scriptcache.cpp

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// Copyright (c) 2017-2020 The Bitcoin developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
#include <script/scriptcache.h>
 
#include <common/system.h>
#include <crypto/sha256.h>
#include <cuckoocache.h>
#include <primitives/transaction.h>
#include <random.h>
#include <script/sigcache.h>
#include <sync.h>
#include <validation.h>
 
struct ScriptCacheElement {
    using KeyType = ScriptCacheKey;
 
    KeyType key;
    int nSigChecks;
 
    ScriptCacheElement() = default;
 
    ScriptCacheElement(const KeyType &keyIn, int nSigChecksIn)
        : key(keyIn), nSigChecks(nSigChecksIn) {}
 
    const KeyType &getKey() const { return key; }
};
 
static_assert(sizeof(ScriptCacheElement) == 32,
              "ScriptCacheElement should be 32 bytes");
 
class ScriptCacheHasher {
public:
    template <uint8_t hash_select>
    uint32_t operator()(const ScriptCacheKey &k) const {
        static_assert(hash_select < 8, "only has 8 hashes available.");
 
        const auto &d = k.data;
 
        static_assert(sizeof(d) == 28,
                      "modify the following if key size changes");
 
        uint32_t u;
        static_assert(sizeof(u) == 4 && sizeof(d[0]) == 1, "basic assumptions");
        if (hash_select < 7) {
            std::memcpy(&u, d.data() + 4 * hash_select, 4);
        } else {
            // We are required to produce 8 subhashes, and all bytes have
            // been used once. We re-use the bytes but mix together different
            // entries (and flip the order) to get a new, distinct value.
            uint8_t arr[4];
            arr[0] = d[3] ^ d[7] ^ d[11] ^ d[15];
            arr[1] = d[6] ^ d[10] ^ d[14] ^ d[18];
            arr[2] = d[9] ^ d[13] ^ d[17] ^ d[21];
            arr[3] = d[12] ^ d[16] ^ d[20] ^ d[24];
            std::memcpy(&u, arr, 4);
        }
        return u;
    }
};
 
static CuckooCache::cache<ScriptCacheElement, ScriptCacheHasher>
    g_scriptExecutionCache;
static CSHA256 g_scriptExecutionCacheHasher;
 
bool InitScriptExecutionCache(size_t max_size_bytes) {
    // Setup the salted hasher
    uint256 nonce = GetRandHash();
    // We want the nonce to be 64 bytes long to force the hasher to process
    // this chunk, which makes later hash computations more efficient. We
    // just write our 32-byte entropy twice to fill the 64 bytes.
    g_scriptExecutionCacheHasher.Write(nonce.begin(), 32);
    g_scriptExecutionCacheHasher.Write(nonce.begin(), 32);
 
    auto setup_results = g_scriptExecutionCache.setup_bytes(max_size_bytes);
    if (!setup_results) {
        return false;
    }
 
    const auto [num_elems, approx_size_bytes] = *setup_results;
    LogPrintf("Using %zu MiB out of %zu MiB requested for script execution "
              "cache, able to store %zu elements\n",
              approx_size_bytes >> 20, max_size_bytes >> 20, num_elems);
    return true;
}
 
ScriptCacheKey::ScriptCacheKey(const CTransaction &tx, uint32_t flags) {
    std::array<uint8_t, 32> hash;
    CSHA256 hasher = g_scriptExecutionCacheHasher;
    hasher.Write(tx.GetHash().begin(), 32)
        .Write((uint8_t *)&flags, sizeof(flags))
        .Finalize(hash.begin());
 
    assert(data.size() < hash.size());
    std::copy(hash.begin(), hash.begin() + data.size(), data.begin());
}
 
bool IsKeyInScriptCache(ScriptCacheKey key, bool erase, int &nSigChecksOut) {
    // TODO: Remove this requirement by making CuckooCache not require external
    // locks
    AssertLockHeld(cs_main);
 
    ScriptCacheElement elem(key, 0);
    bool ret = g_scriptExecutionCache.get(elem, erase);
    nSigChecksOut = elem.nSigChecks;
    return ret;
}
 
void AddKeyInScriptCache(ScriptCacheKey key, int nSigChecks) {
    // TODO: Remove this requirement by making CuckooCache not require external
    // locks
    AssertLockHeld(cs_main);
 
    ScriptCacheElement elem(key, nSigChecks);
    g_scriptExecutionCache.insert(elem);
}