fish-shell/src/fds.h

/** Facilities for working with file descriptors. */

#ifndef FISH_FDS_H
#define FISH_FDS_H

#include "config.h"  // IWYU pragma: keep

#include <sys/types.h>

#include <algorithm>
#include <string>
#include <vector>

#include "maybe.h"

using wcstring = std::wstring;

/// Pipe redirection error message.
#define PIPE_ERROR _(L"An error occurred while setting up pipe")

/// The first "high fd", which is considered outside the range of valid user-specified redirections (like >&5).
extern const int k_first_high_fd;

/// A helper class for managing and automatically closing a file descriptor.
class autoclose_fd_t {
    int fd_;

   public:
    // Closes the fd if not already closed.
    void close();

    // Returns the fd.
    int fd() const { return fd_; }

    // Returns the fd, transferring ownership to the caller.
    int acquire() {
        int temp = fd_;
        fd_ = -1;
        return temp;
    }

    // Resets to a new fd, taking ownership.
    void reset(int fd) {
        if (fd == fd_) return;
        close();
        fd_ = fd;
    }

    // \return if this has a valid fd.
    bool valid() const { return fd_ >= 0; }

    autoclose_fd_t(const autoclose_fd_t &) = delete;
    void operator=(const autoclose_fd_t &) = delete;
    autoclose_fd_t(autoclose_fd_t &&rhs) : fd_(rhs.fd_) { rhs.fd_ = -1; }

    void operator=(autoclose_fd_t &&rhs) {
        close();
        std::swap(this->fd_, rhs.fd_);
    }

    explicit autoclose_fd_t(int fd = -1) : fd_(fd) {}
    ~autoclose_fd_t() { close(); }
};

/// Helper type returned from making autoclose pipes.
struct autoclose_pipes_t {
    /// Read end of the pipe.
    autoclose_fd_t read;

    /// Write end of the pipe.
    autoclose_fd_t write;

    autoclose_pipes_t() = default;
    autoclose_pipes_t(autoclose_fd_t r, autoclose_fd_t w)
        : read(std::move(r)), write(std::move(w)) {}
};

/// Call pipe(), populating autoclose fds.
/// The pipes are marked CLO_EXEC and are placed in the high fd range.
/// \return pipes on success, none() on error.
maybe_t<autoclose_pipes_t> make_autoclose_pipes();

/// An event signaller implemented using a file descriptor, so it can plug into select().
/// This is like a binary semaphore. A call to post() will signal an event, making the fd readable.
/// Multiple calls to post() may be coalesced. On Linux this uses eventfd(); on other systems this
/// uses a pipe. try_consume() may be used to consume the event. Importantly this is async signal
/// safe. Of course it is CLO_EXEC as well.
class fd_event_signaller_t {
   public:
    /// \return the fd to read from, for notification.
    int read_fd() const { return fd_.fd(); }

    /// If an event is signalled, consume it; otherwise return.
    /// This does not block.
    /// This retries on EINTR.
    bool try_consume();

    /// Mark that an event has been received. This may be coalesced.
    /// This retries on EINTR.
    void post();

    /// Perform a poll to see if an event is received.
    /// If \p wait is set, wait until it is readable; this does not consume the event
    /// but guarantees that the next call to wait() will not block.
    /// \return true if readable, false if not readable, or not interrupted by a signal.
    bool poll(bool wait = false) const;

    // The default constructor will abort on failure (fd exhaustion).
    // This should only be used during startup.
    fd_event_signaller_t();
    ~fd_event_signaller_t();

   private:
    // \return the fd to write to.
    int write_fd() const;

    // Always the read end of the fd; maybe the write end as well.
    autoclose_fd_t fd_;

#ifndef HAVE_EVENTFD
    // If using a pipe, then this is its write end.
    autoclose_fd_t write_;
#endif
};

/// Sets CLO_EXEC on a given fd according to the value of \p should_set.
int set_cloexec(int fd, bool should_set = true);

/// Wide character version of open() that also sets the close-on-exec flag (atomically when
/// possible).
int wopen_cloexec(const wcstring &pathname, int flags, mode_t mode = 0);

/// Narrow versions of wopen_cloexec.
int open_cloexec(const std::string &path, int flags, mode_t mode = 0);
int open_cloexec(const char *path, int flags, mode_t mode = 0);

/// Mark an fd as nonblocking; returns errno or 0 on success.
int make_fd_nonblocking(int fd);

/// Mark an fd as blocking; returns errno or 0 on success.
int make_fd_blocking(int fd);

/// Check if an fd is on a remote filesystem (NFS, SMB, CFS)
/// Return 1 if remote, 0 if local, -1 on error or if not implemented on this platform.
int fd_check_is_remote(int fd);

/// Close a file descriptor \p fd, retrying on EINTR.
void exec_close(int fd);

#endif