all: add backward support for Go 1.16 (#13)

.github/workflows/clipboard.yml

@@ -21,6 +21,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
+        go: [ '1.16.x', '1.17.x' ]
     steps:
     - name: Install and run dependencies (xvfb libx11-dev)
       if: ${{ runner.os == 'Linux' }}
@@ -47,24 +48,24 @@ jobs:
     - uses: actions/setup-go@v2
       with:
         stable: 'false'
-        go-version: '1.17.x'
+        go-version: ${{ matrix.go }}
 
-    - name: Build
+    - name: Build (${{ matrix.go }})
       run: |
         go build -o gclip cmd/gclip/main.go
         go build -o gclip-gui cmd/gclip-gui/main.go
 
-    - name: Run Tests with CGO_ENABLED=1
+    - name: Run Tests with CGO_ENABLED=1 (${{ matrix.go }})
       if: ${{ runner.os == 'Linux' || runner.os == 'macOS'}}
       run: |
         CGO_ENABLED=1 go test -v -covermode=atomic .
 
-    - name: Run Tests with CGO_ENABLED=0
+    - name: Run Tests with CGO_ENABLED=0 (${{ matrix.go }})
       if: ${{ runner.os == 'Linux' || runner.os == 'macOS'}}
       run: |
         CGO_ENABLED=0 go test -v -covermode=atomic .
 
-    - name: Run Tests on Windows
+    - name: Run Tests on Windows (${{ matrix.go }})
       if: ${{ runner.os == 'Windows'}}
       run: |
         go test -v -covermode=atomic .

clipboard_android.c

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build android
+// +build android
 
 #include <android/log.h>
 #include <jni.h>

clipboard_android.go

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build android
+// +build android
 
 package clipboard
 

clipboard_darwin.go

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build darwin && !ios
+// +build darwin,!ios
 
 package clipboard
 

clipboard_darwin.m

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build darwin && !ios
+// +build darwin,!ios
 
 // Interact with NSPasteboard using Objective-C
 // https://developer.apple.com/documentation/appkit/nspasteboard?language=objc

clipboard_ios.go

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build ios
+// +build ios
 
 package clipboard
 

clipboard_ios.m

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build ios
+// +build ios
 
 #import <UIKit/UIKit.h>
 #import <MobileCoreServices/MobileCoreServices.h>

clipboard_linux.c

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build linux && !android
+// +build linux,!android
 
 #include <stdlib.h>
 #include <stdio.h>

clipboard_linux.go

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build linux && !android
+// +build linux,!android
 
 package clipboard
 
@@ -34,9 +35,10 @@ import (
 	"fmt"
 	"os"
 	"runtime"
-	"runtime/cgo"
 	"time"
 	"unsafe"
+
+	"golang.design/x/clipboard/internal/cgo"
 )
 
 const errmsg = `Failed to initialize the X11 display, and the clipboard package

clipboard_nocgo.go

@@ -1,4 +1,5 @@
 //go:build !windows && !cgo
+// +build !windows,!cgo
 
 package clipboard
 

clipboard_windows.go

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build windows
+// +build windows
 
 package clipboard
 

cmd/gclip-gui/main.go

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build android || ios || linux || darwin || windows
+// +build android ios linux darwin windows
 
 // This is a very basic example for verification purpose that
 // demonstrates how the golang.design/x/clipboard can interact

example_test.go

@@ -5,6 +5,7 @@
 // Written by Changkun Ou <changkun.de>
 
 //go:build cgo
+// +build cgo
 
 package clipboard_test
 

go.mod

@@ -1,13 +1,8 @@
 module golang.design/x/clipboard
 
-go 1.17
+go 1.16
 
 require (
 	golang.org/x/image v0.0.0-20210628002857-a66eb6448b8d
 	golang.org/x/mobile v0.0.0-20210716004757-34ab1303b554
 )
-
-require (
-	golang.org/x/exp v0.0.0-20190731235908-ec7cb31e5a56 // indirect
-	golang.org/x/sys v0.0.0-20210510120138-977fb7262007 // indirect
-)

