From 49bead83fa38cb8e67459fb6a479122e38388e34 Mon Sep 17 00:00:00 2001
From: Aaron Jacobs <jacobsa@google.com>
Date: Thu, 5 Mar 2015 19:21:20 +1100
Subject: [PATCH] PosixTest.WriteAtDoesntChangeOffset_NotAppendMode

---
 samples/memfs/posix_test.go | 40 ++++++++++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/samples/memfs/posix_test.go b/samples/memfs/posix_test.go
index 93564e9..ab78985 100644
--- a/samples/memfs/posix_test.go
+++ b/samples/memfs/posix_test.go
@@ -29,6 +29,16 @@ import (
 
 func TestPosix(t *testing.T) { RunTests(t) }
 
+////////////////////////////////////////////////////////////////////////
+// Helpers
+////////////////////////////////////////////////////////////////////////
+
+func getFileOffset(f *os.File) (offset int64, err error) {
+	const relativeToCurrent = 1
+	offset, err = f.Seek(0, relativeToCurrent)
+	return
+}
+
 ////////////////////////////////////////////////////////////////////////
 // Boilerplate
 ////////////////////////////////////////////////////////////////////////
@@ -148,11 +158,35 @@ func (t *PosixTest) WriteStartsPastEndOfFile() {
 	ExpectEq("\x00\x00taco", string(contents))
 }
 
-func (t *PosixTest) WriteAtEffectOnOffset_NotAppendMode() {
-	AssertTrue(false, "TODO")
+func (t *PosixTest) WriteAtDoesntChangeOffset_NotAppendMode() {
+	var err error
+	var n int
+
+	// Create a file.
+	f, err := os.Create(path.Join(t.dir, "foo"))
+	t.toClose = append(t.toClose, f)
+	AssertEq(nil, err)
+
+	// Make it 16 bytes long.
+	err = f.Truncate(16)
+	AssertEq(nil, err)
+
+	// Seek to offset 4.
+	_, err = f.Seek(4, 0)
+	AssertEq(nil, err)
+
+	// Write the range [10, 14).
+	n, err = f.WriteAt([]byte("taco"), 2)
+	AssertEq(nil, err)
+	AssertEq(4, n)
+
+	// We should still be at offset 4.
+	offset, err := getFileOffset(f)
+	AssertEq(nil, err)
+	ExpectEq(4, offset)
 }
 
-func (t *PosixTest) WriteAtEffectOnOffset_AppendMode() {
+func (t *PosixTest) WriteAtDoesntChangeOffset_AppendMode() {
 	AssertTrue(false, "TODO")
 }