runtime: for plugins, don't add duplicate itabs

We already do this for shared libraries. Do it for plugins also.
Suggestions on how to test this would be welcome.

I'd like to get this in for 1.8.  It could lead to mysterious
hangs when using plugins.

Fixes #18676

Change-Id: I03209b096149090b9ba171c834c5e59087ed0f92
Reviewed-on: https://go-review.googlesource.com/35117Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

misc/cgo/testplugin/src/issue18676/dynamodbstreamsevt/definition.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 dynamodbstreamsevt
+
+import "encoding/json"
+
+var foo json.RawMessage
+
+type Event struct{}
+
+func (e *Event) Dummy() {}

misc/cgo/testplugin/src/issue18676/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.
+
+// The bug happened like this:
+// 1) The main binary adds an itab for *json.UnsupportedValueError / error
+//    (concrete type / interface type).  This itab goes in hash bucket 0x111.
+// 2) The plugin adds that same itab again.  That makes a cycle in the itab
+//    chain rooted at hash bucket 0x111.
+// 3) The main binary then asks for the itab for *dynamodbstreamsevt.Event /
+//    json.Unmarshaler.  This itab happens to also live in bucket 0x111.
+//    The lookup code goes into an infinite loop searching for this itab.
+// The code is carefully crafted so that the two itabs are both from the
+// same bucket, and so that the second itab doesn't exist in
+// the itab hashmap yet (so the entire linked list must be searched).
+package main
+
+import (
+	"encoding/json"
+	"issue18676/dynamodbstreamsevt"
+	"plugin"
+)
+
+func main() {
+	plugin.Open("plugin.so")
+
+	var x interface{} = (*dynamodbstreamsevt.Event)(nil)
+	if _, ok := x.(json.Unmarshaler); !ok {
+		println("something")
+	}
+}

misc/cgo/testplugin/src/issue18676/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
+
+import "C"
+
+import "issue18676/dynamodbstreamsevt"
+
+func F(evt *dynamodbstreamsevt.Event) {}

misc/cgo/testplugin/test.bash

@@ -16,7 +16,7 @@ goarch=$(go env GOARCH)
 
 function cleanup() {
 	rm -f plugin*.so unnamed*.so iface*.so
-	rm -rf host pkg sub iface
+	rm -rf host pkg sub iface issue18676
 }
 trap cleanup EXIT
 
@@ -38,3 +38,9 @@ GOPATH=$(pwd) go build -buildmode=plugin iface_a
 GOPATH=$(pwd) go build -buildmode=plugin iface_b
 GOPATH=$(pwd) go build iface
 LD_LIBRARY_PATH=$(pwd) ./iface
+
+# Test for issue 18676 - make sure we don't add the same itab twice.
+# The buggy code hangs forever, so use a timeout to check for that.
+GOPATH=$(pwd) go build -buildmode=plugin -o plugin.so src/issue18676/plugin.go
+GOPATH=$(pwd) go build -o issue18676 src/issue18676/main.go
+timeout 10s ./issue18676

src/runtime/plugin.go

@@ -56,8 +56,10 @@ func plugin_lastmoduleinit() (path string, syms map[string]interface{}, mismatch
 
 	lock(&ifaceLock)
 	for _, i := range md.itablinks {
+		if i.inhash == 0 {
 			additab(i, true, false)
 		}
+	}
 	unlock(&ifaceLock)
 
 	// Build a map of symbol names to symbols. Here in the runtime