Ensure ReadStream with direct=true isn't racy

Partially addresses #17. Big thanks to @jonboulle for spotting that
head-slapper.

diskv.go

@@ -175,7 +175,7 @@ func (d *Diskv) Read(key string) ([]byte, error) {
 // ReadStream will use the cached value. Otherwise, it will return a handle to
 // the file on disk, and cache the data on read.
 //
-// If direct is true, ReadStream will always delete any cached value for the
+// If direct is true, ReadStream will lazily delete any cached value for the
 // key, and return a direct handle to the file on disk.
 //
 // If compression is enabled, ReadStream taps into the io.Reader stream prior
@@ -185,16 +185,20 @@ func (d *Diskv) ReadStream(key string, direct bool) (io.ReadCloser, error) {
 	defer d.RUnlock()
 
 	if val, ok := d.cache[key]; ok {
-		if direct {
-			d.cacheSize -= uint64(len(val))
-			delete(d.cache, key)
-		} else {
+		if !direct {
 			buf := bytes.NewBuffer(val)
 			if d.Compression != nil {
 				return d.Compression.Reader(buf)
 			}
 			return ioutil.NopCloser(buf), nil
 		}
+
+		go func() {
+			d.Lock()
+			defer d.Unlock()
+			d.cacheSize -= uint64(len(val))
+			delete(d.cache, key)
+		}()
 	}
 
 	return d.read(key)

issues_test.go

@@ -3,6 +3,7 @@ package diskv
 import (
 	"bytes"
 	"io/ioutil"
+	"sync"
 	"testing"
 	"time"
 )
@@ -72,3 +73,49 @@ func TestIssue2B(t *testing.T) {
 	}
 	t.Logf("ReadStream('abc') returned error: %v", err)
 }
+
+// Ensure ReadStream with direct=true isn't racy.
+func TestIssue17(t *testing.T) {
+	var (
+		basePath = "test-data"
+	)
+
+	dWrite := New(Options{
+		BasePath:     basePath,
+		CacheSizeMax: 0,
+	})
+	defer dWrite.EraseAll()
+
+	dRead := New(Options{
+		BasePath:     basePath,
+		CacheSizeMax: 50,
+	})
+
+	cases := map[string]string{
+		"a": `1234567890`,
+		"b": `2345678901`,
+		"c": `3456789012`,
+		"d": `4567890123`,
+		"e": `5678901234`,
+	}
+
+	for k, v := range cases {
+		if err := dWrite.Write(k, []byte(v)); err != nil {
+			t.Fatalf("during write: %s", err)
+		}
+		dRead.Read(k) // ensure it's added to cache
+	}
+
+	var wg sync.WaitGroup
+	start := make(chan struct{})
+	for k, v := range cases {
+		wg.Add(1)
+		go func(k, v string) {
+			<-start
+			dRead.ReadStream(k, true)
+			wg.Done()
+		}(k, v)
+	}
+	close(start)
+	wg.Wait()
+}