Improved ring buffer read-ahead

CHANGELOG.md

@@ -23,6 +23,7 @@
 * Fixed setting `HostConfig.ApiUser` via `set_value_by_name`
 * Fixed setting `HostConfig.Path` via `set_value_by_name`
 * Fixed setting `HostConfig.Protocol` via `set_value_by_name`
+* Improved ring buffer read-ahead
 * Integrated `renterd` version 2.0.0
 * Prefer using local cache file when opening files
 * Refactored `app_config` unit tests

repertory/librepertory/src/file_manager/ring_buffer_base.cpp

@@ -98,8 +98,8 @@ auto ring_buffer_base::close() -> bool {
   return res;
 }
 
-auto ring_buffer_base::download_chunk(std::size_t chunk,
-                                      bool skip_active) -> api_error {
+auto ring_buffer_base::download_chunk(std::size_t chunk, bool skip_active)
+    -> api_error {
   REPERTORY_USES_FUNCTION_NAME();
 
   unique_mutex_lock chunk_lock(chunk_mtx_);
@@ -225,6 +225,8 @@ auto ring_buffer_base::read(std::size_t read_size, std::uint64_t read_offset,
       forward(chunk - ring_pos_);
     } else if (chunk < ring_pos_) {
       reverse(ring_pos_ - chunk);
+    } else {
+      ring_pos_ = chunk;
     }
 
     res = download_chunk(chunk, false);
@@ -264,15 +266,26 @@ void ring_buffer_base::reader_thread() {
   REPERTORY_USES_FUNCTION_NAME();
 
   unique_mutex_lock chunk_lock(chunk_mtx_);
-  auto next_chunk{ring_pos_};
+  auto last_pos = ring_pos_;
+  auto next_chunk = ring_pos_;
   chunk_notify_.notify_all();
   chunk_lock.unlock();
 
   while (not get_stop_requested()) {
     chunk_lock.lock();
 
-    next_chunk = next_chunk + 1U > ring_end_ ? ring_begin_ : next_chunk + 1U;
-    const auto check_and_wait = [this, &chunk_lock, &next_chunk]() {
+    if (last_pos == ring_pos_) {
+      ++next_chunk;
+    } else {
+      next_chunk = ring_pos_ + 1U;
+      last_pos = ring_pos_;
+    }
+
+    if (next_chunk > ring_end_) {
+      next_chunk = ring_begin_;
+    }
+
+    const auto check_and_wait = [this, &chunk_lock, &last_pos, &next_chunk]() {
       if (get_stop_requested()) {
         chunk_notify_.notify_all();
         chunk_lock.unlock();
@@ -281,6 +294,7 @@ void ring_buffer_base::reader_thread() {
 
       if (get_read_state().all()) {
         chunk_notify_.wait(chunk_lock);
+        last_pos = ring_pos_;
         next_chunk = ring_pos_;
       }