Add memory barriers to data structs that need them for ARM64

This will hopefully finally fix the automated Mac OS test failures.

Author: salcock

debian/changelog

@@ -13,6 +13,8 @@ libtrace4 (4.0.30-1) unstable; urgency=medium
     of one.
   * Improved robustness of ndag read methods in the (now hopefully)
     unlikely event that the data received on the socket is corrupt.
+  * Resolve memory synchronization issues with certain internal
+    data structures (sliding window, ring buffers) when running on ARM64.
 
  -- Shane Alcock <shane@alcock.co.nz>  Wed, 11 Feb 2026 15:58:39 +1300
 

lib/data-struct/ring_buffer.c

@@ -263,6 +263,7 @@ DLLEXPORT void libtrace_ringbuffer_write(libtrace_ringbuffer_t *rb, void *value)
     /* Need an empty to start with */
     wait_for_empty(rb);
     rb->elements[rb->end] = value;
+    __sync_synchronize();
     rb->end = (rb->end + 1) % rb->size;
     notify_full(rb);
 }
@@ -311,6 +312,7 @@ DLLEXPORT size_t libtrace_ringbuffer_write_bulk(libtrace_ringbuffer_t *rb,
             rb->elements[end] = values[i];
             end = (end + 1) % rb->size;
         }
+        __sync_synchronize();
         rb->end = end;
         notify_full(rb);
     } while (i < min_nb_buffers);
@@ -350,8 +352,10 @@ DLLEXPORT void *libtrace_ringbuffer_read(libtrace_ringbuffer_t *rb)
 
     /* We need a full slot */
     wait_for_full(rb);
+    __sync_synchronize();
     value = rb->elements[rb->start];
     rb->start = (rb->start + 1) % rb->size;
+    __sync_synchronize();
     /* Now that's an empty slot */
     notify_empty(rb);
     return value;
@@ -406,6 +410,7 @@ DLLEXPORT size_t libtrace_ringbuffer_read_bulk(libtrace_ringbuffer_t *rb,
             start = (start + 1) % rb->size;
         }
         rb->start = start;
+        __sync_synchronize();
         /* Now that's an empty slot */
         notify_empty(rb);
     } while (i < min_nb_buffers);

lib/data-struct/simple_circular_buffer.c

@@ -116,6 +116,7 @@ DLLEXPORT int libtrace_scb_recv_sock(libtrace_scb_t *buf, int sock,
     if (ret < 0) {
         return ret;
     }
+    __sync_synchronize();
     buf->write_offset += ret;
     return (buf->write_offset - buf->read_offset);
 }
@@ -139,6 +140,7 @@ DLLEXPORT void libtrace_scb_advance_read(libtrace_scb_t *buf, uint32_t forward)
         buf->read_offset -= buf->count_bytes;
         buf->write_offset -= buf->count_bytes;
     }
+    __sync_synchronize();
 }
 
 DLLEXPORT int libtrace_scb_set_recv_thold(libtrace_scb_t *buf, uint32_t thold)

lib/data-struct/sliding_window.c

@@ -85,6 +85,7 @@ int libtrace_slidingwindow_try_write(libtrace_slidingwindow_t *sw,
     if (adjusted_number < sw->size) {
         // Add it
         sw->elements[(adjusted_number + sw->start) % sw->size] = value;
+        __sync_synchronize();
         return 1;
     } else {
         // Out of range don't add it
@@ -118,12 +119,14 @@ int libtrace_slidingwindow_try_read(libtrace_slidingwindow_t *sw, void **value,
                                     uint64_t *number)
 {
     if (sw->elements[sw->start]) {
+        __sync_synchronize();
         *value = sw->elements[sw->start];
         sw->elements[sw->start] = NULL;
         if (number)
             *number = sw->start_number;
         ++sw->start_number;
         sw->start = (sw->start + 1) % sw->size;
+        __sync_synchronize();
         return 1;
     } else {
         return 0;