[bulk] Fix being able to Decompress large payloads

Contrary to Decompress, bulk.Decompress does not retry with stream decompression on failing to Decompress.

#115 introduced preventing to allocate too big buffers when the input zstd header was malicious. This had the side-effect that for highly compressed payloads (> 10x), the dst buffer was still resized and would fail.

This fixes the issue and adds a test

zstd_bulk.go

@@ -112,20 +112,20 @@ func (p *BulkProcessor) Decompress(dst, src []byte) ([]byte, error) {
 
 	contentSize := decompressSizeHint(src)
 	if cap(dst) >= contentSize {
-		dst = dst[0:contentSize]
+		dst = dst[0:cap(dst)]
 	} else {
 		dst = make([]byte, contentSize)
 	}
 
-	if contentSize == 0 {
+	if len(dst) == 0 {
 		return dst, nil
 	}
 
 	dctx := C.ZSTD_createDCtx()
 	cWritten := C.ZSTD_decompress_usingDDict(
 		dctx,
 		unsafe.Pointer(&dst[0]),
-		C.size_t(contentSize),
+		C.size_t(len(dst)),
 		unsafe.Pointer(&src[0]),
 		C.size_t(len(src)),
 		p.dDict,

zstd_bullk_test.go

@@ -216,6 +216,31 @@ func TestBulkCompressAndDecompressInReverseOrder(t *testing.T) {
 	}
 }
 
+func TestDecompressHighlyCompressable(t *testing.T) {
+	p := newBulkProcessor(t, dict, BestSpeed)
+
+	// Generate a big payload
+	msgSize := 10 * 1000 * 1000 // 10 MiB
+	msg := make([]byte, msgSize)
+	compressed, err := Compress(nil, msg)
+	if err != nil {
+		t.Error("failed to compress")
+	}
+
+	// Regular decompression would trigger zipbomb prevention
+	_, err = p.Decompress(nil, compressed)
+	if !IsDstSizeTooSmallError(err) {
+		t.Error("expected too small error")
+	}
+
+	// Passing an output should suceed the decompression
+	dst := make([]byte, 10*msgSize)
+	_, err = p.Decompress(dst, compressed)
+	if err != nil {
+		t.Errorf("failed to decompress: %s", err)
+	}
+}
+
 // BenchmarkBulkCompress-8   	  780148	      1505 ns/op	  61.14 MB/s	     208 B/op	       5 allocs/op
 func BenchmarkBulkCompress(b *testing.B) {
 	p := newBulkProcessor(b, dict, BestSpeed)