From b1c8b4b073c09a297ddb2df3bec09bcda4162d4e Mon Sep 17 00:00:00 2001
From: Christian Muehlhaeuser <muesli@gmail.com>
Date: Fri, 5 Aug 2016 19:23:08 +0200
Subject: [PATCH] Make Join return an error if a reconstruction is required
 first

If one or more required data shards are still nil and we can't correctly join
them before a reconstruction, return ErrReconstructRequired.
---
 reedsolomon.go      | 14 ++++++++++++++
 reedsolomon_test.go |  6 ++++++
 2 files changed, 20 insertions(+)

diff --git a/reedsolomon.go b/reedsolomon.go
index b89a1dc..0c98981 100644
--- a/reedsolomon.go
+++ b/reedsolomon.go
@@ -487,12 +487,18 @@ func (r reedSolomon) Split(data []byte) ([][]byte, error) {
 	return dst, nil
 }
 
+// ErrReconstructRequired is returned if too few data shards are intact and a
+// reconstruction is required before you can successfully join the shards.
+var ErrReconstructRequired = errors.New("reconstruction required as one or more required data shards are nil")
+
 // Join the shards and write the data segment to dst.
 //
 // Only the data shards are considered.
 // You must supply the exact output size you want.
+//
 // If there are to few shards given, ErrTooFewShards will be returned.
 // If the total data size is less than outSize, ErrShortData will be returned.
+// If one or more required data shards are nil, ErrReconstructRequired will be returned.
 func (r reedSolomon) Join(dst io.Writer, shards [][]byte, outSize int) error {
 	// Do we have enough shards?
 	if len(shards) < r.DataShards {
@@ -503,7 +509,15 @@ func (r reedSolomon) Join(dst io.Writer, shards [][]byte, outSize int) error {
 	// Do we have enough data?
 	size := 0
 	for _, shard := range shards {
+		if shard == nil {
+			return ErrReconstructRequired
+		}
 		size += len(shard)
+
+		// Do we have enough data already?
+		if size >= outSize {
+			break
+		}
 	}
 	if size < outSize {
 		return ErrShortData
diff --git a/reedsolomon_test.go b/reedsolomon_test.go
index 6cffc8c..61f8154 100644
--- a/reedsolomon_test.go
+++ b/reedsolomon_test.go
@@ -482,6 +482,12 @@ func TestSplitJoin(t *testing.T) {
 	if err != ErrShortData {
 		t.Errorf("expected %v, got %v", ErrShortData, err)
 	}
+
+	shards[0] = nil
+	err = enc.Join(buf, shards, len(data))
+	if err != ErrReconstructRequired {
+		t.Errorf("expected %v, got %v", ErrReconstructRequired, err)
+	}
 }
 
 func TestCodeSomeShards(t *testing.T) {