cephfs: have Read/ReadAt return io.EOF error when nothing is read

Have Read and ReadAt functions return io.EOF when nothing is read
from the file so that it matches the current behavior of file types
in Go standard library.

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

cephfs/file.go

@@ -9,6 +9,7 @@ package cephfs
 import "C"
 
 import (
+	"io"
 	"unsafe"
 )
 
@@ -85,14 +86,18 @@ func (f *File) read(buf []byte, offset int64) (int, error) {
 	bufptr := (*C.char)(unsafe.Pointer(&buf[0]))
 	ret := C.ceph_read(
 		f.mount.mount, f.fd, bufptr, C.int64_t(len(buf)), C.int64_t(offset))
-	if ret < 0 {
-		return int(ret), getError(ret)
+	switch {
+	case ret < 0:
+		return 0, getError(ret)
+	case ret == 0:
+		return 0, io.EOF
 	}
 	return int(ret), nil
 }
 
 // Read data from file. Up to len(buf) bytes will be read from the file.
 // The number of bytes read will be returned.
+// When nothing is left to read from the file, Read returns, 0, io.EOF.
 func (f *File) Read(buf []byte) (int, error) {
 	// to-consider: should we mimic Go's behavior of returning an
 	// io.ErrShortWrite error if write length < buf size?
@@ -102,6 +107,7 @@ func (f *File) Read(buf []byte) (int, error) {
 // ReadAt will read data from the file starting at the given offset.
 // Up to len(buf) bytes will be read from the file.
 // The number of bytes read will be returned.
+// When nothing is left to read from the file, ReadAt returns, 0, io.EOF.
 func (f *File) ReadAt(buf []byte, offset int64) (int, error) {
 	return f.read(buf, offset)
 }

cephfs/file_test.go

@@ -216,6 +216,8 @@ func TestFileInterfaces(t *testing.T) {
 	t.Run("ioReader", func(t *testing.T) {
 		f1, err := mount.Open(fname, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0666)
 		assert.NoError(t, err)
+		_, err = f1.Write([]byte("foo"))
+		assert.NoError(t, err)
 		assert.NoError(t, f1.Close())
 
 		f1, err = mount.Open(fname, os.O_RDONLY, 0666)
@@ -227,6 +229,9 @@ func TestFileInterfaces(t *testing.T) {
 		buf := make([]byte, 32)
 		_, err = r.Read(buf)
 		assert.NoError(t, err)
+		n, err := r.Read(buf)
+		assert.Equal(t, 0, n)
+		assert.Equal(t, io.EOF, err)
 	})
 }