cmd/go: use build ID as plugin symbol prefix

Updates #17821

Change-Id: Iebd2e88b2d4f3d757ffad72456f4bfc0607d8110
Reviewed-on: https://go-review.googlesource.com/33162
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/host/host.go

@@ -18,6 +18,35 @@ func init() {
 	common.X *= 5
 }
 
+// testUnnamed tests that two plugins built with .go files passed on
+// the command line do not have overlapping symbols. That is,
+// unnamed1.so/FuncInt and unnamed2.so/FuncInt should be distinct functions.
+func testUnnamed() {
+	p, err := plugin.Open("unnamed1.so")
+	if err != nil {
+		log.Fatalf(`plugin.Open("unnamed1.so"): %v`, err)
+	}
+	fn, err := p.Lookup("FuncInt")
+	if err != nil {
+		log.Fatalf(`unnamed1.so: Lookup("FuncInt") failed: %v`, err)
+	}
+	if got, want := fn.(func() int)(), 1; got != want {
+		log.Fatalf("unnamed1.so: FuncInt()=%d, want %d", got, want)
+	}
+
+	p, err = plugin.Open("unnamed2.so")
+	if err != nil {
+		log.Fatalf(`plugin.Open("unnamed2.so"): %v`, err)
+	}
+	fn, err = p.Lookup("FuncInt")
+	if err != nil {
+		log.Fatalf(`unnamed2.so: Lookup("FuncInt") failed: %v`, err)
+	}
+	if got, want := fn.(func() int)(), 2; got != want {
+		log.Fatalf("unnamed2.so: FuncInt()=%d, want %d", got, want)
+	}
+}
+
 func main() {
 	if got, want := common.X, 3*5; got != want {
 		log.Fatalf("before plugin load common.X=%d, want %d", got, want)
@@ -113,5 +142,7 @@ func main() {
 		log.Fatalf(`plugin.Open("plugin-mismatch.so"): error does not mention "different version": %v`, s)
 	}
 
+	testUnnamed()
+
 	fmt.Println("PASS")
 }

misc/cgo/testplugin/test.bash

@@ -15,7 +15,8 @@ goos=$(go env GOOS)
 goarch=$(go env GOARCH)
 
 function cleanup() {
-	rm -rf plugin1.so host pkg sub
+	rm -f plugin*.so unnamed*.so
+	rm -rf host pkg sub
 }
 trap cleanup EXIT
 
@@ -26,6 +27,8 @@ GOPATH=$(pwd) go build -buildmode=plugin plugin1
 GOPATH=$(pwd) go build -buildmode=plugin plugin2
 GOPATH=$(pwd)/altpath go build -buildmode=plugin plugin-mismatch
 GOPATH=$(pwd) go build -buildmode=plugin -o=sub/plugin1.so sub/plugin1
+GOPATH=$(pwd) go build -buildmode=plugin unnamed1.go
+GOPATH=$(pwd) go build -buildmode=plugin unnamed2.go
 GOPATH=$(pwd) go build host
 
 LD_LIBRARY_PATH=$(pwd) ./host

misc/cgo/testplugin/unnamed1.go

@@ -0,0 +1,12 @@
+// 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"
+
+func FuncInt() int { return 1 }
+
+func main() {}

misc/cgo/testplugin/unnamed2.go

@@ -0,0 +1,12 @@
+// 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"
+
+func FuncInt() int { return 2 }
+
+func main() {}

src/cmd/go/build.go

@@ -2580,7 +2580,11 @@ func (gcToolchain) ld(b *builder, root *action, out string, allactions []*action
 		ldflags = append(ldflags, "-w")
 	}
 	if buildBuildmode == "plugin" {
-		ldflags = append(ldflags, "-pluginpath", root.p.ImportPath)
+		pluginpath := root.p.ImportPath
+		if pluginpath == "command-line-arguments" {
+			pluginpath = "plugin/unnamed-" + root.p.buildID
+		}
+		ldflags = append(ldflags, "-pluginpath", pluginpath)
 	}
 
 	// If the user has not specified the -extld option, then specify the