Merge bitcoin#34328: rpc: make uptime monotonic across NTP jumps

14f99cf rpc: make `uptime` monotonic across NTP jumps (Lőrinc)
a9440b1 util: add `TicksSeconds` (Lőrinc)

Pull request description:

  ### Problem
  `bitcoin-cli uptime` was derived from wall-clock time, so it could jump by large amounts when the system clock is corrected after `bitcoind` starts (e.g. on RTC-less systems syncing NTP).
  This breaks the expectation that uptime reflects process runtime.

  ### Fix
  Compute uptime from a [monotonic clock](https://en.cppreference.com/w/cpp/chrono/steady_clock.html) so it is immune to wall-clock jumps, and use that monotonic uptime for the RPC.
  GUI startup time is derived from wall clock time minus monotonic uptime so it remains sensible after clock corrections.

  ### Reproducer
  Revert the fix commit and run the `rpc_uptime` functional test (it should fail with `AssertionError: uptime should not jump with wall clock`):

  Or alternatively:

  ```bash
  cmake -B build && cmake --build build --target bitcoind bitcoin-cli -j$(nproc)
  DATA_DIR=$(mktemp -d)
  ./build/bin/bitcoind -regtest -datadir="$DATA_DIR" -connect=0 -daemon
  ./build/bin/bitcoin-cli -regtest -datadir="$DATA_DIR" -rpcwait uptime
  sleep 1
  ./build/bin/bitcoin-cli -regtest -datadir="$DATA_DIR" setmocktime $(( $(date +%s) + 20000000 ))
  ./build/bin/bitcoin-cli -regtest -datadir="$DATA_DIR" uptime
  ./build/bin/bitcoin-cli -regtest -datadir="$DATA_DIR" stop
  ```

  <details>
  <summary>Before (uptime jumps with wall clock)</summary>

  ```bash
  Bitcoin Core starting
  0
  20000001
  Bitcoin Core stopping
  ```

  </details>

  <details>
  <summary>After (uptime stays monotonic)</summary>

  ```bash
  Bitcoin Core starting
  0
  1
  Bitcoin Core stopping
  ```
  </details>

  ----------

  Issue: bitcoin#34326

ACKs for top commit:
  maflcko:
    review ACK 14f99cf 🎦
  willcl-ark:
    tACK 14f99cf
  w0xlt:
    ACK 14f99cf
  sedited:
    ACK 14f99cf

Tree-SHA512: 3909973f58666ffa0b784a6df087031b9e34d2022d354900a4dbb6cbe1d36285cd92770ee71350ebf64d6e8ab212d8ff0cd851f7dca1ec46ee2f19b417f53984

Author: sedited

src/common/system.cpp

@@ -37,9 +37,6 @@
 
 using util::ReplaceAll;
 
-// Application startup time (used for uptime calculation)
-const int64_t nStartupTime = GetTime();
-
 #ifndef WIN32
 std::string ShellEscape(const std::string& arg)
 {
@@ -130,8 +127,8 @@ std::optional<size_t> GetTotalRAM()
     return std::nullopt;
 }
 
-// Obtain the application startup time (used for uptime calculation)
-int64_t GetStartupTime()
+SteadyClock::duration GetUptime()
 {
-    return nStartupTime;
+    static const auto g_startup_time{SteadyClock::now()};
+    return SteadyClock::now() - g_startup_time;
 }

src/common/system.h

@@ -7,13 +7,15 @@
 #define BITCOIN_COMMON_SYSTEM_H
 
 #include <bitcoin-build-config.h> // IWYU pragma: keep
+#include <util/time.h>
 
+#include <chrono>
 #include <cstdint>
 #include <optional>
 #include <string>
 
-// Application startup time (used for uptime calculation)
-int64_t GetStartupTime();
+/// Monotonic uptime (not affected by system time changes).
+SteadyClock::duration GetUptime();
 
 void SetupEnvironment();
 [[nodiscard]] bool SetupNetworking();

src/qt/clientmodel.cpp

@@ -210,7 +210,7 @@ bool ClientModel::isReleaseVersion() const
 
 QString ClientModel::formatClientStartupTime() const
 {
-    return QDateTime::fromSecsSinceEpoch(GetStartupTime()).toString();
+    return QDateTime::currentDateTime().addSecs(-TicksSeconds(GetUptime())).toString();
 }
 
 QString ClientModel::dataDir() const

src/rpc/server.cpp

@@ -184,7 +184,7 @@ static RPCHelpMan uptime()
                 },
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
 {
-    return GetTime() - GetStartupTime();
+    return TicksSeconds(GetUptime());
 }
     };
 }

src/test/util_tests.cpp

@@ -600,6 +600,15 @@ BOOST_AUTO_TEST_CASE(util_mocktime)
     SetMockTime(0s);
 }
 
+BOOST_AUTO_TEST_CASE(util_ticksseconds)
+{
+    BOOST_CHECK_EQUAL(TicksSeconds(0s), 0);
+    BOOST_CHECK_EQUAL(TicksSeconds(1s), 1);
+    BOOST_CHECK_EQUAL(TicksSeconds(999ms), 0);
+    BOOST_CHECK_EQUAL(TicksSeconds(1000ms), 1);
+    BOOST_CHECK_EQUAL(TicksSeconds(1500ms), 1);
+}
+
 BOOST_AUTO_TEST_CASE(test_IsDigit)
 {
     BOOST_CHECK_EQUAL(IsDigit('0'), true);

src/util/time.h

@@ -74,6 +74,12 @@ constexpr auto Ticks(Dur2 d)
 {
     return std::chrono::duration_cast<Dur1>(d).count();
 }
+
+template <typename Duration>
+constexpr int64_t TicksSeconds(Duration d)
+{
+    return int64_t{Ticks<std::chrono::seconds>(d)};
+}
 template <typename Duration, typename Timepoint>
 constexpr auto TicksSinceEpoch(Timepoint t)
 {

test/functional/rpc_uptime.py

@@ -26,9 +26,12 @@ def _test_negative_time(self):
         assert_raises_rpc_error(-8, "Mocktime must be in the range [0, 9223372036], not -1.", self.nodes[0].setmocktime, -1)
 
     def _test_uptime(self):
-        wait_time = 10
-        self.nodes[0].setmocktime(int(time.time() + wait_time))
-        assert self.nodes[0].uptime() >= wait_time
+        wait_time = 20_000
+        uptime_before = self.nodes[0].uptime()
+        self.nodes[0].setmocktime(int(time.time()) + wait_time)
+        uptime_after = self.nodes[0].uptime()
+        self.nodes[0].setmocktime(0)
+        assert uptime_after - uptime_before < wait_time, "uptime should not jump with wall clock"
 
 
 if __name__ == '__main__':