cutil: add SyncBuffer abstraction layer to PtrGuard

In order to be able to swich on and off PtrGuard usage, SyncBuffer
implements a typical usage pattern of PtrGuard, that is a data buffer
referenced from inside a C allocated struct, in two ways: 1) using a
PtrGuard to dierctly store a pointer to the Go buffer in C memory, or
2) allocating a C buffer of same size and syncing data back with
C.memcpy().  The implementation can be chosen with the with_ptrguard
build tag.

Signed-off-by: Sven Anderson <sven@redhat.com>

internal/cutil/sync_buffer.go

@@ -0,0 +1,28 @@
+// +build ptrguard
+
+package cutil
+
+import (
+	"unsafe"
+)
+
+// SyncBuffer is a C buffer connected to a data slice
+type SyncBuffer struct {
+	pg *PtrGuard
+}
+
+// NewSyncBuffer creates a C buffer from a data slice and stores it at CPtr
+func NewSyncBuffer(cPtr CPtr, data []byte) *SyncBuffer {
+	var v SyncBuffer
+	v.pg = NewPtrGuard(cPtr, unsafe.Pointer(&data[0]))
+	return &v
+}
+
+// Release releases the C buffer and nulls its stored pointer
+func (v *SyncBuffer) Release() {
+	v.pg.Release()
+}
+
+// Sync asserts that changes in the C buffer are available in the data
+// slice
+func (v *SyncBuffer) Sync() {}

internal/cutil/sync_buffer_memcpy.go

@@ -0,0 +1,37 @@
+// +build !ptrguard
+
+package cutil
+
+// SyncBuffer is a C buffer connected to a data slice
+type SyncBuffer struct {
+	data []byte
+	cPtr *CPtr
+}
+
+// NewSyncBuffer creates a C buffer from a data slice and stores it at CPtr
+func NewSyncBuffer(cPtr CPtr, data []byte) *SyncBuffer {
+	var v SyncBuffer
+	v.data = data
+	v.cPtr = (*CPtr)(cPtr)
+	*v.cPtr = CBytes(data)
+	return &v
+}
+
+// Release releases the C buffer and nulls its stored pointer
+func (v *SyncBuffer) Release() {
+	if v.cPtr != nil {
+		Free(*v.cPtr)
+		*v.cPtr = nil
+		v.cPtr = nil
+	}
+	v.data = nil
+}
+
+// Sync asserts that changes in the C buffer are available in the data
+// slice
+func (v *SyncBuffer) Sync() {
+	if v.cPtr == nil {
+		return
+	}
+	Memcpy(CPtr(&v.data[0]), CPtr(*v.cPtr), SizeT(len(v.data)))
+}