Function Templates with callback functions in Go (rogchap#68)

* refactor to extend from private template struct

* refactor the C++ side to also match base class template

* Basic callbacks with arguments

* fix stat now there is an internal context

* deal with Go -> C pointer madness

* apply formatting and add examples

* add tests to the function template and the registries

* simplify, bug fixes and add comments

Author: rogchap

CHANGELOG.md

@@ -10,12 +10,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Support for the BigInt value to the big.Int Go type
 - Create Object Templates with primitive values, including other Object Templates
 - Configure Object Template as the global object of any new Context
+- Function Templates with callbacks to Go
 
 ### Changed
 - NewContext() API has been improved to handle optional global object, as well as optional Isolate
 - Package error messages are now prefixed with `v8go` rather than the struct name
-
-### Changed
+- Deprecated `iso.Close()` in favor of `iso.Dispose()` to keep consistancy with the C++ API
 - Upgraded V8 to 8.8.278.14
 
 ## [v0.4.0] - 2021-01-14

context.go

@@ -6,12 +6,28 @@ import "C"
 import (
 	"fmt"
 	"runtime"
+	"sync"
 	"unsafe"
 )
 
+// Due to the limitations of passing pointers to C from Go we need to create
+// a registry so that we can lookup the Context from any given callback from V8.
+// This is similar to what is described here: https://github.com/golang/go/wiki/cgo#function-variables
+// To make sure we can still GC *Context we register the context only when we are
+// running a script inside the context and then deregister.
+type ctxRef struct {
+	ctx      *Context
+	refCount int
+}
+
+var ctxMutex sync.RWMutex
+var ctxRegistry = make(map[int]*ctxRef)
+var ctxSeq = 0
+
 // Context is a global root execution environment that allows separate,
 // unrelated, JavaScript applications to run in a single instance of V8.
 type Context struct {
+	ref int
 	ptr C.ContextPtr
 	iso *Isolate
 }
@@ -45,12 +61,18 @@ func NewContext(opt ...ContextOption) (*Context, error) {
 	}
 
 	if opts.gTmpl == nil {
-		opts.gTmpl = &ObjectTemplate{}
+		opts.gTmpl = &ObjectTemplate{&template{}}
 	}
 
+	ctxMutex.Lock()
+	ctxSeq++
+	ref := ctxSeq
+	ctxMutex.Unlock()
+
 	ctx := &Context{
+		ref: ref,
+		ptr: C.NewContext(opts.iso.ptr, opts.gTmpl.ptr, C.int(ref)),
 		iso: opts.iso,
-		ptr: C.NewContext(opts.iso.ptr, opts.gTmpl.ptr),
 	}
 	runtime.SetFinalizer(ctx, (*Context).finalizer)
 	// TODO: [RC] catch any C++ exceptions and return as error
@@ -73,7 +95,10 @@ func (c *Context) RunScript(source string, origin string) (*Value, error) {
 	defer C.free(unsafe.Pointer(cSource))
 	defer C.free(unsafe.Pointer(cOrigin))
 
+	c.register()
 	rtn := C.RunScript(c.ptr, cSource, cOrigin)
+	c.deregister()
+
 	return getValue(c, rtn), getError(rtn)
 }
 
@@ -83,11 +108,45 @@ func (c *Context) Close() {
 }
 
 func (c *Context) finalizer() {
-	C.ContextDispose(c.ptr)
+	C.ContextFree(c.ptr)
 	c.ptr = nil
 	runtime.SetFinalizer(c, nil)
 }
 
+func (c *Context) register() {
+	ctxMutex.Lock()
+	r := ctxRegistry[c.ref]
+	if r == nil {
+		r = &ctxRef{ctx: c}
+		ctxRegistry[c.ref] = r
+	}
+	r.refCount++
+	ctxMutex.Unlock()
+}
+
+func (c *Context) deregister() {
+	ctxMutex.Lock()
+	defer ctxMutex.Unlock()
+	r := ctxRegistry[c.ref]
+	if r == nil {
+		return
+	}
+	r.refCount--
+	if r.refCount <= 0 {
+		delete(ctxRegistry, c.ref)
+	}
+}
+
+func getContext(ref int) *Context {
+	ctxMutex.RLock()
+	defer ctxMutex.RUnlock()
+	r := ctxRegistry[ref]
+	if r == nil {
+		return nil
+	}
+	return r.ctx
+}
+
 func getValue(ctx *Context, rtn C.RtnValue) *Value {
 	if rtn.value == nil {
 		return nil

context_test.go

@@ -61,6 +61,33 @@ func TestJSExceptions(t *testing.T) {
 	}
 }
 
+func TestContextRegistry(t *testing.T) {
+	t.Parallel()
+
+	ctx, _ := v8go.NewContext()
+	ctxref := ctx.Ref()
+
+	c1 := v8go.GetContext(ctxref)
+	if c1 != nil {
+		t.Error("expected context to be <nil>")
+	}
+
+	ctx.Register()
+	c2 := v8go.GetContext(ctxref)
+	if c2 == nil {
+		t.Error("expected context, but got <nil>")
+	}
+	if c2 != ctx {
+		t.Errorf("contexts should match %p != %p", c2, ctx)
+	}
+	ctx.Deregister()
+
+	c3 := v8go.GetContext(ctxref)
+	if c3 != nil {
+		t.Error("expected context to be <nil>")
+	}
+}
+
 func BenchmarkContext(b *testing.B) {
 	b.ReportAllocs()
 	vm, _ := v8go.NewIsolate()

