-class AccessRXCache {
-public:
- AccessRXCache(IOUringEngine *engine)
- : engine(engine) {}
- void check_access(const char *filename, bool allow_async, function<void(bool)> cb);
-
-private:
- unordered_map<string, bool> cache;
- struct PendingStat {
- string filename;
- function<void(bool)> cb;
- };
- map<string, vector<PendingStat>> pending_stats;
- IOUringEngine *engine;
- mutex mu;
-};
-
-void AccessRXCache::check_access(const char *filename, bool allow_async, function<void(bool)> cb)
-{
- lock_guard<mutex> lock(mu);
- if (engine == nullptr || !engine->get_supports_stat()) {
- allow_async = false;
- }
-
- for (const char *end = strchr(filename + 1, '/'); end != nullptr; end = strchr(end + 1, '/')) {
- string parent_path(filename, end - filename); // string_view from C++20.
- auto cache_it = cache.find(parent_path);
- if (cache_it != cache.end()) {
- // Found in the cache.
- if (!cache_it->second) {
- cb(false);
- return;
- }
- continue;
- }
-
- if (!allow_async) {
- bool ok = access(parent_path.c_str(), R_OK | X_OK) == 0;
- cache.emplace(parent_path, ok);
- if (!ok) {
- cb(false);
- return;
- }
- continue;
- }
-
- // We want to call access(), but it could block on I/O. io_uring doesn't support
- // access(), but we can do a dummy asynchonous statx() to populate the kernel's cache,
- // which nearly always makes the next access() instantaneous.
-
- // See if there's already a pending stat that matches this,
- // or is a subdirectory.
- auto it = pending_stats.lower_bound(parent_path);
- if (it != pending_stats.end() && it->first.size() >= parent_path.size() &&
- it->first.compare(0, parent_path.size(), parent_path) == 0) {
- it->second.emplace_back(PendingStat{ filename, move(cb) });
- } else {
- it = pending_stats.emplace(filename, vector<PendingStat>{}).first;
- engine->submit_stat(filename, [this, it, filename{ strdup(filename) }, cb{ move(cb) }] {
- // The stat returned, so now do the actual access() calls.
- // All of them should be in cache, so don't fire off new statx()
- // calls during that check.
- check_access(filename, /*allow_async=*/false, move(cb));
- free(filename);
-
- // Call all others that waited for the same stat() to finish.
- // They may fire off new stat() calls if needed.
- vector<PendingStat> pending = move(it->second);
- pending_stats.erase(it);
- for (PendingStat &ps : pending) {
- check_access(ps.filename.c_str(), /*allow_async=*/true, move(ps.cb));
- }
- });
- }
- return; // The rest will happen in async context.
- }
-
- // Passed all checks.
- cb(true);
-}
-