Method read

src/networkfilemanager.cpp:1349–1474

// ---------------------------------------------------------------------------

size_t NetworkFile::read(void *buffer, size_t sizeBytes) {

    if (sizeBytes == 0)
        return 0;

    auto iterOffset = m_pos;
    while (sizeBytes) {
        const auto chunkIdxToDownload = iterOffset / DOWNLOAD_CHUNK_SIZE;
        const auto offsetToDownload = chunkIdxToDownload * DOWNLOAD_CHUNK_SIZE;
        std::vector<unsigned char> region;
        auto pChunk = gNetworkChunkCache.get(m_ctx, m_url, chunkIdxToDownload);
        if (pChunk != nullptr) {
            region = *pChunk;
        } else {
            if (offsetToDownload == m_lastDownloadedOffset) {
                // In case of consecutive reads (of small size), we use a
                // heuristic that we will read the file sequentially, so
                // we double the requested size to decrease the number of
                // client/server roundtrips.
                if (m_nBlocksToDownload < 100)
                    m_nBlocksToDownload *= 2;
            } else {
                // Random reads. Cancel the above heuristics.
                m_nBlocksToDownload = 1;
            }

            // Ensure that we will request at least the number of blocks
            // to satisfy the remaining buffer size to read.
            const auto endOffsetToDownload =
                ((iterOffset + sizeBytes + DOWNLOAD_CHUNK_SIZE - 1) /
                 DOWNLOAD_CHUNK_SIZE) *
                DOWNLOAD_CHUNK_SIZE;
            const auto nMinBlocksToDownload = static_cast<size_t>(
                (endOffsetToDownload - offsetToDownload) / DOWNLOAD_CHUNK_SIZE);
            if (m_nBlocksToDownload < nMinBlocksToDownload)
                m_nBlocksToDownload = nMinBlocksToDownload;

            // Avoid reading already cached data.
            // Note: this might get evicted if concurrent reads are done, but
            // this should not cause bugs. Just missed optimization.
            for (size_t i = 1; i < m_nBlocksToDownload; i++) {
                if (gNetworkChunkCache.get(m_ctx, m_url,
                                           chunkIdxToDownload + i) != nullptr) {
                    m_nBlocksToDownload = i;
                    break;
                }
            }

            if (m_nBlocksToDownload > MAX_CHUNKS)
                m_nBlocksToDownload = MAX_CHUNKS;

            region.resize(m_nBlocksToDownload * DOWNLOAD_CHUNK_SIZE);
            size_t nRead = 0;
            std::string errorBuffer;
            errorBuffer.resize(1024);
            if (!m_handle) {
                m_handle = m_ctx->networking.open(
                    m_ctx, m_url.c_str(), offsetToDownload,