cmd/link: support plugins with no exported symbols

A plugin with no exported symbols is still potentially very useful.
Its init functions are called on load, and it so it can have visible
side effects.

Fixes #17681

Change-Id: Icdca31f48e5ab13c99020a2ef724f3de47dcd74b
Reviewed-on: https://go-review.googlesource.com/32437
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testplugin/src/host/host.go

@@ -84,5 +84,13 @@ func main() {
 		log.Fatalf(`plugin1.F()=%d, want 17`, gotf)
 	}
 
+	// plugin2 has no exported symbols, only an init function.
+	if _, err := plugin.Open("plugin2.so"); err != nil {
+		log.Fatalf("plugin.Open failed: %v", err)
+	}
+	if got, want := common.X, 2; got != want {
+		log.Fatalf("after loading plugin2, common.X=%d, want %d", got, want)
+	}
+
 	fmt.Println("PASS")
 }

misc/cgo/testplugin/src/plugin2/plugin2.go

+// Copyright 2016 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
+
+// // No C code required.
+import "C"
+
+import "common"
+
+func init() {
+	common.X = 2
+}
+
+func main() {
+	panic("plugin1.main called")
+}

misc/cgo/testplugin/test.bash

@@ -23,6 +23,7 @@ rm -rf pkg sub
 mkdir sub
 
 GOPATH=$(pwd) go build -buildmode=plugin plugin1
+GOPATH=$(pwd) go build -buildmode=plugin plugin2
 GOPATH=$(pwd) go build -buildmode=plugin -o=sub/plugin1.so sub/plugin1
 GOPATH=$(pwd) go build host
 

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

@@ -252,8 +252,10 @@ func (d *deadcodepass) init() {
 			// We don't keep the go.plugin.exports symbol,
 			// but we do keep the symbols it refers to.
 			exports := d.ctxt.Syms.ROLookup("go.plugin.exports", 0)
-			for _, r := range exports.R {
-				d.mark(r.Sym, nil)
+			if exports != nil {
+				for _, r := range exports.R {
+					d.mark(r.Sym, nil)
+				}
 			}
 		}
 		for _, name := range markextra {

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

@@ -588,8 +588,7 @@ func (ctxt *Link) symtab() {
 	adduint(ctxt, moduledata, uint64(nitablinks))
 	adduint(ctxt, moduledata, uint64(nitablinks))
 	// The ptab slice
-	if Buildmode == BuildmodePlugin {
-		ptab := ctxt.Syms.ROLookup("go.plugin.tabs", 0)
+	if ptab := ctxt.Syms.ROLookup("go.plugin.tabs", 0); ptab != nil {
 		ptab.Attr |= AttrReachable
 		ptab.Attr |= AttrLocal
 		ptab.Type = obj.SRODATA