bench: make setup() use single-iteration epochs

`setup()` in nanobench runs once per epoch, not once per timed call.
If an epoch executes the benchmark body multiple times, `setup()` can silently leave later iterations with different preconditions.

Make `setup()` force `epochIterations(1)` itself: keep rejecting incompatible larger explicit epoch sizes, but allow existing single-iteration callers such as `-sanity-check`.

With `setup()` handling this centrally, remove the redundant `epochIterations(1)` calls from the benchmarks that use it.

Co-authored-by: David Gumberg <davidzgumberg@gmail.com>

src/bench/addrman.cpp

@@ -162,8 +162,7 @@ static void AddrManAddThenGood(benchmark::Bench& bench)
     CreateAddresses();
 
     std::optional<AddrMan> addrman;
-    bench.epochIterations(1)
-        .setup([&] {
+    bench.setup([&] {
             addrman.emplace(EMPTY_NETGROUPMAN, /*deterministic=*/false, ADDRMAN_CONSISTENCY_CHECK_RATIO);
             AddAddressesToAddrMan(*addrman);
         })

src/bench/checkblock.cpp

@@ -33,7 +33,7 @@ static void CheckBlockTest(benchmark::Bench& bench)
     const auto block_data{benchmark::data::block413567};
 
     CBlock block;
-    bench.unit("block").epochIterations(1)
+    bench.unit("block")
         .setup([&] {
             block = CBlock{};
             SpanReader{block_data} >> TX_WITH_WITNESS(block);

src/bench/coin_selection.cpp

@@ -115,8 +115,7 @@ static void BnBExhaustion(benchmark::Bench& bench)
 {
     std::vector<OutputGroup> utxo_pool;
     CAmount target;
-    bench.epochIterations(1)
-        .setup([&] { target = make_hard_case(17, utxo_pool); })
+    bench.setup([&] { target = make_hard_case(17, utxo_pool); })
         .run([&] {
             auto res{SelectCoinsBnB(utxo_pool, target, /*cost_of_change=*/0, MAX_STANDARD_TX_WEIGHT)}; // Should exhaust
             ankerl::nanobench::doNotOptimizeAway(res);

src/bench/load_external.cpp

@@ -62,8 +62,7 @@ static void LoadExternalBlockFile(benchmark::Bench& bench)
 
     std::multimap<uint256, FlatFilePos> blocks_with_unknown_parent;
     FlatFilePos pos;
-    bench.epochIterations(1)
-        .setup([&] {
+    bench.setup([&] {
             blocks_with_unknown_parent.clear();
             pos = FlatFilePos{};
         })

src/bench/nanobench.h

@@ -40,6 +40,7 @@
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 #include <chrono>        // high_resolution_clock
+#include <cassert>       // assert
 #include <cstring>       // memcpy
 #include <iosfwd>        // for std::ostream* custom output target in Config
 #include <string>        // all names
@@ -1013,7 +1014,7 @@ public:
     ANKERL_NANOBENCH(NODISCARD) Config const& config() const noexcept;
 
     /**
-     * @brief Configure an untimed setup step per epoch (fluent API).
+     * @brief Configure an untimed setup step per epoch (forces single-iteration epochs).
      *
      * Example: `bench.setup(...).run(...);`
      */
@@ -1238,6 +1239,9 @@ public:
     template <typename Op>
     ANKERL_NANOBENCH_NO_SANITIZE("integer")
     Bench& run(Op&& op) {
+        assert((mBench.epochIterations() <= 1) &&
+               "setup() runs once per epoch, not once per iteration; it requires epochIterations(1)");
+        mBench.epochIterations(1);
         return mBench.runImpl(mSetupOp, std::forward<Op>(op));
     }
 

src/bench/streams_findbyte.cpp

@@ -22,8 +22,7 @@ static void FindByte(benchmark::Bench& bench)
     file.seek(0, SEEK_SET);
     BufferedFile bf{file, /*nBufSize=*/file_size + 1, /*nRewindIn=*/file_size};
 
-    bench.epochIterations(1)
-        .setup([&] { bf.SetPos(0); })
+    bench.setup([&] { bf.SetPos(0); })
         .run([&] { bf.FindByte(std::byte(1)); });
 
     assert(file.fclose() == 0);

src/bench/verify_script.cpp

@@ -115,7 +115,7 @@ static void VerifyNestedIfScript(benchmark::Bench& bench)
     for (int i = 0; i < 100; ++i) {
         script << OP_ENDIF;
     }
-    bench.unit("script").epochIterations(1)
+    bench.unit("script")
         .setup([&] { stack.clear(); })
         .run([&] {
             ScriptError error;