modules/zstd/frame_header_test: Fix absl::Span handling

Internal-tag: [#64294]
Signed-off-by: Pawel Czarnecki <[email protected]>

Author: lpawelcz

xls/modules/zstd/frame_header_test.cc

@@ -57,10 +57,21 @@ enum FrameHeaderStatus : uint8_t {
   UNSUPPORTED_WINDOW_SIZE
 };
 
-struct ZstdFrameHeader {
-  absl::Span<const uint8_t> kBuffer;
+class ZstdFrameHeader {
+ public:
   ZSTD_frameHeader kHeader;
   size_t kResult;
+
+  absl::Span<const uint8_t> buffer() const {
+    return absl::MakeConstSpan(buffer_);
+  }
+
+  ZstdFrameHeader(absl::Span<const uint8_t> buffer, ZSTD_frameHeader h,
+                  size_t r)
+      : kHeader(std::move(h)), kResult(r) {
+    std::vector<uint8_t> v(buffer.begin(), buffer.end());
+    buffer_ = v;
+  }
   // Parse a frame header from an arbitrary buffer with the ZSTD library.
   static absl::StatusOr<ZstdFrameHeader> Parse(
       absl::Span<const uint8_t> buffer) {
@@ -69,8 +80,11 @@ struct ZstdFrameHeader {
     ZSTD_frameHeader zstd_fh;
     size_t result = ZSTD_getFrameHeader_advanced(
         &zstd_fh, buffer.data(), buffer.size(), ZSTD_f_zstd1_magicless);
-    return ZstdFrameHeader(buffer, zstd_fh, result);
+    return ZstdFrameHeader(buffer, std::move(zstd_fh), result);
   }
+
+ private:
+  std::vector<uint8_t> buffer_;
 };
 
 class FrameHeaderTest : public xls::IrTestBase {
@@ -113,7 +127,7 @@ class FrameHeaderTest : public xls::IrTestBase {
   // expected values.
   void RunAndExpectFrameHeader(const ZstdFrameHeader& zstd_frame_header) {
     // Extend buffer contents to 128 bits if necessary.
-    const absl::Span<const uint8_t>& buffer = zstd_frame_header.kBuffer;
+    const absl::Span<const uint8_t> buffer = zstd_frame_header.buffer();
     std::vector<uint8_t> buffer_extended(kDslxBufferSizeBytes, 0);
     absl::Span<const uint8_t> input_buffer;
     if (buffer.size() < kDslxBufferSizeBytes) {