From d0d47c3781674db571b18df0544f66d93decf342 Mon Sep 17 00:00:00 2001
From: Kane York <kanepyork@gmail.com>
Date: Sat, 4 Jul 2020 19:58:38 -0700
Subject: memoryobject: make blob reader seekable

Replace the bytes.Buffer with a bytes.Reader wrapped in a custom NopCloser, so that the extra reading methods are sill accessible.
---
 plumbing/memory.go      | 17 ++++++++++++++---
 plumbing/memory_test.go | 28 ++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 3 deletions(-)

diff --git a/plumbing/memory.go b/plumbing/memory.go
index b8e1e1b..21337cc 100644
--- a/plumbing/memory.go
+++ b/plumbing/memory.go
@@ -3,7 +3,6 @@ package plumbing
 import (
 	"bytes"
 	"io"
-	"io/ioutil"
 )
 
 // MemoryObject on memory Object implementation
@@ -39,9 +38,11 @@ func (o *MemoryObject) Size() int64 { return o.sz }
 // afterwards
 func (o *MemoryObject) SetSize(s int64) { o.sz = s }
 
-// Reader returns a ObjectReader used to read the object's content.
+// Reader returns an io.ReadCloser used to read the object's content.
+//
+// For a MemoryObject, this reader is seekable.
 func (o *MemoryObject) Reader() (io.ReadCloser, error) {
-	return ioutil.NopCloser(bytes.NewBuffer(o.cont)), nil
+	return nopCloser{bytes.NewReader(o.cont)}, nil
 }
 
 // Writer returns a ObjectWriter used to write the object's content.
@@ -59,3 +60,13 @@ func (o *MemoryObject) Write(p []byte) (n int, err error) {
 // Close releases any resources consumed by the object when it is acting as a
 // ObjectWriter.
 func (o *MemoryObject) Close() error { return nil }
+
+// nopCloser exposes the extra methods of bytes.Reader while nopping Close().
+//
+// This allows clients to attempt seeking in a cached Blob's Reader.
+type nopCloser struct {
+	*bytes.Reader
+}
+
+// Close does nothing.
+func (nc nopCloser) Close() error { return nil }
diff --git a/plumbing/memory_test.go b/plumbing/memory_test.go
index 879ed37..2a141f4 100644
--- a/plumbing/memory_test.go
+++ b/plumbing/memory_test.go
@@ -1,6 +1,7 @@
 package plumbing
 
 import (
+	"io"
 	"io/ioutil"
 
 	. "gopkg.in/check.v1"
@@ -56,6 +57,33 @@ func (s *MemoryObjectSuite) TestReader(c *C) {
 	c.Assert(b, DeepEquals, []byte("foo"))
 }
 
+func (s *MemoryObjectSuite) TestSeekableReader(c *C) {
+	const pageSize = 4096
+	const payload = "foo"
+	content := make([]byte, pageSize+len(payload))
+	copy(content[pageSize:], []byte(payload))
+
+	o := &MemoryObject{cont: content}
+
+	reader, err := o.Reader()
+	c.Assert(err, IsNil)
+	defer func() { c.Assert(reader.Close(), IsNil) }()
+
+	rs, ok := reader.(io.ReadSeeker)
+	c.Assert(ok, Equals, true)
+
+	_, err = rs.Seek(pageSize, io.SeekStart)
+	c.Assert(err, IsNil)
+
+	b, err := ioutil.ReadAll(rs)
+	c.Assert(err, IsNil)
+	c.Assert(b, DeepEquals, []byte(payload))
+
+	// Check that our Reader isn't also accidentally writable
+	_, ok = reader.(io.WriteSeeker)
+	c.Assert(ok, Equals, false)
+}
+
 func (s *MemoryObjectSuite) TestWriter(c *C) {
 	o := &MemoryObject{}
 
-- 
cgit