tests: add OpenTruncateRead test

This is a regression test for the issue that was fixed by the
last commit.

tests/defaults/main_test.go

@@ -2,6 +2,7 @@
 package defaults
 
 import (
+	"bytes"
 	"os"
 	"os/exec"
 	"syscall"
@@ -57,3 +58,51 @@ func TestCtlSock(t *testing.T) {
 		t.Errorf("incorrect error handling: %+v", response)
 	}
 }
+
+// In gocryptfs before v1.2, the file header was only read once for each
+// open. But truncating a file to zero will generate a new random file ID.
+// The sequence below caused an I/O error to be returned.
+func TestOpenTruncateRead(t *testing.T) {
+	fn := test_helpers.DefaultPlainDir + "/TestTruncateWrite"
+	// First FD is used for write and trucate.
+	writeFd, err := os.Create(fn)
+	if err != nil {
+		t.Fatal(err)
+	}
+	abc := []byte("abc")
+	_, err = writeFd.WriteAt(abc, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Second FD is just for reading.
+	readFd, err := os.Open(fn)
+	if err != nil {
+		t.Fatal(err)
+	}
+	content := make([]byte, 3)
+	_, err = readFd.ReadAt(content, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(content, abc) {
+		t.Fatalf("wrong content: %s", string(content))
+	}
+	// Truncate to zero to generate a new file ID and write new content.
+	err = writeFd.Truncate(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	xyz := []byte("xyz")
+	_, err = writeFd.WriteAt(xyz, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Try to read from the other FD.
+	_, err = readFd.ReadAt(content, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(content, xyz) {
+		t.Fatalf("wrong content: %s", string(content))
+	}
+}