system/lib/wasmfs/paths.h

// Copyright 2022 The Emscripten Authors.  All rights reserved.
// Emscripten is available under two separate licenses, the MIT license and the
// University of Illinois/NCSA Open Source License.  Both these licenses can be
// found in the LICENSE file.

#pragma once

#include <cassert>
#include <fcntl.h>
#include <memory>
#include <string_view>
#include <variant>

#include "file.h"

namespace wasmfs::path {

// Typically -ENOTDIR or -ENOENT.
using Error = long;

// The parent directory and the name of an entry within it. The returned string
// view is either backed by the same memory as the view passed to `parseParent`
// or is a view into a static string.
struct ParentChild {
  std::shared_ptr<Directory> parent;
  std::string_view child;
  bool hasTrailingSlash;
};

// If the path refers to a link, whether we should follow that link. Links among
// the parent directories in the path are always followed.
enum LinkBehavior { FollowLinks, NoFollowLinks };

struct ParsedParent {
private:
  std::variant<Error, ParentChild> val;

public:
  ParsedParent(Error err) : val(err) {}
  ParsedParent(ParentChild pc) : val(pc) {}
  ParsedParent(std::shared_ptr<Directory> parent,
               std::string_view child,
               bool hasTrailingSlash)
    : val(ParentChild{parent, child, hasTrailingSlash}) {}
  // Always ok to call, returns 0 if there is no error.
  long getError() const {
    if (auto* err = std::get_if<Error>(&val)) {
      assert(*err != 0 && "Unexpected zero error value");
      return *err;
    }
    return 0;
  }
  // Call only after checking for an error.
  const ParentChild& getParentChild() const {
    auto* ptr = std::get_if<ParentChild>(&val);
    assert(ptr && "Unhandled path parsing error!");
    return *ptr;
  }
};

ParsedParent parseParent(std::string_view path, __wasi_fd_t basefd = AT_FDCWD);

struct ParsedFile {
private:
  std::variant<Error, std::shared_ptr<File>> val;

public:
  ParsedFile(Error err) : val(err) {}
  ParsedFile(std::shared_ptr<File> file) : val(std::move(file)) {}
  // Always ok to call. Returns 0 if there is no error.
  long getError() {
    if (auto* err = std::get_if<Error>(&val)) {
      assert(*err != 0 && "Unexpected zero error value");
      return *err;
    }
    return 0;
  }
  // Call only after checking for an error.
  std::shared_ptr<File>& getFile() {
    auto* ptr = std::get_if<std::shared_ptr<File>>(&val);
    return *ptr;
  }
};

ParsedFile parseFile(std::string_view path,
                     __wasi_fd_t basefd = AT_FDCWD,
                     LinkBehavior links = FollowLinks);

// Like `parseFile`, but handle the cases where flags include `AT_EMPTY_PATH` or
// AT_SYMLINK_NOFOLLOW. Does not validate the flags since different callers have
// different allowed flags.
ParsedFile getFileAt(__wasi_fd_t fd, std::string_view path, int flags);

// Like `parseFile`, but parse the path relative to the given directory.
ParsedFile getFileFrom(std::shared_ptr<Directory> base, std::string_view path);

} // namespace wasmfs::path