rpc/timeoutconn: platform-independent sizes for computing syscall.Iovec.Len

refs #184

rpc/dataconn/timeoutconn/timeoutconn.go

@@ -162,10 +162,14 @@ func buildIovecs(buffers net.Buffers) (totalLen int64, vecs []syscall.Iovec) {
 		if len(buffers[i]) == 0 {
 			continue
 		}
-		vecs = append(vecs, syscall.Iovec{
+
+		v := syscall.Iovec{
 			Base: &buffers[i][0],
-			Len:  uint64(len(buffers[i])),
-		})
+		}
+		// syscall.Iovec.Len has platform-dependent size, thus use SetLen
+		v.SetLen(len(buffers[i]))
+
+		vecs = append(vecs, v)
 	}
 	return totalLen, vecs
 }
@@ -268,14 +272,22 @@ func (c Conn) doOneReadv(rawConn syscall.RawConn, iovecs *[]syscall.Iovec) (n in
 		if int(thisReadN) < 0 {
 			panic("unexpected return value")
 		}
-		n += int64(thisReadN)
+		n += int64(thisReadN) // TODO check overflow
+
 		// shift iovecs forward
-		for left := int64(thisReadN); left > 0; {
-			curVecNewLength := int64((*iovecs)[0].Len) - left // TODO assert conversion
-			if curVecNewLength <= 0 {
-				left -= int64((*iovecs)[0].Len)
+		for left := int(thisReadN); left > 0; {
+			// conversion to uint does not change value, see TestIovecLenFieldIsMachineUint, and left > 0
+			thisIovecConsumedCompletely := uint((*iovecs)[0].Len) <= uint(left)
+			if thisIovecConsumedCompletely {
+				// Update left, cannot go below 0 due to
+				// a) definition of thisIovecConsumedCompletely
+				// b) left > 0 due to loop invariant
+				// Convertion .Len to int64 is thus also safe now, because it is < left < INT_MAX
+				left -= int((*iovecs)[0].Len)
 				*iovecs = (*iovecs)[1:]
 			} else {
+				// trim this iovec to remaining length
+
 				// NOTE: unsafe.Pointer safety rules
 				// 		https://tip.golang.org/pkg/unsafe/#Pointer
 				// 		(3) Conversion of a Pointer to a uintptr and back, with arithmetic.
@@ -283,7 +295,9 @@ func (c Conn) doOneReadv(rawConn syscall.RawConn, iovecs *[]syscall.Iovec) (n in
 				//		Note that both conversions must appear in the same expression,
 				//		with only the intervening arithmetic between them:
 				(*iovecs)[0].Base = (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer((*iovecs)[0].Base)) + uintptr(left)))
-				(*iovecs)[0].Len = uint64(curVecNewLength)
+				curVecNewLength := uint((*iovecs)[0].Len) - uint(left) // casts to uint do not change value
+				(*iovecs)[0].SetLen(int(curVecNewLength))              // int and uint have the same size, no change of value
+
 				break // inner
 			}
 		}

rpc/dataconn/timeoutconn/timeoutconn_test.go

@@ -5,8 +5,10 @@ import (
 	"io"
 	"net"
 	"sync"
+	"syscall"
 	"testing"
 	"time"
+	"unsafe"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -176,3 +178,14 @@ func TestNoPartialWritesDueToDeadline(t *testing.T) {
 	require.True(t, ok)
 	assert.True(t, netErr.Timeout())
 }
+
+func TestIovecLenFieldIsMachineUint(t *testing.T) {
+	iov := syscall.Iovec{}
+	_ = iov // make linter happy (unsafe.Sizeof not recognized as usage)
+	size_t := unsafe.Sizeof(iov.Len)
+	if size_t != unsafe.Sizeof(uint(23)) {
+		t.Fatalf("expecting (struct iov)->Len to be sizeof(uint)")
+	}
+	// ssize_t is defined to be the signed version of size_t,
+	// so we know sizeof(ssize_t) == sizeof(int)
+}