acamd: do not acquire on a frame exposed before the acquisition started

acamd/acam_interface.cpp

@@ -3394,6 +3394,7 @@ logwrite( function, message.str() );
       this->sequential_failures = 0;
       this->allow_large_offset.store(false);  // no stale deliberate-offset allowance
       this->is_acquired.store( false, std::memory_order_release );
+      this->acquire_start_mjd.store( mjd_now() );
 
       // Start the timeout clock, initialized as the time now plus the
       // configured timeout in seconds (duration defaults to seconds).
@@ -3451,6 +3452,18 @@ logwrite( function, message.str() );
       return NO_ERROR;
     }
 
+    // Skip a frame that began exposing before the acquisition started
+    // (mjd0 is the readout time, so allow one frame cycle).
+    //
+    if ( this->acquire_mode == Acam::TARGET_ACQUIRE || this->acquire_mode == Acam::TARGET_ACQUIRE_HERE ) {
+      const double cycle = ( iface->camera.andor.camera_info.kintime > 0 ? iface->camera.andor.camera_info.kintime
+                                                                         : iface->camera.andor.camera_info.exptime );
+      if ( iface->camera.andor.camera_info.mjd0 < this->acquire_start_mjd.load() + cycle / 86400.0 ) {
+        logwrite( function, "frame exposed before the acquisition started: not used" );
+        return NO_ERROR;
+      }
+    }
+
     // Here is the acquisition sequence, called after every frame grab.
     //
     // Using that frame, call the solver and send offsets to the TCS. After this

acamd/acam_interface.h

@@ -346,6 +346,7 @@ namespace Acam {
       int nacquired;
       int attempts;
       int sequential_failures;
+      std::atomic<double> acquire_start_mjd{0};  ///< when the current acquisition was started
 
       std::atomic<bool> is_acquired;       ///< set if target acquired successfully
       std::atomic<bool> stop_acquisition;  ///< set if the acquisition sequence should stop