rpc: make uptime monotonic across NTP jumps

Compute `uptime` from `SteadyClock` so it is unaffected by system time changes after startup.

Derive GUI startup time by subtracting the monotonic uptime from the wall clock time.

Add a functional test covering a large `setmocktime` jump.

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>

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());
 }
     };
 }

test/functional/rpc_uptime.py

@@ -26,9 +26,12 @@ class UptimeTest(BitcoinTestFramework):
         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__':