refactor(headerssync): Process spans of headers

diff --git a/src/headerssync.cpp b/src/headerssync.cpp
index ae7187f4..f966b13a 100644
--- a/src/headerssync.cpp
+++ b/src/headerssync.cpp
@@ -1,4 +1,4 @@
-// Copyright (c) 2022 The Bitcoin Core developers
+// Copyright (c) 2022-present The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -66,8 +66,8 @@ void HeadersSyncState::Finalize()
 /** Process the next batch of headers received from our peer.
  *  Validate and store commitments, and compare total chainwork to our target to
  *  see if we can switch to REDOWNLOAD mode.  */
-HeadersSyncState::ProcessingResult HeadersSyncState::ProcessNextHeaders(const
-        std::vector<CBlockHeader>& received_headers, const bool full_headers_message)
+HeadersSyncState::ProcessingResult HeadersSyncState::ProcessNextHeaders(
+        std::span<const CBlockHeader> received_headers, const bool full_headers_message)
 {
     ProcessingResult ret;
 
@@ -137,7 +137,7 @@ HeadersSyncState::ProcessingResult HeadersSyncState::ProcessNextHeaders(const
     return ret;
 }
 
-bool HeadersSyncState::ValidateAndStoreHeadersCommitments(const std::vector<CBlockHeader>& headers)
+bool HeadersSyncState::ValidateAndStoreHeadersCommitments(std::span<const CBlockHeader> headers)
 {
     // The caller should not give us an empty set of headers.
     Assume(headers.size() > 0);
diff --git a/src/headerssync.h b/src/headerssync.h
index 56380c66..12afe660 100644
--- a/src/headerssync.h
+++ b/src/headerssync.h
@@ -165,7 +165,7 @@ public:
      * ProcessingResult.request_more: if true, the caller is suggested to call
      *                       NextHeadersRequestLocator and send a getheaders message using it.
      */
-    ProcessingResult ProcessNextHeaders(const std::vector<CBlockHeader>&
+    ProcessingResult ProcessNextHeaders(std::span<const CBlockHeader>
             received_headers, bool full_headers_message);
 
     /** Issue the next GETHEADERS message to our peer.
@@ -195,7 +195,7 @@ private:
      *  processed headers.
      *  On failure, this invokes Finalize() and returns false.
      */
-    bool ValidateAndStoreHeadersCommitments(const std::vector<CBlockHeader>& headers);
+    bool ValidateAndStoreHeadersCommitments(std::span<const CBlockHeader> headers);
 
     /** In PRESYNC, process and update state for a single header */
     bool ValidateAndProcessSingleHeader(const CBlockHeader& current);
diff --git a/src/test/headers_sync_chainwork_tests.cpp b/src/test/headers_sync_chainwork_tests.cpp
index db58970f..51193328 100644
--- a/src/test/headers_sync_chainwork_tests.cpp
+++ b/src/test/headers_sync_chainwork_tests.cpp
@@ -92,16 +92,12 @@ BOOST_AUTO_TEST_CASE(headers_sync_state)
 
     std::unique_ptr<HeadersSyncState> hss;
 
-    std::vector<CBlockHeader> headers_batch;
-
     // Feed the first chain to HeadersSyncState, by delivering 1 header
     // initially and then the rest.
-    headers_batch.insert(headers_batch.end(), std::next(first_chain.begin()), first_chain.end());
-
     hss.reset(new HeadersSyncState(0, Params().GetConsensus(), chain_start, CHAIN_WORK));
-    (void)hss->ProcessNextHeaders({first_chain.front()}, true);
+    (void)hss->ProcessNextHeaders({{first_chain.front()}}, true);
     // Pretend the first header is still "full", so we don't abort.
-    auto result = hss->ProcessNextHeaders(headers_batch, true);
+    auto result = hss->ProcessNextHeaders(std::span{first_chain}.subspan(1), true);
 
     // This chain should look valid, and we should have met the proof-of-work
     // requirement.
@@ -132,15 +128,13 @@ BOOST_AUTO_TEST_CASE(headers_sync_state)
     hss.reset(new HeadersSyncState(0, Params().GetConsensus(), chain_start, CHAIN_WORK));
     BOOST_CHECK(hss->GetState() == HeadersSyncState::State::PRESYNC);
      // Pretend just the first message is "full", so we don't abort.
-    (void)hss->ProcessNextHeaders({second_chain.front()}, true);
+    (void)hss->ProcessNextHeaders({{second_chain.front()}}, true);
     BOOST_CHECK(hss->GetState() == HeadersSyncState::State::PRESYNC);
 
-    headers_batch.clear();
-    headers_batch.insert(headers_batch.end(), std::next(second_chain.begin(), 1), second_chain.end());
     // Tell the sync logic that the headers message was not full, implying no
     // more headers can be requested. For a low-work-chain, this should causes
     // the sync to end with no headers for acceptance.
-    result = hss->ProcessNextHeaders(headers_batch, false);
+    result = hss->ProcessNextHeaders(std::span{second_chain}.subspan(1), false);
     BOOST_CHECK(hss->GetState() == HeadersSyncState::State::FINAL);
     BOOST_CHECK(result.pow_validated_headers.empty());
     BOOST_CHECK(!result.request_more);