i#5329: Fix tool.drcachesim.scattergather test failure.

Avoids going over the atomic write size while waiting for enough
entries to write to IPC pipe. If we find we went over, we just
write till the last ok-to-split ref.

Fixes: #5329

clients/drcachesim/tracer/output.cpp

@@ -731,6 +731,7 @@ output_buffer(void *drcontext, per_thread_t *data, byte *buf_base, byte *buf_ptr
     byte *pipe_end = pipe_start;
     if (!op_offline.get_value()) {
         byte *post_header = buf_base + header_size;
+        byte *last_ok_to_split_ref = nullptr;
         // Pipe split headers are just the tid.
         header_size = instru->sizeof_entry();
         for (byte *mem_ref = post_header; mem_ref < buf_ptr;
@@ -752,8 +753,23 @@ output_buffer(void *drcontext, per_thread_t *data, byte *buf_base, byte *buf_ptr
                     DR_ASSERT(is_ok_to_split_before(
                         instru->get_entry_type(pipe_start + header_size),
                         instru->get_entry_size(pipe_start + header_size)));
-                    pipe_start = atomic_pipe_write(drcontext, pipe_start, pipe_end,
-                                                   get_local_window(data));
+                    // Check if we went over the edge waiting for enough entries to
+                    // write. If we did, we simply write till the last ok-to-split ref.
+                    if (pipe_end - pipe_start > ipc_pipe.get_atomic_write_size()) {
+                        // If the followin assert triggers, we found too many entries
+                        // without an ok-to-split point.
+                        DR_ASSERT(last_ok_to_split_ref != nullptr);
+                        pipe_start =
+                            atomic_pipe_write(drcontext, pipe_start, last_ok_to_split_ref,
+                                              get_local_window(data));
+                        last_ok_to_split_ref = pipe_end;
+                    } else {
+                        pipe_start = atomic_pipe_write(drcontext, pipe_start, pipe_end,
+                                                       get_local_window(data));
+                        last_ok_to_split_ref = nullptr;
+                    }
+                } else {
+                    last_ok_to_split_ref = pipe_end;
                 }
             }
         }