add support for GstPromise

gst/cgo_exports.go

@@ -275,9 +275,18 @@ func goLogFunction(
 	}
 }
 
-// goDestroyBusWatch frees the go memory associated with the bus watch.
+// goUnrefGopointerUserData is a GDestroyNotify used to unref the gopointer from the userdata, used for callback functions
 //
-//export goDestroyBusWatch
-func goDestroyBusWatch(fPtr C.gpointer) {
+//export goUnrefGopointerUserData
+func goUnrefGopointerUserData(fPtr C.gpointer) {
 	gopointer.Unref(unsafe.Pointer(fPtr))
 }
+
+// goPromiseChangeFunc is the function the GstPromise calls when it changes state
+//
+//export goPromiseChangeFunc
+func goPromiseChangeFunc(_ *C.GstPromise, fPtr C.gpointer) {
+	f := gopointer.Restore(unsafe.Pointer(fPtr)).(func())
+
+	f()
+}

gst/gst_bus.go

@@ -5,16 +5,16 @@ package gst
 
 extern GstBusSyncReply   goBusSyncHandler  (GstBus * bus, GstMessage * message, gpointer user_data);
 extern gboolean          goBusFunc         (GstBus * bus, GstMessage * msg, gpointer user_data);
-extern void              goDestroyBusWatch (gpointer);
+extern void              goUnrefGopointerUserData (gpointer);
 
 gboolean cgoBusFunc (GstBus * bus, GstMessage * msg, gpointer user_data)
 {
 	return goBusFunc(bus, msg, user_data);
 }
 
-void cgoDestroyBusWatch (gpointer data)
+void cgoUnrefGopointerUserData (gpointer data)
 {
-	goDestroyBusWatch(data);
+	goUnrefGopointerUserData(data);
 }
 
 GstBusSyncReply cgoBusSyncHandler (GstBus * bus, GstMessage * message, gpointer user_data)
@@ -141,7 +141,7 @@ func (b *Bus) AddWatch(busFunc BusWatchFunc) bool {
 			C.G_PRIORITY_DEFAULT,
 			C.GstBusFunc(C.cgoBusFunc),
 			(C.gpointer)(unsafe.Pointer(fPtr)),
-			C.GDestroyNotify(C.cgoDestroyBusWatch),
+			C.GDestroyNotify(C.cgoUnrefGopointerUserData),
 		)),
 	)
 }
@@ -308,7 +308,7 @@ func (b *Bus) SetSyncHandler(f BusSyncHandler) {
 		b.Instance(),
 		C.GstBusSyncHandler(C.cgoBusSyncHandler),
 		(C.gpointer)(unsafe.Pointer(ptr)),
-		C.GDestroyNotify(C.cgoDestroyBusWatch),
+		C.GDestroyNotify(C.cgoUnrefGopointerUserData),
 	)
 }
 

