tests: add TestConcurrentReadWrite

Another attempt to find out what is going on behind
https://github.com/rfjakob/gocryptfs/issues/363

tests/matrix/matrix_test.go

@@ -15,7 +15,9 @@ import (
 	"bytes"
 	"flag"
 	"fmt"
+	"io"
 	"io/ioutil"
+	"log"
 	"os"
 	"os/exec"
 	"runtime"
@@ -186,6 +188,67 @@ func TestWrite10Tight(t *testing.T) {
 	}
 }
 
+// https://github.com/rfjakob/gocryptfs/issues/363
+//
+// Note: this test calls log.Fatal() instead of t.Fatal() because apperently,
+// calling t.Fatal() from a goroutine hangs the test.
+func TestConcurrentReadWrite(t *testing.T) {
+	var wg sync.WaitGroup
+	fn := test_helpers.DefaultPlainDir + "/TestConcurrentReadWrite"
+	if f, err := os.Create(fn); err != nil {
+		t.Fatal(err)
+	} else {
+		f.Close()
+	}
+	buf := make([]byte, 100)
+	content := []byte("1234567890")
+	threads := 30
+	loops := 30
+	for i := 0; i < threads; i++ {
+		// Reader thread
+		wg.Add(1)
+		go func() {
+			fRd, err := os.Open(fn)
+			if err != nil {
+				log.Fatal(err)
+			}
+			for j := 0; j < loops; j++ {
+				n, err := fRd.ReadAt(buf, 0)
+				if err != nil && err != io.EOF {
+					log.Fatal(err)
+				}
+				if n != 0 && n != 10 {
+					log.Fatalf("strange read length: %d", n)
+				}
+			}
+			fRd.Close()
+			wg.Done()
+		}()
+
+		// Writer thread
+		wg.Add(1)
+		go func() {
+			fWr, err := os.OpenFile(fn, os.O_RDWR, 0700)
+			if err != nil {
+				log.Fatal(err)
+			}
+			for j := 0; j < loops; j++ {
+				err = fWr.Truncate(0)
+				if err != nil {
+					log.Fatal(err)
+				}
+				_, err = fWr.WriteAt(content, 0)
+				if err != nil {
+					log.Fatal(err)
+				}
+			}
+			fWr.Close()
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+}
+
 // Hint for calculating reference md5sums:
 // dd if=/dev/zero count=1 bs=XYZ | md5sum
 func TestTruncate(t *testing.T) {