internal/cgo/handle.go

@@ -0,0 +1,159 @@
+// Copyright 2021 The golang.design Initiative Authors.
+// All rights reserved. Use of this source code is governed
+// by a MIT license that can be found in the LICENSE file.
+//
+// Written by Changkun Ou <changkun.de>
+
+//go:build !go1.17
+// +build !go1.17
+
+// Package cgo is an implementation of golang.org/issue/37033.
+//
+// See golang.org/cl/294670 for code review discussion.
+package cgo
+
+import (
+	"reflect"
+	"sync"
+)
+
+// Handle provides a safe representation to pass Go values between C and
+// Go back and forth. The zero value of a handle is not a valid handle,
+// and thus safe to use as a sentinel in C APIs.
+//
+// The underlying type of Handle may change, but the value is guaranteed
+// to fit in an integer type that is large enough to hold the bit pattern
+// of any pointer. For instance, on the Go side:
+//
+// 	package main
+//
+// 	/*
+// 	extern void MyGoPrint(unsigned long long handle);
+// 	void myprint(unsigned long long handle);
+// 	*/
+// 	import "C"
+// 	import "runtime/cgo"
+//
+// 	//export MyGoPrint
+// 	func MyGoPrint(handle C.ulonglong) {
+// 		h := cgo.Handle(handle)
+// 		val := h.Value().(int)
+// 		println(val)
+// 		h.Delete()
+// 	}
+//
+// 	func main() {
+// 		val := 42
+// 		C.myprint(C.ulonglong(cgo.NewHandle(val)))
+// 		// Output: 42
+// 	}
+//
+// and on the C side:
+//
+// 	// A Go function
+// 	extern void MyGoPrint(unsigned long long handle);
+//
+// 	// A C function
+// 	void myprint(unsigned long long handle) {
+// 	    MyGoPrint(handle);
+// 	}
+type Handle uintptr
+
+// NewHandle returns a handle for a given value. If a given value is a
+// pointer, slice, map, channel, or function that refers to the same
+// object, the returned handle will also be the same. Besides, nil value
+// must not be used.
+//
+// The handle is valid until the program calls Delete on it. The handle
+// uses resources, and this package assumes that C code may hold on to
+// the handle, so a program must explicitly call Delete when the handle
+// is no longer needed.
+//
+// The intended use is to pass the returned handle to C code, which
+// passes it back to Go, which calls Value. See an example in the
+// comments of the Handle definition.
+func NewHandle(v interface{}) Handle {
+	var k uintptr
+
+	rv := reflect.ValueOf(v)
+	switch rv.Kind() {
+	case reflect.Ptr, reflect.UnsafePointer, reflect.Slice,
+		reflect.Map, reflect.Chan, reflect.Func:
+		if rv.IsNil() {
+			panic("cgo: cannot use Handle for nil value")
+		}
+
+		k = rv.Pointer()
+	default:
+		// Wrap and turn a value parameter into a pointer. This enables
+		// us to always store the passing object as a pointer, and helps
+		// to identify which of whose are initially pointers or values
+		// when Value is called.
+		v = &wrap{v}
+		k = reflect.ValueOf(v).Pointer()
+	}
+
+	// v was escaped to the heap because of reflection. As Go do not have
+	// a moving GC (and possibly lasts true for a long future), it is
+	// safe to use its pointer address as the key of the global map at
+	// this moment. The implementation must be reconsidered if moving GC
+	// is introduced internally in the runtime.
+	actual, loaded := m.LoadOrStore(k, v)
+	if !loaded {
+		return Handle(k)
+	}
+
+	arv := reflect.ValueOf(actual)
+	switch arv.Kind() {
+	case reflect.Ptr, reflect.UnsafePointer, reflect.Slice,
+		reflect.Map, reflect.Chan, reflect.Func:
+		// The underlying object of the given Go value already have
+		// its existing handle.
+		if arv.Pointer() == k {
+			return Handle(k)
+		}
+
+		// If the loaded pointer is inconsistent with the new pointer,
+		// it means the address has been used for different objects
+		// because of GC and its address is reused for a new Go object,
+		// meaning that the Handle does not call Delete explicitly when
+		// the old Go value is not needed. Consider this as a misuse of
+		// a handle, do panic.
+		panic("cgo: misuse of a Handle")
+	default:
+		panic("cgo: Handle implementation has an internal bug")
+	}
+}
+
+// Delete invalidates a handle. This method must be called when C code no
+// longer has a copy of the handle, and the program no longer needs the
+// Go value that associated with the handle.
+//
+// The method panics if the handle is invalid already.
+func (h Handle) Delete() {
+	_, ok := m.LoadAndDelete(uintptr(h))
+	if !ok {
+		panic("cgo: misuse of an invalid Handle")
+	}
+}
+
+// Value returns the associated Go value for a valid handle.
+//
+// The method panics if the handle is invalid already.
+func (h Handle) Value() interface{} {
+	v, ok := m.Load(uintptr(h))
+	if !ok {
+		panic("cgo: misuse of an invalid Handle")
+	}
+	if wv, ok := v.(*wrap); ok {
+		return wv.v
+	}
+	return v
+}
+
+var m = &sync.Map{} // map[uintptr]interface{}
+
+// wrap wraps a Go value.
+type wrap struct {
+	v interface{}
+}

internal/cgo/handle_117.go

@@ -0,0 +1,16 @@
+// Copyright 2021 The golang.design Initiative Authors.
+// All rights reserved. Use of this source code is governed
+// by a MIT license that can be found in the LICENSE file.
+//
+// Written by Changkun Ou <changkun.de>
+
+//go:build go1.17
+// +build go1.17
+
+package cgo
+
+import "runtime/cgo"
+
+type Handle = cgo.Handle
+
+var NewHandle = cgo.NewHandle