file.go

package sys

import "github.com/tetratelabs/wazero/sys"

// File is a writeable fs.File bridge backed by syscall functions needed for ABI
// including WASI.
//
// Implementations should embed UnimplementedFile for forward compatibility. Any
// unsupported method or parameter should return ENOSYS.
//
// # Errors
//
// All methods that can return an error return a Errno, which is zero
// on success.
//
// Restricting to Errno matches current WebAssembly host functions,
// which are constrained to well-known error codes. For example, WASI maps syscall
// errors to u32 numeric values.
//
// # Notes
//
//   - You must call Close to avoid file resource conflicts. For example,
//     Windows cannot delete the underlying directory while a handle to it
//     remains open.
//   - A writable filesystem abstraction is not yet implemented as of Go 1.20.
//     See https://github.com/golang/go/issues/45757
type File interface {
	// Dev returns the device ID (Stat_t.Dev) of this file, zero if unknown or
	// an error retrieving it.
	//
	// # Errors
	//
	// Possible errors are those from Stat, except ENOSYS should not
	// be returned. Zero should be returned if there is no implementation.
	//
	// # Notes
	//
	//   - Implementations should cache this result.
	//   - This combined with Ino can implement os.SameFile.
	Dev() (uint64, Errno)

	// Ino returns the serial number (Stat_t.Ino) of this file, zero if unknown
	// or an error retrieving it.
	//
	// # Errors
	//
	// Possible errors are those from Stat, except ENOSYS should not
	// be returned. Zero should be returned if there is no implementation.
	//
	// # Notes
	//
	//   - Implementations should cache this result.
	//   - This combined with Dev can implement os.SameFile.
	Ino() (sys.Inode, Errno)

	// IsDir returns true if this file is a directory or an error there was an
	// error retrieving this information.
	//
	// # Errors
	//
	// Possible errors are those from Stat, except ENOSYS should not
	// be returned. false should be returned if there is no implementation.
	//
	// # Notes
	//
	//   - Implementations should cache this result.
	IsDir() (bool, Errno)

	// IsAppend returns true if the file was opened with O_APPEND, or
	// SetAppend was successfully enabled on this file.
	//
	// # Notes
	//
	//   - This might not match the underlying state of the file descriptor if
	//     the file was not opened via OpenFile.
	IsAppend() bool

	// SetAppend toggles the append mode (O_APPEND) of this file.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file or directory was closed.
	//
	// # Notes
	//
	//   - There is no `O_APPEND` for `fcntl` in POSIX, so implementations may
	//     have to re-open the underlying file to apply this. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html
	SetAppend(enable bool) Errno

	// Stat is similar to syscall.Fstat.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file or directory was closed.
	//
	// # Notes
	//
	//   - This is like syscall.Fstat and `fstatat` with `AT_FDCWD` in POSIX.
	//     See https://pubs.opengroup.org/onlinepubs/9699919799/functions/stat.html
	//   - A fs.FileInfo backed implementation sets atim, mtim and ctim to the
	//     same value.
	//   - Windows allows you to stat a closed directory.
	Stat() (sys.Stat_t, Errno)

	// Read attempts to read all bytes in the file into `buf`, and returns the
	// count read even on error.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file or directory was closed or not readable.
	//   - EISDIR: the file was a directory.
	//
	// # Notes
	//
	//   - This is like io.Reader and `read` in POSIX, preferring semantics of
	//     io.Reader. See https://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html
	//   - Unlike io.Reader, there is no io.EOF returned on end-of-file. To
	//     read the file completely, the caller must repeat until `n` is zero.
	Read(buf []byte) (n int, errno Errno)

	// Pread attempts to read all bytes in the file into `p`, starting at the
	// offset `off`, and returns the count read even on error.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file or directory was closed or not readable.
	//   - EINVAL: the offset was negative.
	//   - EISDIR: the file was a directory.
	//
	// # Notes
	//
	//   - This is like io.ReaderAt and `pread` in POSIX, preferring semantics
	//     of io.ReaderAt. See https://pubs.opengroup.org/onlinepubs/9699919799/functions/pread.html
	//   - Unlike io.ReaderAt, there is no io.EOF returned on end-of-file. To
	//     read the file completely, the caller must repeat until `n` is zero.
	Pread(buf []byte, off int64) (n int, errno Errno)