export_test.go

@@ -0,0 +1,29 @@
+package v8go
+
+// RegisterCallback is exported for testing only.
+func (i *Isolate) RegisterCallback(cb FunctionCallback) int {
+	return i.registerCallback(cb)
+}
+
+// GetCallback is exported for testing only.
+func (i *Isolate) GetCallback(ref int) FunctionCallback {
+	return i.getCallback(ref)
+}
+
+// Register is exported for testing only.
+func (c *Context) Register() {
+	c.register()
+}
+
+// Deregister is exported for testing only.
+func (c *Context) Deregister() {
+	c.deregister()
+}
+
+// GetContext is exported for testing only.
+var GetContext = getContext
+
+// Ref is exported for testing only.
+func (c *Context) Ref() int {
+	return c.ref
+}

function_template.go

@@ -0,0 +1,78 @@
+package v8go
+
+// #include <stdlib.h>
+// #include "v8go.h"
+import "C"
+import (
+	"errors"
+	"runtime"
+	"unsafe"
+)
+
+// FunctionCallback is a callback that is executed in Go when a function is executed in JS.
+type FunctionCallback func(info *FunctionCallbackInfo) *Value
+
+// FunctionCallbackInfo is the argument that is passed to a FunctionCallback.
+type FunctionCallbackInfo struct {
+	ctx  *Context
+	args []*Value
+}
+
+// Context is the current context that the callback is being executed in.
+func (i *FunctionCallbackInfo) Context() *Context {
+	return i.ctx
+}
+
+// Args returns a slice of the value arguments that are passed to the JS function.
+func (i *FunctionCallbackInfo) Args() []*Value {
+	return i.args
+}
+
+// FunctionTemplate is used to create functions at runtime.
+// There can only be one function created from a FunctionTemplate in a context.
+// The lifetime of the created function is equal to the lifetime of the context.
+type FunctionTemplate struct {
+	*template
+}
+
+// NewFunctionTemplate creates a FunctionTemplate for a given callback.
+func NewFunctionTemplate(iso *Isolate, callback FunctionCallback) (*FunctionTemplate, error) {
+	if iso == nil {
+		return nil, errors.New("v8go: failed to create new FunctionTemplate: Isolate cannot be <nil>")
+	}
+	if callback == nil {
+		return nil, errors.New("v8go: failed to create new FunctionTemplate: FunctionCallback cannot be <nil>")
+	}
+
+	cbref := iso.registerCallback(callback)
+
+	tmpl := &template{
+		ptr: C.NewFunctionTemplate(iso.ptr, C.int(cbref)),
+		iso: iso,
+	}
+	runtime.SetFinalizer(tmpl, (*template).finalizer)
+	return &FunctionTemplate{tmpl}, nil
+}
+
+//export goFunctionCallback
+func goFunctionCallback(ctxref int, cbref int, args *C.ValuePtr, argsCount int) C.ValuePtr {
+	ctx := getContext(ctxref)
+
+	info := &FunctionCallbackInfo{
+		ctx:  ctx,
+		args: make([]*Value, argsCount),
+	}
+
+	argv := (*[1 << 30]C.ValuePtr)(unsafe.Pointer(args))[:argsCount:argsCount]
+	for i, v := range argv {
+		val := &Value{ptr: v}
+		runtime.SetFinalizer(val, (*Value).finalizer)
+		info.args[i] = val
+	}
+
+	callbackFunc := ctx.iso.getCallback(cbref)
+	if val := callbackFunc(info); val != nil {
+		return val.ptr
+	}
+	return nil
+}

function_template_test.go

@@ -0,0 +1,65 @@
+package v8go_test
+
+import (
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"strings"
+	"testing"
+
+	"rogchap.com/v8go"
+)
+
+func TestFunctionTemplate(t *testing.T) {
+	t.Parallel()
+
+	if _, err := v8go.NewFunctionTemplate(nil, func(*v8go.FunctionCallbackInfo) *v8go.Value { return nil }); err == nil {
+		t.Error("expected error but got <nil>")
+	}
+
+	iso, _ := v8go.NewIsolate()
+	if _, err := v8go.NewFunctionTemplate(iso, nil); err == nil {
+		t.Error("expected error but got <nil>")
+	}
+
+	fn, err := v8go.NewFunctionTemplate(iso, func(*v8go.FunctionCallbackInfo) *v8go.Value { return nil })
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if fn == nil {
+		t.Error("expected FunctionTemplate, but got <nil>")
+	}
+}
+
+func ExampleFunctionTemplate() {
+	iso, _ := v8go.NewIsolate()
+	global, _ := v8go.NewObjectTemplate(iso)
+	printfn, _ := v8go.NewFunctionTemplate(iso, func(info *v8go.FunctionCallbackInfo) *v8go.Value {
+		fmt.Printf("%+v\n", info.Args())
+		return nil
+	})
+	global.Set("print", printfn, v8go.ReadOnly)
+	ctx, _ := v8go.NewContext(iso, global)
+	ctx.RunScript("print('foo', 'bar', 0, 1)", "")
+	// Output:
+	// [foo bar 0 1]
+}
+
+func ExampleFunctionTemplate_fetch() {
+	iso, _ := v8go.NewIsolate()
+	global, _ := v8go.NewObjectTemplate(iso)
+	fetchfn, _ := v8go.NewFunctionTemplate(iso, func(info *v8go.FunctionCallbackInfo) *v8go.Value {
+		args := info.Args()
+		url := args[0].String()
+		res, _ := http.Get(url)
+		body, _ := ioutil.ReadAll(res.Body)
+		val, _ := v8go.NewValue(iso, string(body))
+		return val
+	})
+	global.Set("fetch", fetchfn, v8go.ReadOnly)
+	ctx, _ := v8go.NewContext(iso, global)
+	val, _ := ctx.RunScript("fetch('https://rogchap.com/v8go')", "")
+	fmt.Printf("%s\n", strings.Split(val.String(), "\n")[0])
+	// Output:
+	// <!DOCTYPE html>
+}