memfs: fix trunc-append case

memfs/memory.go

@@ -310,14 +310,14 @@ func (f *file) Duplicate(filename string, mode os.FileMode, flag int) billy.File
 		flag:    flag,
 	}
 
-	if isAppend(flag) {
-		new.position = int64(new.content.Len())
-	}
-
 	if isTruncate(flag) {
 		new.content.Truncate()
 	}
 
+	if isAppend(flag) {
+		new.position = int64(new.content.Len())
+	}
+
 	return new
 }
 

memfs/memory_test.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/go-git/go-billy/v5"
 	"github.com/go-git/go-billy/v5/test"
+	"github.com/go-git/go-billy/v5/util"
 
 	. "gopkg.in/check.v1"
 )
@@ -83,3 +84,22 @@ func (s *MemorySuite) TestOrder(c *C) {
 		}
 	}
 }
+
+func (s *MemorySuite) TestTruncateAppend(c *C) {
+	err := util.WriteFile(s.FS, "truncate_append", []byte("file-content"), 0666)
+	c.Assert(err, IsNil)
+
+	f, err := s.FS.OpenFile("truncate_append", os.O_WRONLY|os.O_TRUNC|os.O_APPEND, 0666)
+	c.Assert(err, IsNil)
+
+	n, err := f.Write([]byte("replace"))
+	c.Assert(err, IsNil)
+	c.Assert(n, Equals, len("replace"))
+
+	err = f.Close()
+	c.Assert(err, IsNil)
+
+	data, err := util.ReadFile(s.FS, "truncate_append")
+	c.Assert(err, IsNil)
+	c.Assert(string(data), Equals, "replace")
+}