	// Seek attempts to set the next offset for Read or Write and returns the
	// resulting absolute offset or an error.
	//
	// # Parameters
	//
	// The `offset` parameters is interpreted in terms of `whence`:
	//   - io.SeekStart: relative to the start of the file, e.g. offset=0 sets
	//     the next Read or Write to the beginning of the file.
	//   - io.SeekCurrent: relative to the current offset, e.g. offset=16 sets
	//     the next Read or Write 16 bytes past the prior.
	//   - io.SeekEnd: relative to the end of the file, e.g. offset=-1 sets the
	//     next Read or Write to the last byte in the file.
	//
	// # Behavior when a directory
	//
	// The only supported use case for a directory is seeking to `offset` zero
	// (`whence` = io.SeekStart). This should have the same behavior as
	// os.File, which resets any internal state used by Readdir.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file or directory was closed or not readable.
	//   - EINVAL: the offset was negative.
	//
	// # Notes
	//
	//   - This is like io.Seeker and `fseek` in POSIX, preferring semantics
	//     of io.Seeker. See https://pubs.opengroup.org/onlinepubs/9699919799/functions/fseek.html
	Seek(offset int64, whence int) (newOffset int64, errno Errno)

	// Readdir reads the contents of the directory associated with file and
	// returns a slice of up to n Dirent values in an arbitrary order. This is
	// a stateful function, so subsequent calls return any next values.
	//
	// If n > 0, Readdir returns at most n entries or an error.
	// If n <= 0, Readdir returns all remaining entries or an error.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file was closed or not a directory.
	//   - ENOENT: the directory could not be read (e.g. deleted).
	//
	// # Notes
	//
	//   - This is like `Readdir` on os.File, but unlike `readdir` in POSIX.
	//     See https://pubs.opengroup.org/onlinepubs/9699919799/functions/readdir.html
	//   - Unlike os.File, there is no io.EOF returned on end-of-directory. To
	//     read the directory completely, the caller must repeat until the
	//     count read (`len(dirents)`) is less than `n`.
	//   - See /RATIONALE.md for design notes.
	Readdir(n int) (dirents []Dirent, errno Errno)

	// Write attempts to write all bytes in `p` to the file, and returns the
	// count written even on error.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file was closed, not writeable, or a directory.
	//
	// # Notes
	//
	//   - This is like io.Writer and `write` in POSIX, preferring semantics of
	//     io.Writer. See https://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html
	Write(buf []byte) (n int, errno Errno)

	// Pwrite attempts to write all bytes in `p` to the file at the given
	// offset `off`, and returns the count written even on error.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file or directory was closed or not writeable.
	//   - EINVAL: the offset was negative.
	//   - EISDIR: the file was a directory.
	//
	// # Notes
	//
	//   - This is like io.WriterAt and `pwrite` in POSIX, preferring semantics
	//     of io.WriterAt. See https://pubs.opengroup.org/onlinepubs/9699919799/functions/pwrite.html
	Pwrite(buf []byte, off int64) (n int, errno Errno)

	// Truncate truncates a file to a specified length.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file or directory was closed.
	//   - EINVAL: the `size` is negative.
	//   - EISDIR: the file was a directory.
	//
	// # Notes
	//
	//   - This is like syscall.Ftruncate and `ftruncate` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/ftruncate.html
	//   - Windows does not error when calling Truncate on a closed file.
	Truncate(size int64) Errno

	// Sync synchronizes changes to the file.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - EBADF: the file or directory was closed.
	//
	// # Notes
	//
	//   - This is like syscall.Fsync and `fsync` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/fsync.html
	//   - This returns with no error instead of ENOSYS when
	//     unimplemented. This prevents fake filesystems from erring.
	//   - Windows does not error when calling Sync on a closed file.
	Sync() Errno

	// Datasync synchronizes the data of a file.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - EBADF: the file or directory was closed.
	//
	// # Notes
	//
	//   - This is like syscall.Fdatasync and `fdatasync` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/fdatasync.html
	//   - This returns with no error instead of ENOSYS when
	//     unimplemented. This prevents fake filesystems from erring.
	//   - As this is commonly missing, some implementations dispatch to Sync.
	Datasync() Errno

	// Utimens set file access and modification times of this file, at
	// nanosecond precision.
	//
	// # Parameters
	//
	// The `atim` and `mtim` parameters refer to access and modification time
	// stamps as defined in sys.Stat_t. To retain one or the other, substitute
	// it with the pseudo-timestamp UTIME_OMIT.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EBADF: the file or directory was closed.
	//
	// # Notes
	//
	//   - This is like syscall.UtimesNano and `futimens` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/futimens.html
	//   - Windows requires files to be open with O_RDWR, which means you
	//     cannot use this to update timestamps on a directory (EPERM).
	Utimens(atim, mtim int64) Errno

	// Close closes the underlying file.
	//
	// A zero Errno is returned if unimplemented or success.
	//
	// # Notes
	//
	//   - This is like syscall.Close and `close` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/close.html
	Close() Errno
}