910

5 files changed, 65 insertions, 7 deletions

diff --git a/misc/cgo/testplugin/plugin_test.go b/misc/cgo/testplugin/plugin_test.go
index 9055dbda04..2d991012c8 100644
--- a/misc/cgo/testplugin/plugin_test.go
+++ b/misc/cgo/testplugin/plugin_test.go
@@ -201,12 +201,11 @@ func TestMethod(t *testing.T) {
 	// Exported symbol's method must be live.
 	goCmd(t, "build", "-buildmode=plugin", "-o", "plugin.so", "./method/plugin.go")
 	goCmd(t, "build", "-o", "method.exe", "./method/main.go")
+	run(t, "./method.exe")
+}
 
-	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
-	defer cancel()
-	cmd := exec.CommandContext(ctx, "./method.exe")
-	out, err := cmd.CombinedOutput()
-	if err != nil {
-		t.Fatalf("%s: %v\n%s", strings.Join(cmd.Args, " "), err, out)
-	}
+func TestMethod2(t *testing.T) {
+	goCmd(t, "build", "-buildmode=plugin", "-o", "method2.so", "./method2/plugin.go")
+	goCmd(t, "build", "-o", "method2.exe", "./method2/main.go")
+	run(t, "./method2.exe")
 }
diff --git a/misc/cgo/testplugin/testdata/method2/main.go b/misc/cgo/testplugin/testdata/method2/main.go
new file mode 100644
index 0000000000..6a87e7b6a0
--- /dev/null
+++ b/misc/cgo/testplugin/testdata/method2/main.go
@@ -0,0 +1,32 @@
+// Copyright 2021 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.
+
+// A type can be passed to a plugin and converted to interface
+// there. So its methods need to be live.
+
+package main
+
+import (
+	"plugin"
+
+	"testplugin/method2/p"
+)
+
+var t p.T
+
+type I interface { M() }
+
+func main() {
+	pl, err := plugin.Open("method2.so")
+	if err != nil {
+		panic(err)
+	}
+
+	f, err := pl.Lookup("F")
+	if err != nil {
+		panic(err)
+	}
+
+	f.(func(p.T) interface{})(t).(I).M()
+}
diff --git a/misc/cgo/testplugin/testdata/method2/p/p.go b/misc/cgo/testplugin/testdata/method2/p/p.go
new file mode 100644
index 0000000000..acb526acec
--- /dev/null
+++ b/misc/cgo/testplugin/testdata/method2/p/p.go
@@ -0,0 +1,9 @@
+// Copyright 2021 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") }
diff --git a/misc/cgo/testplugin/testdata/method2/plugin.go b/misc/cgo/testplugin/testdata/method2/plugin.go
new file mode 100644
index 0000000000..6198e7648e
--- /dev/null
+++ b/misc/cgo/testplugin/testdata/method2/plugin.go
@@ -0,0 +1,11 @@
+// Copyright 2021 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/method2/p"
+
+func main() {}
+
+func F(t p.T) interface{} { return t }
diff --git a/src/cmd/link/internal/ld/deadcode.go b/src/cmd/link/internal/ld/deadcode.go
index 245076a83a..bfa8640ba9 100644
--- a/src/cmd/link/internal/ld/deadcode.go
+++ b/src/cmd/link/internal/ld/deadcode.go
@@ -24,6 +24,7 @@ type deadcodePass struct {
 	ifaceMethod     map[methodsig]bool // methods declared in reached interfaces
 	markableMethods []methodref        // methods of reached types
 	reflectSeen     bool               // whether we have seen a reflect method call
+	dynlink         bool
 
 	methodsigstmp []methodsig // scratch buffer for decoding method signatures
 }
@@ -34,6 +35,7 @@ func (d *deadcodePass) init() {
 	if objabi.Fieldtrack_enabled != 0 {
 		d.ldr.Reachparent = make([]loader.Sym, d.ldr.NSym())
 	}
+	d.dynlink = d.ctxt.DynlinkingGo()
 
 	if d.ctxt.BuildMode == BuildModeShared {
 		// Mark all symbols defined in this library as reachable when
@@ -111,6 +113,11 @@ func (d *deadcodePass) flood() {
 		var usedInIface bool
 
 		if isgotype {
+			if d.dynlink {
+				// When dynaamic linking, a type may be passed across DSO
+				// boundary and get converted to interface at the other side.
+				d.ldr.SetAttrUsedInIface(symIdx, true)
+			}
 			usedInIface = d.ldr.AttrUsedInIface(symIdx)
 		}