From e6545a24b2ba04deec37b18ad24eb05fc7e2a3ff Mon Sep 17 00:00:00 2001
From: Steven Allen <steven@stebalien.com>
Date: Thu, 21 Dec 2017 17:33:27 -0800
Subject: [PATCH] fix truncating when already at the correct size

fixes #4518

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>
---
 mfs/mfs_test.go           | 25 +++++++++++++++++++++++++
 unixfs/mod/dagmodifier.go |  3 +++
 2 files changed, 28 insertions(+)

diff --git a/mfs/mfs_test.go b/mfs/mfs_test.go
index 228c5bdc03e..42a87fde46d 100644
--- a/mfs/mfs_test.go
+++ b/mfs/mfs_test.go
@@ -1111,3 +1111,28 @@ func TestFileDescriptors(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestTruncateAtSize(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	ds, rt := setupRoot(ctx, t)
+
+	dir := rt.GetDirectory()
+
+	nd := dag.NodeWithData(ft.FilePBData(nil, 0))
+	fi, err := NewFile("test", nd, dir, ds)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fd, err := fi.Open(OpenReadWrite, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer fd.Close()
+	_, err = fd.Write([]byte("test"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	fd.Truncate(4)
+}
diff --git a/unixfs/mod/dagmodifier.go b/unixfs/mod/dagmodifier.go
index 0a1ae0a9647..28de7228d61 100644
--- a/unixfs/mod/dagmodifier.go
+++ b/unixfs/mod/dagmodifier.go
@@ -481,6 +481,9 @@ func (dm *DagModifier) Truncate(size int64) error {
 	if err != nil {
 		return err
 	}
+	if size == int64(realSize) {
+		return nil
+	}
 
 	// Truncate can also be used to expand the file
 	if size > int64(realSize) {