fs.go

package sys

import (
	"io/fs"

	"github.com/bananabytelabs/wazero/sys"
)

// FS is a writeable fs.FS bridge backed by syscall functions needed for ABI
// including WASI and runtime.GOOS=js.
//
// Implementations should embed UnimplementedFS for forward compatability. Any
// unsupported method or parameter should return ENO
//
// # 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, `GOOS=js` maps
// hard coded values and panics otherwise. More commonly, WASI maps syscall
// errors to u32 numeric values.
//
// # Notes
//
// A writable filesystem abstraction is not yet implemented as of Go 1.20. See
// https://github.com/golang/go/issues/45757
type FS interface {
	// OpenFile opens a file. It should be closed via Close on File.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EINVAL: `path` or `flag` is invalid.
	//   - EISDIR: the path was a directory, but flag included O_RDWR or
	//     O_WRONLY
	//   - ENOENT: `path` doesn't exist and `flag` doesn't contain O_CREAT.
	//
	// # Constraints on the returned file
	//
	// Implementations that can read flags should enforce them regardless of
	// the type returned. For example, while os.File implements io.Writer,
	// attempts to write to a directory or a file opened with O_RDONLY fail
	// with a EBADF.
	//
	// Some implementations choose whether to enforce read-only opens, namely
	// fs.FS. While fs.FS is supported (Adapt), wazero cannot runtime enforce
	// open flags. Instead, we encourage good behavior and test our built-in
	// implementations.
	//
	// # Notes
	//
	//   - This is like os.OpenFile, except the path is relative to this file
	//     system, and Errno is returned instead of os.PathError.
	//   - Implications of permissions when O_CREAT are described in Chmod notes.
	//   - This is like `open` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html
	OpenFile(path string, flag Oflag, perm fs.FileMode) (File, Errno)

	// Lstat gets file status without following symbolic links.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - ENOENT: `path` doesn't exist.
	//
	// # Notes
	//
	//   - This is like syscall.Lstat, except the `path` is relative to this
	//     file system.
	//   - This is like `lstat` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/lstat.html
	//   - An fs.FileInfo backed implementation sets atim, mtim and ctim to the
	//     same value.
	//   - When the path is a symbolic link, the stat returned is for the link,
	//     not the file it refers to.
	Lstat(path string) (sys.Stat_t, Errno)

	// Stat gets file status.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - ENOENT: `path` doesn't exist.
	//
	// # Notes
	//
	//   - This is like syscall.Stat, except the `path` is relative to this
	//     file system.
	//   - This is like `stat` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/stat.html
	//   - An fs.FileInfo backed implementation sets atim, mtim and ctim to the
	//     same value.
	//   - When the path is a symbolic link, the stat returned is for the file
	//     it refers to.
	Stat(path string) (sys.Stat_t, Errno)

	// Mkdir makes a directory.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EINVAL: `path` is invalid.
	//   - EEXIST: `path` exists and is a directory.
	//   - ENOTDIR: `path` exists and is a file.
	//
	// # Notes
	//
	//   - This is like syscall.Mkdir, except the `path` is relative to this
	//     file system.
	//   - This is like `mkdir` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/mkdir.html
	//   - Implications of permissions are described in Chmod notes.
	Mkdir(path string, perm fs.FileMode) Errno

	// Chmod changes the mode of the file.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EINVAL: `path` is invalid.
	//   - ENOENT: `path` does not exist.
	//
	// # Notes
	//
	//   - This is like syscall.Chmod, except the `path` is relative to this
	//     file system.
	//   - This is like `chmod` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/chmod.html
	//   - Windows ignores the execute bit, and any permissions come back as
	//     group and world. For example, chmod of 0400 reads back as 0444, and
	//     0700 0666. Also, permissions on directories aren't supported at all.
	Chmod(path string, perm fs.FileMode) Errno

	// Rename renames file or directory.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EINVAL: `from` or `to` is invalid.
	//   - ENOENT: `from` or `to` don't exist.
	//   - ENOTDIR: `from` is a directory and `to` exists as a file.
	//   - EISDIR: `from` is a file and `to` exists as a directory.
	//   - ENOTEMPTY: `both from` and `to` are existing directory, but
	//    `to` is not empty.
	//
	// # Notes
	//
	//   - This is like syscall.Rename, except the paths are relative to this
	//     file system.
	//   - This is like `rename` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/rename.html
	//   -  Windows doesn't let you overwrite an existing directory.
	Rename(from, to string) Errno

