fix: uptime RPC returns 0 on first call

The monotonic uptime fix (#34328) used a function-local static for `g_startup_time`, which was initialized on first `GetUptime()` call instead of app startup time.
This caused the first `uptime()` call to always return 0.

Move `g_startup_time` to namespace scope so it initializes at program start, ensuring the first call returns actual elapsed time. Note that we don't need to make it `static` anymore because it is just used in this single translation unit.

Test was updated to simulate some work before the first call.

Co-authored-by: Carlo Antinarella <carloantinarella@users.noreply.github.com>

src/common/system.cpp

@@ -127,8 +127,8 @@ std::optional<size_t> GetTotalRAM()
     return std::nullopt;
 }
 
-SteadyClock::duration GetUptime()
-{
-    static const auto g_startup_time{SteadyClock::now()};
-    return SteadyClock::now() - g_startup_time;
-}
+namespace {
+    const auto g_startup_time{SteadyClock::now()};
+} // namespace
+
+SteadyClock::duration GetUptime() { return SteadyClock::now() - g_startup_time; }

test/functional/rpc_uptime.py

@@ -26,8 +26,11 @@ 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 = 20_000
+        time.sleep(1) # Do some work before checking uptime
         uptime_before = self.nodes[0].uptime()
+        assert uptime_before > 0, "uptime should begin at app start"
+
+        wait_time = 20_000
         self.nodes[0].setmocktime(int(time.time()) + wait_time)
         uptime_after = self.nodes[0].uptime()
         self.nodes[0].setmocktime(0)