cutil: add Iovec type wrapping C struct iovec arrays

This type is useful for passing disparate buffers to be read or
written in a single call. Functions using this type exist in cephfs
and rbd. Currently this is needed for cephfs calls.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

internal/cutil/iovec.go

@@ -0,0 +1,78 @@
+package cutil
+
+/*
+#include <stdlib.h>
+#include <sys/uio.h>
+*/
+import "C"
+
+import (
+	"unsafe"
+)
+
+var iovecSize uintptr
+
+// StructIovecPtr is an unsafe pointer wrapping C's `*struct iovec`.
+type StructIovecPtr unsafe.Pointer
+
+// Iovec helps manage struct iovec arrays needed by some C functions.
+type Iovec struct {
+	// cvec represents an array of struct iovec C memory
+	cvec unsafe.Pointer
+	// length of the array (in elements)
+	length int
+}
+
+// NewIovec creates an Iovec, and underlying C memory, of the specified size.
+func NewIovec(l int) *Iovec {
+	r := &Iovec{
+		cvec:   C.malloc(C.size_t(l) * C.size_t(iovecSize)),
+		length: l,
+	}
+	return r
+}
+
+// ByteSlicesToIovec takes a slice of byte slices and returns a new iovec that
+// maps the slice data to struct iovec entries.
+func ByteSlicesToIovec(data [][]byte) *Iovec {
+	iov := NewIovec(len(data))
+	for i := range data {
+		iov.Set(i, data[i])
+	}
+	return iov
+}
+
+// Pointer returns a StructIovecPtr that represents the C memory of the
+// underlying array.
+func (v *Iovec) Pointer() StructIovecPtr {
+	return StructIovecPtr(unsafe.Pointer(v.cvec))
+}
+
+// Len returns the number of entries in the Iovec.
+func (v *Iovec) Len() int {
+	return v.length
+}
+
+// Free the C memory in the Iovec.
+func (v *Iovec) Free() {
+	if v.cvec != nil {
+		C.free(v.cvec)
+		v.cvec = nil
+		v.length = 0
+	}
+}
+
+// Set will map the memory of the given byte slice to the iovec at the
+// specified position.
+func (v *Iovec) Set(i int, buf []byte) {
+	offset := uintptr(i) * iovecSize
+	iov := (*C.struct_iovec)(unsafe.Pointer(
+		uintptr(unsafe.Pointer(v.cvec)) + offset))
+	iov.iov_base = unsafe.Pointer(&buf[0])
+	iov.iov_len = C.size_t(len(buf))
+}
+
+func init() {
+	var iovec C.struct_iovec
+	iovecSize = unsafe.Sizeof(iovec)
+}

internal/cutil/iovec_test.go

@@ -0,0 +1,57 @@
+package cutil
+
+import (
+	"testing"
+	"unsafe"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIovec(t *testing.T) {
+	t.Run("newAndFree", func(t *testing.T) {
+		iov := NewIovec(3)
+		iov.Free()
+	})
+	t.Run("setBufs", func(t *testing.T) {
+		b1 := []byte("foo")
+		b2 := []byte("barbar")
+		b3 := []byte("bazbazbaz")
+		iov := NewIovec(3)
+		iov.Set(0, b1)
+		iov.Set(1, b2)
+		iov.Set(2, b3)
+		iov.Free()
+		// free also unsets internal values
+		assert.Equal(t, unsafe.Pointer(nil), iov.cvec)
+		assert.Equal(t, 0, iov.length)
+	})
+	t.Run("testGetters", func(t *testing.T) {
+		b1 := []byte("foo")
+		b2 := []byte("barbar")
+		b3 := []byte("bazbazbaz")
+		b4 := []byte("zonk")
+		iov := NewIovec(4)
+		defer iov.Free()
+		iov.Set(0, b1)
+		iov.Set(1, b2)
+		iov.Set(2, b3)
+		iov.Set(3, b4)
+
+		assert.NotNil(t, iov.Pointer())
+		assert.Equal(t, 4, iov.Len())
+	})
+}
+
+func TestByteSlicesToIovec(t *testing.T) {
+	d := [][]byte{
+		[]byte("ramekin"),
+		[]byte("shuffleboard"),
+		[]byte("tranche"),
+		[]byte("phycobilisomes"),
+	}
+	iov := ByteSlicesToIovec(d)
+	defer iov.Free()
+
+	assert.NotNil(t, iov.Pointer())
+	assert.Equal(t, 4, iov.Len())
+}