cmd/compile: mark plugin-exported types as used in interface

Plugin exports symbols as interfaces. Mark their types as used in
interfaces, so their methods will be kept alive by the linker.

Fixes #42579.

Change-Id: If1b5aacc21510c20c25f88bb131bca61db6f1d56
Reviewed-on: https://go-review.googlesource.com/c/go/+/269819
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

misc/cgo/testplugin/plugin_test.go

@@ -196,3 +196,17 @@ func TestIssue25756(t *testing.T) {
 		})
 	}
 }
+
+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")
+
+	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)
+	}
+}

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

@@ -0,0 +1,26 @@
+// Copyright 2020 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.
+
+// Issue 42579: methods of symbols exported from plugin must be live.
+
+package main
+
+import (
+	"plugin"
+	"reflect"
+)
+
+func main() {
+	p, err := plugin.Open("plugin.so")
+	if err != nil {
+		panic(err)
+	}
+
+	x, err := p.Lookup("X")
+	if err != nil {
+		panic(err)
+	}
+
+	reflect.ValueOf(x).Elem().MethodByName("M").Call(nil)
+}

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

@@ -0,0 +1,13 @@
+// Copyright 2020 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
+
+func main() {}
+
+type T int
+
+func (T) M() { println("M") }
+
+var X T

src/cmd/compile/internal/gc/reflect.go

@@ -1591,8 +1591,12 @@ func dumptabs() {
 			//	typ  typeOff // pointer to symbol
 			// }
 			nsym := dname(p.s.Name, "", nil, true)
+			tsym := dtypesym(p.t)
 			ot = dsymptrOff(s, ot, nsym)
-			ot = dsymptrOff(s, ot, dtypesym(p.t))
+			ot = dsymptrOff(s, ot, tsym)
+			// Plugin exports symbols as interfaces. Mark their types
+			// as UsedInIface.
+			tsym.Set(obj.AttrUsedInIface, true)
 		}
 		ggloblsym(s, int32(ot), int16(obj.RODATA))