From 8a2434ab7c4f9846dd33506612702aacb28d7278 Mon Sep 17 00:00:00 2001
From: Taru Karttunen <taruti@taruti.net>
Date: Tue, 12 Sep 2017 19:47:57 +0300
Subject: [PATCH 1/3] Add support for Truncate on files

---
 fs.go           | 2 ++
 memfs/memory.go | 7 +++++++
 test/mock.go    | 4 ++++
 3 files changed, 13 insertions(+)

diff --git a/fs.go b/fs.go
index cd1a4e7..00bb7a1 100644
--- a/fs.go
+++ b/fs.go
@@ -140,4 +140,6 @@ type File interface {
 	Lock() error
 	// Unlock unlocks the file.
 	Unlock() error
+	// Truncate the file.
+	Truncate(size int64) error
 }
diff --git a/memfs/memory.go b/memfs/memory.go
index 0bb278c..1d1a0e2 100644
--- a/memfs/memory.go
+++ b/memfs/memory.go
@@ -270,6 +270,13 @@ func (f *file) Close() error {
 	return nil
 }
 
+func (f *file) Truncate(size int64) error {
+	if size < int64(len(f.content.bytes)) {
+		f.content.bytes = f.content.bytes[:size]
+	}
+	return nil
+}
+
 func (f *file) Duplicate(filename string, mode os.FileMode, flag int) billy.File {
 	new := &file{
 		name:    filename,
diff --git a/test/mock.go b/test/mock.go
index 0f00a92..2cf7397 100644
--- a/test/mock.go
+++ b/test/mock.go
@@ -130,3 +130,7 @@ func (*FileMock) Lock() error {
 func (*FileMock) Unlock() error {
 	return nil
 }
+
+func (*FileMock) Truncate(size int64) error {
+	return nil
+}

From 2db7bbcd0c9b0aa1195a417d29700f366d65a47b Mon Sep 17 00:00:00 2001
From: Taru Karttunen <taruti@taruti.net>
Date: Wed, 13 Sep 2017 22:37:21 +0300
Subject: [PATCH 2/3] Support extending files in memoryfs truncate

---
 memfs/memory.go | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/memfs/memory.go b/memfs/memory.go
index 1d1a0e2..2f8dcae 100644
--- a/memfs/memory.go
+++ b/memfs/memory.go
@@ -273,7 +273,10 @@ func (f *file) Close() error {
 func (f *file) Truncate(size int64) error {
 	if size < int64(len(f.content.bytes)) {
 		f.content.bytes = f.content.bytes[:size]
+	} else if more := int(size)-len(f.content.bytes); more > 0 {
+		f.content.bytes = append(f.content.bytes, make([]byte, more)...)
 	}
+
 	return nil
 }
 

From 0aa82040a3fbb8e4e76847b33e122c5f4cad1837 Mon Sep 17 00:00:00 2001
From: Taru Karttunen <taruti@taruti.net>
Date: Wed, 13 Sep 2017 22:37:46 +0300
Subject: [PATCH 3/3] Add test for truncating files

---
 test/basic.go | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/test/basic.go b/test/basic.go
index f714b1c..1d5428b 100644
--- a/test/basic.go
+++ b/test/basic.go
@@ -562,3 +562,22 @@ func (s *BasicSuite) TestWriteFile(c *C) {
 
 	c.Assert(f.Close(), IsNil)
 }
+
+func (s *BasicSuite) TestTruncate(c *C) {
+	f, err := s.FS.Create("foo")
+	c.Assert(err, IsNil)
+
+	for _, sz := range []int64{4, 7, 2, 30, 0, 1} {
+		err = f.Truncate(sz)
+		c.Assert(err, IsNil)
+
+		bs, err := ioutil.ReadAll(f)
+		c.Assert(err, IsNil)
+		c.Assert(len(bs), Equals, int(sz))
+
+		_, err = f.Seek(0, io.SeekStart)
+		c.Assert(err, IsNil)
+	}
+
+	c.Assert(f.Close(), IsNil)
+}