Take the server clock from CLOCK_MONOTONIC_RAW on Linux

include/sendspin/server/server.h

@@ -324,8 +324,8 @@ class SendspinServer {
     /// and after its source's open stream is reported stopped.
     void stop();
 
-    /// @brief The clock of every timestamp the server sends, in microseconds: monotonic, and
-    /// counting through system sleep
+    /// @brief The clock of every timestamp the server sends, in microseconds: monotonic, at the
+    /// clock's own rate, never slewed by NTP
     int64_t now_us() const;
 
     /// @brief The server_id clients see (spec connection.md "Identities"); empty while stopped

src/clock.cpp

@@ -49,7 +49,12 @@ int64_t now_us() {
 int64_t now_us() {
     constexpr int64_t US_PER_S = 1000000;
     timespec ts{};
+#if defined(__linux__)
+    // A slewed clock would change rate under the clients' time filters
+    clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
+#else
     clock_gettime(CLOCK_MONOTONIC, &ts);
+#endif
     return (static_cast<int64_t>(ts.tv_sec) * US_PER_S) + (ts.tv_nsec / NS_PER_US);
 }
 

src/clock.h

@@ -21,8 +21,9 @@
 
 namespace sendspin::server {
 
-/// @brief Monotonic microseconds that keep counting while the system sleeps: `CLOCK_MONOTONIC`
-/// on Linux, `mach_continuous_time` on macOS (whose `steady_clock` stops during sleep)
+/// @brief Monotonic microseconds at the clock's own rate, never slewed by NTP:
+/// `CLOCK_MONOTONIC_RAW` on Linux, `mach_continuous_time` on macOS (which keeps counting while the
+/// system sleeps), the esp_timer's `CLOCK_MONOTONIC` on ESP-IDF
 int64_t now_us();
 
 }  // namespace sendspin::server

src/transport.h

@@ -396,7 +396,7 @@ class Timer {
     Timer& operator=(const Timer&) = delete;
 
     /// @brief Calls on_fire from the loop once now_us() reaches about at_us, replacing any
-    /// pending call. The wait runs on the steady clock, which can fall behind now_us().
+    /// pending call. The wait runs on the steady clock, which can drift from now_us().
     void start(int64_t at_us, std::function<void()> on_fire);
 
     void cancel();