Add LoadExternalBlockFile() benchmark

src/Makefile.bench.include

@@ -32,6 +32,7 @@ bench_bench_bitcoin_SOURCES = \
   bench/examples.cpp \
   bench/gcs_filter.cpp \
   bench/hashpadding.cpp \
+  bench/load_external.cpp \
   bench/lockedpool.cpp \
   bench/logging.cpp \
   bench/mempool_eviction.cpp \

src/bench/load_external.cpp

@@ -0,0 +1,63 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://www.opensource.org/licenses/mit-license.php.
+
+#include <bench/bench.h>
+#include <bench/data.h>
+#include <chainparams.h>
+#include <test/util/setup_common.h>
+#include <validation.h>
+
+/**
+ * The LoadExternalBlockFile() function is used during -reindex and -loadblock.
+ *
+ * Create a test file that's similar to a datadir/blocks/blk?????.dat file,
+ * It contains around 134 copies of the same block (typical size of real block files).
+ * For each block in the file, LoadExternalBlockFile() won't find its parent,
+ * and so will skip the block. (In the real system, it will re-read the block
+ * from disk later when it encounters its parent.)
+ *
+ * This benchmark measures the performance of deserializing the block (or just
+ * its header, beginning with PR 16981).
+ */
+static void LoadExternalBlockFile(benchmark::Bench& bench)
+{
+    const auto testing_setup{MakeNoLogFileContext<const TestingSetup>(CBaseChainParams::MAIN)};
+
+    // Create a single block as in the blocks files (magic bytes, block size,
+    // block data) as a stream object.
+    const fs::path blkfile{testing_setup.get()->m_path_root / "blk.dat"};
+    CDataStream ss(SER_DISK, 0);
+    auto params{testing_setup->m_node.chainman->GetParams()};
+    ss << params.MessageStart();
+    ss << static_cast<uint32_t>(benchmark::data::block413567.size());
+    // We can't use the streaming serialization (ss << benchmark::data::block413567)
+    // because that first writes a compact size.
+    ss.write(MakeByteSpan(benchmark::data::block413567));
+
+    // Create the test file.
+    {
+        // "wb+" is "binary, O_RDWR | O_CREAT | O_TRUNC".
+        FILE* file{fsbridge::fopen(blkfile, "wb+")};
+        // Make the test block file about 128 MB in length.
+        for (size_t i = 0; i < node::MAX_BLOCKFILE_SIZE / ss.size(); ++i) {
+            if (fwrite(ss.data(), 1, ss.size(), file) != ss.size()) {
+                throw std::runtime_error("write to test file failed\n");
+            }
+        }
+        fclose(file);
+    }
+
+    Chainstate& chainstate{testing_setup->m_node.chainman->ActiveChainstate()};
+    std::multimap<uint256, FlatFilePos> blocks_with_unknown_parent;
+    FlatFilePos pos;
+    bench.run([&] {
+        // "rb" is "binary, O_RDONLY", positioned to the start of the file.
+        // The file will be closed by LoadExternalBlockFile().
+        FILE* file{fsbridge::fopen(blkfile, "rb")};
+        chainstate.LoadExternalBlockFile(file, &pos, &blocks_with_unknown_parent);
+    });
+    fs::remove(blkfile);
+}
+
+BENCHMARK(LoadExternalBlockFile, benchmark::PriorityLevel::HIGH);