Repository: bitcoin/bitcoin
File: src/node/caches.cpp (C++)

Code before:
#include <common/args.h>
#include <index/txindex.h>
#include <kernel/caches.h>
#include <logging.h>

#include <algorithm>
#include <optional>
#include <string>

// Unlike for the UTXO database, for the txindex scenario the leveldb cache make
// a meaningful difference: https://github.com/bitcoin/bitcoin/pull/8273#issuecomment-229601991
//! Max memory allocated to tx index DB specific cache in MiB.
static constexpr int64_t MAX_TX_INDEX_CACHE{1024};
//! Max memory allocated to all block filter index caches combined in MiB.
static constexpr int64_t MAX_FILTER_INDEX_CACHE{1024};

namespace node {
std::optional<CacheSizes> CalculateCacheSizes(const ArgsManager& args, size_t n_indexes)
{
    int64_t db_cache = args.GetIntArg("-dbcache", DEFAULT_DB_CACHE);
    if (static_cast<uint64_t>(db_cache << 20) > std::numeric_limits<size_t>::max()) {
        LogWarning("Cannot allocate more than %d MiB in total for db caches.", static_cast<double>(std::numeric_limits<size_t>::max()) * (1.0 / 1024 / 1024));
        return std::nullopt;
    }

    // negative values are permitted, but interpreted as zero.
    db_cache = std::max(int64_t{0}, db_cache);
    size_t total_cache = std::max(MiBToBytes(db_cache), MiBToBytes(MIN_DB_CACHE));

    IndexCacheSizes index_sizes;
    index_sizes.tx_index = std::min(total_cache / 8, args.GetBoolArg("-txindex", DEFAULT_TXINDEX) ? MiBToBytes(MAX_TX_INDEX_CACHE) : 0);
    total_cache -= index_sizes.tx_index;
    index_sizes.filter_index = 0;
    if (n_indexes > 0) {
        int64_t max_cache = std::min(total_cache / 8, MiBToBytes(MAX_FILTER_INDEX_CACHE));
        index_sizes.filter_index = max_cache / n_indexes;
        total_cache -= index_sizes.filter_index * n_indexes;
    }
    return {{index_sizes, kernel::CacheSizes{total_cache}}};
}
} // namespace node


Review comment (bug):
This check seems pretty broken to me. Left shifting a negative value is UB, and if db_cache is indeed too large it would already overflow in `static_cast<uint64_t>(db_cache << 20)`.

On my machine, it seems this whole block is compiled away entirely, I'm not getting any warnings for either negative or too large values.

Code after:
#include <common/args.h>
#include <index/txindex.h>
#include <kernel/caches.h>
#include <logging.h>
#include <util/byte_units.h>

#include <algorithm>
#include <string>

// Unlike for the UTXO database, for the txindex scenario the leveldb cache make
// a meaningful difference: https://github.com/bitcoin/bitcoin/pull/8273#issuecomment-229601991
//! Max memory allocated to tx index DB specific cache in bytes.
static constexpr size_t MAX_TX_INDEX_CACHE{1024_MiB};
//! Max memory allocated to all block filter index caches combined in bytes.
static constexpr size_t MAX_FILTER_INDEX_CACHE{1024_MiB};

namespace node {
CacheSizes CalculateCacheSizes(const ArgsManager& args, size_t n_indexes)
{
    // Convert -dbcache from MiB units to bytes. The total cache is floored by MIN_DB_CACHE and capped by max size_t value.
    size_t total_cache{DEFAULT_DB_CACHE};
    if (std::optional<int64_t> db_cache = args.GetIntArg("-dbcache")) {
        if (*db_cache < 0) db_cache = 0;
        uint64_t db_cache_bytes = SaturatingLeftShift<uint64_t>(*db_cache, 20);
        total_cache = std::max<size_t>(MIN_DB_CACHE, std::min<uint64_t>(db_cache_bytes, std::numeric_limits<size_t>::max()));
    }

    IndexCacheSizes index_sizes;
    index_sizes.tx_index = std::min(total_cache / 8, args.GetBoolArg("-txindex", DEFAULT_TXINDEX) ? MAX_TX_INDEX_CACHE : 0);
    total_cache -= index_sizes.tx_index;
    if (n_indexes > 0) {
        size_t max_cache = std::min(total_cache / 8, MAX_FILTER_INDEX_CACHE);
        index_sizes.filter_index = max_cache / n_indexes;
        total_cache -= index_sizes.filter_index * n_indexes;
    }
    return {index_sizes, kernel::CacheSizes{total_cache}};
}
} // namespace node