cmd/link: support -X values for main.* in plugins

Fixes #19418

Change-Id: I98205f40c1915cd68a5d20438469ba06f1efb160
Reviewed-on: https://go-review.googlesource.com/67432
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testplugin/src/issue19418/main.go

+// Copyright 2017 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 (
+	"fmt"
+	"os"
+	"plugin"
+)
+
+func main() {
+	p, err := plugin.Open("plugin.so")
+	if err != nil {
+		panic(err)
+	}
+
+	val, err := p.Lookup("Val")
+	if err != nil {
+		panic(err)
+	}
+	got := *val.(*string)
+	const want = "linkstr"
+	if got != want {
+		fmt.Fprintf(os.Stderr, "issue19418 value is %q, want %q\n", got, want)
+		os.Exit(2)
+	}
+}

misc/cgo/testplugin/src/issue19418/plugin.go

+// Copyright 2017 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
+
+var Val = "val-unset"

misc/cgo/testplugin/test.bash

@@ -66,3 +66,8 @@ GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -o issue19534 src/issue19534/main.
 GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -buildmode=plugin -o plugin.so src/issue18584/plugin.go
 GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -o issue18584 src/issue18584/main.go
 ./issue18584
+
+# Test for issue 19418
+GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -buildmode=plugin "-ldflags=-X main.Val=linkstr" -o plugin.so src/issue19418/plugin.go
+GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -o issue19418 src/issue19418/main.go
+./issue19418

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

@@ -1045,7 +1045,11 @@ func addstrdata1(ctxt *Link, arg string) {
 	if eq < 0 || dot < 0 {
 		Exitf("-X flag requires argument of the form importpath.name=value")
 	}
-	addstrdata(ctxt, objabi.PathToPrefix(arg[:dot])+arg[dot:eq], arg[eq+1:])
+	pkg := objabi.PathToPrefix(arg[:dot])
+	if Buildmode == BuildmodePlugin && pkg == "main" {
+		pkg = *flagPluginPath
+	}
+	addstrdata(ctxt, pkg+arg[dot:eq], arg[eq+1:])
 }
 
 func addstrdata(ctxt *Link, name string, value string) {