gst/gst_promise.go

@@ -0,0 +1,202 @@
+package gst
+
+/*
+#include "gst.go.h"
+
+extern void goPromiseChangeFunc (GstPromise*, gpointer user_data);
+extern void cgoUnrefGopointerUserData (gpointer);
+
+void cgoPromiseChangeFunc (GstPromise *promise, gpointer data)
+{
+	goPromiseChangeFunc(promise, data);
+}
+*/
+import "C"
+import (
+	"context"
+	"errors"
+	"fmt"
+	"runtime"
+	"unsafe"
+
+	"github.com/go-gst/go-glib/glib"
+	gopointer "github.com/mattn/go-pointer"
+)
+
+type PromiseResult int
+
+func (pr PromiseResult) String() string {
+	switch pr {
+	case PromiseResultPending:
+		return "PENDING"
+	case PromiseResultInterrupted:
+		return "INTERRUPTED"
+	case PromiseResultReplied:
+		return "REPLIED"
+	case PromiseResultExpired:
+		return "EXPIRED"
+	default:
+		return "UNKNOWN"
+	}
+}
+
+const (
+	//Initial state. Waiting for transition to any other state.
+	PromiseResultPending = C.GST_PROMISE_RESULT_PENDING
+	// Interrupted by the consumer as it doesn't want the value anymore.
+	PromiseResultInterrupted = C.GST_PROMISE_RESULT_INTERRUPTED
+	// A producer marked a reply
+	PromiseResultReplied = C.GST_PROMISE_RESULT_REPLIED
+	// The promise expired (the carrying object lost all refs) and the promise will never be fulfilled.
+	PromiseResultExpired = C.GST_PROMISE_RESULT_EXPIRED
+)
+
+// Promise is a go wrapper around a GstPromise.
+// See: https://gstreamer.freedesktop.org/documentation/gstreamer/gstpromise.html
+//
+// it can be awaited on-blocking using Await, given the promise was constructed in go and not received from FFI.
+type Promise struct {
+	ptr *C.GstPromise
+
+	// done will be closed when the GstPromise has changed state
+	done <-chan struct{}
+}
+
+func NewPromise() *Promise {
+	done := make(chan struct{})
+
+	fPtr := gopointer.Save(func() {
+		close(done)
+	})
+
+	cprom := C.gst_promise_new_with_change_func(
+		C.GstPromiseChangeFunc(C.cgoPromiseChangeFunc),
+		C.gpointer(fPtr),
+		C.GDestroyNotify(C.cgoUnrefGopointerUserData),
+	)
+
+	prom := &Promise{
+		ptr:  cprom,
+		done: done,
+	}
+
+	runtime.SetFinalizer(prom, func(prom *Promise) {
+		prom.Unref()
+	})
+
+	return prom
+}
+
+func (p *Promise) Instance() *C.GstPromise {
+	return p.ptr
+}
+
+// Ref increases the ref count on the promise. Exposed for completeness sake. Should not be called
+// by application code
+func (p *Promise) Ref() {
+	C.gst_promise_ref(p.ptr)
+}
+
+// Unref decreases the ref count on the promise. Exposed for completeness sake. Should not be called
+// by application code
+func (p *Promise) Unref() {
+	C.gst_promise_unref(p.ptr)
+}
+
+var ErrPromiseNotReplied = errors.New("promise was not replied")
+var ErrNilPromiseReply = errors.New("promise returned a nil reply")
+
+// ErrCannotAwaitPromise signifies that we do not have a channel that we can await.
+//
+// this happens if the promise was marshaled from a GValue coming from C
+var ErrCannotAwaitPromise = errors.New("promises received from FFI cannot be awaited")
+
+// Await awaits the promise without blocking the thread. It returns the reply returned by the GstPromise
+//
+// its implementation is preferred over the blocking gst_promise_wait, which would lock a thread until the
+// promise has changed state.
+func (p *Promise) Await(ctx context.Context) (*Structure, error) {
+	if p.done == nil {
+		return nil, ErrCannotAwaitPromise
+	}
+
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case <-p.done:
+	}
+
+	// gst_promise_wait will not block here, because the promise has already changed state
+	result := PromiseResult(C.gst_promise_wait(p.ptr))
+
+	if result != PromiseResultReplied {
+		return nil, fmt.Errorf("%w: got %s", ErrPromiseNotReplied, result)
+	}
+
+	structure := p.GetReply()
+
+	if structure == nil {
+		return nil, ErrNilPromiseReply
+	}
+
+	return structure, nil
+}
+
+// GetReply wraps gst_promise_get_reply and returns the structure, which can be nil.
+func (p *Promise) GetReply() *Structure {
+	cstruct := C.gst_promise_get_reply(p.ptr)
+
+	if cstruct == nil {
+		return nil
+	}
+
+	structure := wrapStructure(cstruct)
+
+	// the structure is owned by the promise, so we keep the promise alive
+	// until the structure gets GC'ed
+	p.Ref()
+	runtime.SetFinalizer(structure, func(_ *Structure) {
+		p.Unref()
+	})
+
+	return structure
+}
+
+// Expire wraps gst_promise_expire
+func (p *Promise) Expire() {
+	C.gst_promise_expire(p.ptr)
+}
+
+// Interrupt wraps gst_promise_interrupt
+func (p *Promise) Interrupt() {
+	C.gst_promise_interrupt(p.ptr)
+}
+
+// Reply wraps gst_promise_reply
+func (p *Promise) Reply(answer *Structure) {
+	C.gst_promise_reply(p.ptr, answer.Instance())
+}
+
+var TypePromise = glib.Type(C.GST_TYPE_PROMISE)
+
+// ToGValue implements glib.ValueTransformer
+func (p *Promise) ToGValue() (*glib.Value, error) {
+	val, err := glib.ValueInit(TypePromise)
+	if err != nil {
+		return nil, err
+	}
+	val.SetInstance(unsafe.Pointer(p.Instance()))
+	return val, nil
+}
+
+func marshalPromise(p unsafe.Pointer) (interface{}, error) {
+	c := C.g_value_get_object(toGValue(p))
+	obj := (*C.GstPromise)(unsafe.Pointer(c))
+
+	prom := &Promise{
+		ptr:  obj,
+		done: nil, // cannot be awaited if received from FFI
+	}
+
+	return prom, nil
+}

