Do not use std::vector = {} to release memory

Github-Pull: bitcoin#28452
Rebased-From: 3fcd7fc

Author: sipa

src/headerssync.cpp

@@ -7,6 +7,7 @@
 #include <pow.h>
 #include <timedata.h>
 #include <util/check.h>
+#include <util/vector.h>
 
 // The two constants below are computed using the simulation script on
 // https://gist.github.com/sipa/016ae445c132cdf65a2791534dfb7ae1
@@ -51,9 +52,9 @@ HeadersSyncState::HeadersSyncState(NodeId id, const Consensus::Params& consensus
 void HeadersSyncState::Finalize()
 {
     Assume(m_download_state != State::FINAL);
-    m_header_commitments = {};
+    ClearShrink(m_header_commitments);
     m_last_header_received.SetNull();
-    m_redownloaded_headers = {};
+    ClearShrink(m_redownloaded_headers);
     m_redownload_buffer_last_hash.SetNull();
     m_redownload_buffer_first_prev_hash.SetNull();
     m_process_all_remaining_headers = false;

src/test/util_tests.cpp

@@ -1791,4 +1791,29 @@ BOOST_AUTO_TEST_CASE(util_WriteBinaryFile)
     BOOST_CHECK(valid);
     BOOST_CHECK_EQUAL(actual_text, expected_text);
 }
+
+BOOST_AUTO_TEST_CASE(clearshrink_test)
+{
+    {
+        std::vector<uint8_t> v = {1, 2, 3};
+        ClearShrink(v);
+        BOOST_CHECK_EQUAL(v.size(), 0);
+        BOOST_CHECK_EQUAL(v.capacity(), 0);
+    }
+
+    {
+        std::vector<bool> v = {false, true, false, false, true, true};
+        ClearShrink(v);
+        BOOST_CHECK_EQUAL(v.size(), 0);
+        BOOST_CHECK_EQUAL(v.capacity(), 0);
+    }
+
+    {
+        std::deque<int> v = {1, 3, 3, 7};
+        ClearShrink(v);
+        BOOST_CHECK_EQUAL(v.size(), 0);
+        // std::deque has no capacity() we can observe.
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()

src/util/vector.h

@@ -49,4 +49,22 @@ inline V Cat(V v1, const V& v2)
     return v1;
 }
 
+/** Clear a vector (or std::deque) and release its allocated memory. */
+template<typename V>
+inline void ClearShrink(V& v) noexcept
+{
+    // There are various ways to clear a vector and release its memory:
+    //
+    // 1. V{}.swap(v)
+    // 2. v = V{}
+    // 3. v = {}; v.shrink_to_fit();
+    // 4. v.clear(); v.shrink_to_fit();
+    //
+    // (2) does not appear to release memory in glibc debug mode, even if v.shrink_to_fit()
+    // follows. (3) and (4) rely on std::vector::shrink_to_fit, which is only a non-binding
+    // request. Therefore, we use method (1).
+
+    V{}.swap(v);
+}
+
 #endif // BITCOIN_UTIL_VECTOR_H