#pragma once
#include <functional>
#include <string>
// Cross-process exclusive lock (flock on POSIX, LockFileEx on Windows).
// Lazy: acquire() opens the lock file, so a read-only cache works if the lock is never acquired.
class common_file_lock {
public:
common_file_lock(const std::string & lock_path);
~common_file_lock();
// take the lock, waiting while another process holds it;
// keep_waiting is polled before each retry, false cancels the wait.
// Also returns false when locking is unavailable.
bool acquire(const std::function<bool()> & keep_waiting = {});
void close();
common_file_lock(const common_file_lock &) = delete;
common_file_lock & operator=(const common_file_lock &) = delete;
private:
enum class status {
acquired,
busy, // another process holds the lock
error, // file locking is unavailable: no exclusion is possible
};
status try_lock();
std::string lock_path;
int fd = -1;
bool locked = false;
};