gst/gst_promise_test.go

@@ -0,0 +1,123 @@
+package gst
+
+import (
+	"context"
+	"errors"
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+	"unsafe"
+)
+
+//go:noinline
+func awaitGC() {
+	type tmp struct{ v string }
+
+	setup := make(chan struct{})
+	done := make(chan struct{})
+
+	go func() {
+		v := &tmp{"foo"}
+
+		runtime.SetFinalizer(v, func(v *tmp) {
+			close(done)
+		})
+
+		close(setup)
+	}()
+
+	<-setup
+	runtime.GC()
+	<-done
+	runtime.GC()
+	time.Sleep(1 * time.Second)
+}
+
+func TestPromise(t *testing.T) {
+	initOnce.Do(func() {
+		Init(nil)
+	})
+
+	prom := NewPromise()
+	cprom := prom.Instance()
+
+	reply := NewStructure("foo/bar")
+	errchan := make(chan error)
+
+	go func() {
+		res, err := prom.Await(context.Background())
+
+		if err != nil {
+			errchan <- err
+		}
+
+		// even though we don't use the promise, the result structure should be still accessible.
+		// the returned structure is owned by the promise, so the promise must not get GC'ed until
+		// we don't use the structure anymore
+		awaitGC()
+
+		if res.Name() != reply.Name() {
+			errchan <- errors.New("name mismatch")
+		}
+
+		runtime.GC()
+		runtime.GC()
+		runtime.GC()
+
+		close(errchan)
+	}()
+
+	prom.Reply(reply)
+
+	err := <-errchan
+
+	if err != nil {
+		t.FailNow()
+	}
+
+	awaitGC()
+
+	if cprom.parent.refcount != 1 {
+		panic("refcount too high")
+	}
+
+	runtime.KeepAlive(prom)
+
+	awaitGC()
+}
+
+var initOnce sync.Once
+
+func TestPromiseMarshal(t *testing.T) {
+	initOnce.Do(func() {
+		Init(nil)
+	})
+
+	prom := NewPromise()
+
+	gv, err := prom.ToGValue()
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	receivedPromI, err := marshalPromise(unsafe.Pointer(gv.GValue))
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	receivedProm, ok := receivedPromI.(*Promise)
+
+	if !ok {
+		t.Fatal("could not cast")
+	}
+
+	// Awaiting received promise should error immediately
+	_, err = receivedProm.Await(context.Background())
+
+	if err == nil {
+		t.FailNow()
+	}
+}

gst/gst_wrappers.go

@@ -234,6 +234,10 @@ func registerMarshalers() {
 			T: TypeValueList,
 			F: marshalValueList,
 		},
+		{
+			T: TypePromise,
+			F: marshalPromise,
+		},
 		{
 			T: glib.Type(C.gst_sample_get_type()),
 			F: marshalSample,