From c59aaa536e009a6e84ca5ccf9c1869dbf0ec73f0 Mon Sep 17 00:00:00 2001
From: John Floren <john@jfloren.net>
Date: Mon, 3 May 2021 16:14:19 -0700
Subject: [PATCH 1/2] Handle possible race where WriteStream can mess up the
 data being read from ReadStream.

---
 diskv.go | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/diskv.go b/diskv.go
index 9f07b85..4ff6200 100644
--- a/diskv.go
+++ b/diskv.go
@@ -216,7 +216,16 @@ func (d *Diskv) createKeyFileWithLock(pathKey *PathKey) (*os.File, error) {
 		return f, nil
 	}
 
-	mode := os.O_WRONLY | os.O_CREATE | os.O_TRUNC // overwrite if exists
+	// We call os.Remove before the call to OpenFile because otherwise we can
+	// mess with existing readers. If someone calls ReadStream, then proceeds
+	// to read from it slowly, then someone else calls WriteStream, the reader
+	// will start to get the *new* resource contents part-way through.
+	// By calling os.Remove, we unlink the existing file, but since the process
+	// still has a handle on the file, the existing readers can continue to
+	// read from it. Meanwhile we're free to update what's actually on-disk.
+	os.Remove(d.completeFilename(pathKey))
+
+	mode := os.O_WRONLY | os.O_CREATE | os.O_TRUNC // overwrite if exists somehow
 	f, err := os.OpenFile(d.completeFilename(pathKey), mode, d.FilePerm)
 	if err != nil {
 		return nil, fmt.Errorf("open file: %s", err)

From 5a1fbbe382821c4f6d9dd2cea7127761e67df66e Mon Sep 17 00:00:00 2001
From: John Floren <john@jfloren.net>
Date: Tue, 4 May 2021 08:41:20 -0700
Subject: [PATCH 2/2] Add test for issue #63

---
 issues_test.go | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/issues_test.go b/issues_test.go
index 5bf9e35..4c21de9 100644
--- a/issues_test.go
+++ b/issues_test.go
@@ -189,3 +189,55 @@ func TestIssue40(t *testing.T) {
 	// is no room in the cache for this entry and it panics.
 	d.Read(k2)
 }
+
+// Test issue #63, where a reader obtained from ReadStream will start
+// to return invalid data if WriteStream is called before you finish
+// reading.
+func TestIssue63(t *testing.T) {
+	var (
+		basePath = "test-data"
+	)
+	// Simplest transform function: put all the data files into the base dir.
+	flatTransform := func(s string) []string { return []string{} }
+
+	// Initialize a new diskv store, rooted at "my-data-dir",
+	// with no cache.
+	d := New(Options{
+		BasePath:     basePath,
+		Transform:    flatTransform,
+		CacheSizeMax: 0,
+	})
+
+	defer d.EraseAll()
+
+	// Write a big entry
+	k1 := "key1"
+	d1 := make([]byte, 1024*1024)
+	rand.Read(d1)
+	d.Write(k1, d1)
+
+	// Open a reader. We set the direct flag to be sure we're going straight to disk.
+	s1, err := d.ReadStream(k1, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Now generate a second big entry and put it in the *same* key
+	d2 := make([]byte, 1024*1024)
+	rand.Read(d2)
+	d.Write(k1, d2)
+
+	// Now read from that stream we opened
+	out, err := ioutil.ReadAll(s1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(out) != len(d1) {
+		t.Fatalf("Invalid read: got %v bytes expected %v\n", len(out), len(d1))
+	}
+	for i := range out {
+		if out[i] != d1[i] {
+			t.Fatalf("Output differs from expected at byte %v", i)
+		}
+	}
+}
\ No newline at end of file