Make readahead a bit more robust for codecs with different io speeds

src/audio/readahead_source.cpp

@@ -42,43 +42,42 @@ ReadaheadSource::~ReadaheadSource() {
 }
 
 auto ReadaheadSource::Read(cpp::span<std::byte> dest) -> ssize_t {
-  // Optismise for the most frequent case: the buffer already contains enough
-  // data for this call.
-  size_t bytes_read =
-      xStreamBufferReceive(buffer_, dest.data(), dest.size_bytes(), 0);
-
+  size_t bytes_written = 0;
+  // Fill the destination from our buffer, until either the buffer is drained
+  // or the destination is full.
+  while (!dest.empty() && (is_refilling_ || !xStreamBufferIsEmpty(buffer_))) {
+    size_t bytes_read = xStreamBufferReceive(buffer_, dest.data(),
+                                             dest.size_bytes(), portMAX_DELAY);
     tell_ += bytes_read;
-  if (bytes_read == dest.size_bytes()) {
-    return bytes_read;
-  }
-
+    bytes_written += bytes_read;
     dest = dest.subspan(bytes_read);
-
-  // Are we currently fetching more bytes?
-  ssize_t extra_bytes = 0;
-  if (!is_refilling_) {
-    // No! Pass through directly to the wrapped source for the fastest
-    // response.
-    extra_bytes = wrapped_->Read(dest);
-  } else {
-    // Yes! Wait for the refill to catch up, then try again.
-    is_refilling_.wait(true);
-    extra_bytes =
-        xStreamBufferReceive(buffer_, dest.data(), dest.size_bytes(), 0);
   }
 
-  // No need to check whether the dest buffer is actually filled, since at this
-  // point we've read as many bytes as were available.
+  // After the loop, we've either written everything that was asked for, or
+  // we're out of data.
+  if (!dest.empty()) {
+    // Out of data in the buffer. Finish using the wrapped stream.
+    size_t extra_bytes = wrapped_->Read(dest);
     tell_ += extra_bytes;
-  bytes_read += extra_bytes;
+    bytes_written += extra_bytes;
 
-  // Before returning, make sure the readahead task is kicked off again.
-  ESP_LOGI(kTag, "triggering readahead");
+    // Check for EOF in the wrapped stream.
+    if (extra_bytes < dest.size_bytes()) {
+      return bytes_written;
+    }
+  }
+  // After this point, we're done writing to `dest`. It's either empty, or the
+  // underlying source is EOF.
+
+  // If we're here, then there is more data to be read from the wrapped stream.
+  // Ensure the readahead is running.
+  if (!is_refilling_ &&
+      xStreamBufferBytesAvailable(buffer_) < kBufferSize / 4) {
     is_refilling_ = true;
     std::function<void(void)> refill = [this]() {
       // Try to keep larger than most reasonable FAT sector sizes for more
       // efficient disk reads.
-    constexpr size_t kMaxSingleRead = 1024 * 64;
+      constexpr size_t kMaxSingleRead = 1024 * 16;
       std::byte working_buf[kMaxSingleRead];
       for (;;) {
         size_t bytes_to_read = std::min<size_t>(
@@ -98,8 +97,9 @@ auto ReadaheadSource::Read(cpp::span<std::byte> dest) -> ssize_t {
       is_refilling_.notify_all();
     };
     worker_.Dispatch(refill);
+  }
 
-  return bytes_read;
+  return bytes_written;
 }
 
 auto ReadaheadSource::CanSeek() -> bool {
@@ -109,6 +109,7 @@ auto ReadaheadSource::CanSeek() -> bool {
 auto ReadaheadSource::SeekTo(int64_t destination, SeekFrom from) -> void {
   // Seeking blows away all of our prefetched data. To do this safely, we
   // first need to wait for the refill task to finish.
+  ESP_LOGI(kTag, "dropping readahead due to seek");
   is_refilling_.wait(true);
   // It's now safe to clear out the buffer.
   xStreamBufferReset(buffer_);