APIv2: remove tx_history and ledger_header (#4759)

Remove `tx_history` and `ledger_header` methods from API version 2.

Update `RPC::Handler` to allow for methods (or method implementations)
to be API version specific. This partially resolves #4727. We can now
store multiple handlers with the same name, as long as they belong to
different (non-overlapping) API versions. This necessarily impacts the
handler lookup algorithm and its complexity; however, there is no
performance loss on x86_64 architecture, and only minimal performance
loss on arm64 (around 10ns). This design change gives us extra
flexibility evolving the API in the future, including other parts of

In API version 2, `tx_history` and `ledger_header` are no longer
recognised; if they are called, `rippled` will return error
`unknownCmd`

Resolve #3638

Resolve #3539

src/test/rpc/Handler_test.cpp

@@ -0,0 +1,132 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2023 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include <ripple/beast/unit_test.h>
+#include <ripple/rpc/impl/Handler.h>
+#include <test/jtx.h>
+
+#include <chrono>
+#include <iostream>
+#include <limits>
+#include <numeric>
+#include <random>
+
+namespace ripple::test {
+
+// NOTE: there should be no need for this function;
+// `std::cout << some_duration` should just work if built with a compliant
+// C++20 compiler. Sadly, we are not using one, as of today
+// TODO: remove this operator<< overload when we bump compiler version
+std::ostream&
+operator<<(std::ostream& os, std::chrono::nanoseconds ns)
+{
+    return (os << ns.count() << "ns");
+}
+
+// NOTE This is a rather naive effort at a microbenchmark. Ideally we want
+// Google Benchmark, or something similar. Also, this actually does not belong
+// to unit tests, as it makes little sense to run it in conditions very
+// dissimilar to how rippled will normally work.
+// TODO as https://github.com/XRPLF/rippled/issues/4765
+
+class Handler_test : public beast::unit_test::suite
+{
+    auto
+    time(std::size_t n, auto f, auto prng) -> auto
+    {
+        using clock = std::chrono::steady_clock;
+        assert(n > 0);
+        double sum = 0;
+        double sum_squared = 0;
+        std::size_t j = 0;
+        while (j < n)
+        {
+            // Generate 100 inputs upfront, separated from the inner loop
+            std::array<decltype(prng()), 100> inputs = {};
+            for (auto& i : inputs)
+            {
+                i = prng();
+            }
+
+            // Take 100 samples, then sort and throw away 35 from each end,
+            // using only middle 30. This helps to reduce measurement noise.
+            std::array<long, 100> samples = {};
+            for (std::size_t k = 0; k < 100; ++k)
+            {
+                auto start = std::chrono::steady_clock::now();
+                f(inputs[k]);
+                samples[k] = (std::chrono::steady_clock::now() - start).count();
+            }
+
+            std::sort(samples.begin(), samples.end());
+            for (std::size_t k = 35; k < 65; ++k)
+            {
+                j += 1;
+                sum += samples[k];
+                sum_squared += (samples[k] * samples[k]);
+            }
+        }
+
+        const double mean_squared = (sum * sum) / (j * j);
+        return std::make_tuple(
+            clock::duration{static_cast<long>(sum / j)},
+            clock::duration{
+                static_cast<long>(std::sqrt((sum_squared / j) - mean_squared))},
+            j);
+    }
+
+    void
+    reportLookupPerformance()
+    {
+        testcase("Handler lookup performance");
+
+        std::random_device dev;
+        std::ranlux48 prng(dev());
+
+        std::vector<const char*> names =
+            test::jtx::make_vector(ripple::RPC::getHandlerNames());
+
+        std::uniform_int_distribution<std::size_t> distr{0, names.size() - 1};
+
+        std::size_t dummy = 0;
+        auto const [mean, stdev, n] = time(
+            1'000'000,
+            [&](std::size_t i) {
+                auto const d = RPC::getHandler(1, false, names[i]);
+                dummy = dummy + i + (int)d->role_;
+            },
+            [&]() -> std::size_t { return distr(prng); });
+
+        std::cout << "mean=" << mean << " stdev=" << stdev << " N=" << n
+                  << '\n';
+
+        BEAST_EXPECT(dummy != 0);
+    }
+
+public:
+    void
+    run() override
+    {
+        reportLookupPerformance();
+    }
+};
+
+BEAST_DEFINE_TESTSUITE_MANUAL(Handler, test, ripple);
+
+}  // namespace ripple::test