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
#4727.

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