rados: implement binding for rados_write_op_cmpext

This commit implements binding for rados_write_op_cmpext RADOS Write operation.
Includes a unit test.

Signed-off-by: Robert Vasek <[email protected]>

rados/write_op_preview.go

@@ -0,0 +1,31 @@
+// +build ceph_preview
+
+package rados
+
+// #cgo LDFLAGS: -lrados
+// #include <rados/librados.h>
+//
+import "C"
+
+import (
+	"unsafe"
+)
+
+// CmpExt ensures that given object range (extent) satisfies comparison (PREVIEW).
+//
+// Implements:
+//  void rados_write_op_cmpext(rados_write_op_t write_op,
+//                             const char * cmp_buf,
+//                             size_t cmp_len,
+//                             uint64_t off,
+//                             int * prval);
+func (w *WriteOp) CmpExt(b []byte, offset uint64, prval *int32) {
+	oe := newWriteStep(b, 0, offset)
+	w.steps = append(w.steps, oe)
+	C.rados_write_op_cmpext(
+		w.op,
+		oe.cBuffer,
+		oe.cDataLen,
+		oe.cOffset,
+		(*C.int)(unsafe.Pointer(prval)))
+}

rados/write_op_preview_test.go

@@ -0,0 +1,40 @@
+// +build ceph_preview
+
+package rados
+
+import (
+	"github.com/stretchr/testify/assert"
+)
+
+func (suite *RadosTestSuite) TestWriteOpCmpExt() {
+	suite.SetupConnection()
+	ta := assert.New(suite.T())
+
+	oid := "TestWriteOpCmpExt"
+	data := []byte("compare this")
+	var prval int32
+
+	// Create an object and populate it with data.
+	op1 := CreateWriteOp()
+	defer op1.Release()
+	op1.Create(CreateIdempotent)
+	op1.WriteFull([]byte(data))
+	err := op1.Operate(suite.ioctx, oid, OperationNoFlag)
+	ta.NoError(err)
+
+	// Compare contents of the object. Should succeed.
+	op2 := CreateWriteOp()
+	defer op2.Release()
+	op2.CmpExt(data, 0, &prval)
+	err = op2.Operate(suite.ioctx, oid, OperationNoFlag)
+	ta.NoError(err)
+	ta.Equal(prval, int32(0))
+
+	// Compare contents of the object. Should fail.
+	op3 := CreateWriteOp()
+	defer op3.Release()
+	op3.CmpExt([]byte("xxx"), 0, &prval)
+	err = op3.Operate(suite.ioctx, oid, OperationNoFlag)
+	ta.Error(err)
+	ta.NotEqual(prval, int32(0))
+}