From 78517ac908a2400abb3c94493f1f7465098500c3 Mon Sep 17 00:00:00 2001
From: jawr <jess@lawrence.pm>
Date: Mon, 14 Sep 2020 20:45:54 +0900
Subject: [PATCH] Check to see if OpenFile has the O_EXCL flag and return the
 appropriate error if it does.

---
 memfs/memory.go      |  8 ++++++++
 memfs/memory_test.go | 15 +++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/memfs/memory.go b/memfs/memory.go
index d4ac056..e471993 100644
--- a/memfs/memory.go
+++ b/memfs/memory.go
@@ -51,6 +51,10 @@ func (fs *Memory) OpenFile(filename string, flag int, perm os.FileMode) (billy.F
 			return nil, err
 		}
 	} else {
+		if isExclusive(flag) {
+			return nil, os.ErrExist
+		}
+
 		if target, isLink := fs.resolveLink(filename, f); isLink {
 			return fs.OpenFile(target, flag, perm)
 		}
@@ -368,6 +372,10 @@ func isCreate(flag int) bool {
 	return flag&os.O_CREATE != 0
 }
 
+func isExclusive(flag int) bool {
+	return flag&os.O_EXCL != 0
+}
+
 func isAppend(flag int) bool {
 	return flag&os.O_APPEND != 0
 }
diff --git a/memfs/memory_test.go b/memfs/memory_test.go
index 1ec74b8..320b7c4 100644
--- a/memfs/memory_test.go
+++ b/memfs/memory_test.go
@@ -1,7 +1,9 @@
 package memfs
 
 import (
+	"fmt"
 	"io"
+	"os"
 	"testing"
 
 	"github.com/go-git/go-billy/v5"
@@ -44,3 +46,16 @@ func (s *MemorySuite) TestNegativeOffsets(c *C) {
 	_, err = f.Write(buf)
 	c.Assert(err, ErrorMatches, "writeat negative: negative offset")
 }
+
+func (s *MemorySuite) TestExclusive(c *C) {
+	f, err := s.FS.OpenFile("exclusive", os.O_CREATE|os.O_EXCL|os.O_RDWR, 0666)
+	c.Assert(err, IsNil)
+
+	fmt.Fprint(f, "mememememe")
+
+	err = f.Close()
+	c.Assert(err, IsNil)
+
+	_, err = s.FS.OpenFile("exclusive", os.O_CREATE|os.O_EXCL|os.O_RDWR, 0666)
+	c.Assert(err, ErrorMatches, os.ErrExist.Error())
+}