	// Rmdir removes a directory.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EINVAL: `path` is invalid.
	//   - ENOENT: `path` doesn't exist.
	//   - ENOTDIR: `path` exists, but isn't a directory.
	//   - ENOTEMPTY: `path` exists, but isn't empty.
	//
	// # Notes
	//
	//   - This is like syscall.Rmdir, except the `path` is relative to this
	//     file system.
	//   - This is like `rmdir` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/rmdir.html
	//   - As of Go 1.19, Windows maps ENOTDIR to ENOENT.
	Rmdir(path string) Errno

	// Unlink removes a directory entry.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EINVAL: `path` is invalid.
	//   - ENOENT: `path` doesn't exist.
	//   - EISDIR: `path` exists, but is a directory.
	//
	// # Notes
	//
	//   - This is like syscall.Unlink, except the `path` is relative to this
	//     file system.
	//   - This is like `unlink` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/unlink.html
	//   - On Windows, syscall.Unlink doesn't delete symlink to directory unlike other platforms. Implementations might
	//     want to combine syscall.RemoveDirectory with syscall.Unlink in order to delete such links on Windows.
	//     See https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-removedirectorya
	Unlink(path string) Errno

	// Link creates a "hard" link from oldPath to newPath, in contrast to a
	// soft link (via Symlink).
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EPERM: `oldPath` is invalid.
	//   - ENOENT: `oldPath` doesn't exist.
	//   - EISDIR: `newPath` exists, but is a directory.
	//
	// # Notes
	//
	//   - This is like syscall.Link, except the `oldPath` is relative to this
	//     file system.
	//   - This is like `link` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/link.html
	Link(oldPath, newPath string) Errno

	// Symlink creates a "soft" link from oldPath to newPath, in contrast to a
	// hard link (via Link).
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EPERM: `oldPath` or `newPath` is invalid.
	//   - EEXIST: `newPath` exists.
	//
	// # Notes
	//
	//   - This is like syscall.Symlink, except the `oldPath` is relative to
	//     this file system.
	//   - This is like `symlink` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/symlink.html
	//   - Only `newPath` is relative to this file system and `oldPath` is kept
	//     as-is. That is because the link is only resolved relative to the
	//     directory when dereferencing it (e.g. ReadLink).
	//     See https://github.com/bytecodealliance/cap-std/blob/v1.0.4/cap-std/src/fs/dir.rs#L404-L409
	//     for how others implement this.
	//   - Symlinks in Windows requires `SeCreateSymbolicLinkPrivilege`.
	//     Otherwise, EPERM results.
	//     See https://learn.microsoft.com/en-us/windows/security/threat-protection/security-policy-settings/create-symbolic-links
	Symlink(oldPath, linkName string) Errno

	// Readlink reads the contents of a symbolic link.
	//
	// # Errors
	//
	// A zero Errno is success. The below are expected otherwise:
	//   - ENOSYS: the implementation does not support this function.
	//   - EINVAL: `path` is invalid.
	//
	// # Notes
	//
	//   - This is like syscall.Readlink, except the path is relative to this
	//     filesystem.
	//   - This is like `readlink` in POSIX. See
	//     https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html
	//   - On Windows, the path separator is different from other platforms,
	//     but to provide consistent results to Wasm, this normalizes to a "/"
	//     separator.
	Readlink(path string) (string, Errno)

	// Utimens set file access and modification times on a path relative to
	// this file system, at nanosecond precision.
	//
	// # Parameters
	//
	// If the path is a symbolic link, the target of expanding that link is
	// updated.
	//
	// 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.
	//   - EINVAL: `path` is invalid.
	//   - EEXIST: `path` exists and is a directory.
	//   - ENOTDIR: `path` exists and is a file.
	//
	// # Notes
	//
	//   - This is like syscall.UtimesNano and `utimensat` with `AT_FDCWD` in
	//     POSIX. See https://pubs.opengroup.org/onlinepubs/9699919799/functions/futimens.html
	Utimens(path string, atim, mtim int64) Errno
}