system/lib/wasmfs/file.h

// Copyright 2021 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.
// This file defines the file object of the new file system.
// Current Status: Work in Progress.
// See https://github.com/emscripten-core/emscripten/issues/15041.

#pragma once

#include <assert.h>
#include <emscripten/html5.h>
#include <map>
#include <mutex>
#include <sys/stat.h>
#include <vector>
#include <wasi/api.h>

namespace wasmfs {

// Note: The general locking strategy for all Files is to only hold 1 lock at a
// time to prevent deadlock. This methodology can be seen in getDirs().

class Backend;
// This represents an opaque pointer to a Backend. A user may use this to
// specify a backend in file operations.
using backend_t = Backend*;

class File : public std::enable_shared_from_this<File> {

public:
  enum FileKind { DataFileKind = 0, DirectoryKind, SymlinkKind };

  template<class T> bool is() const {
    static_assert(std::is_base_of<File, T>::value,
                  "File is not a base of destination type T");
    return int(kind) == int(T::expectedKind);
  }

  template<class T> std::shared_ptr<T> dynCast() {
    static_assert(std::is_base_of<File, T>::value,
                  "File is not a base of destination type T");
    if (int(kind) == int(T::expectedKind)) {
      return std::static_pointer_cast<T>(shared_from_this());
    } else {
      return nullptr;
    }
  }

  template<class T> std::shared_ptr<T> cast() {
    static_assert(std::is_base_of<File, T>::value,
                  "File is not a base of destination type T");
    assert(int(kind) == int(T::expectedKind));
    return std::static_pointer_cast<T>(shared_from_this());
  }

  ino_t getIno() {
    // Set inode number to the file pointer. This gives a unique inode number.
    // TODO: For security it would be better to use an indirect mapping.
    // Ensure that the pointer will not overflow an ino_t.
    static_assert(sizeof(this) <= sizeof(ino_t));
    return (ino_t)this;
  }

  class Handle {

  protected:
    // This mutex is needed when one needs to access access a previously locked
    // file in the same thread. For example, rename will need to traverse
    // 2 paths and access the same locked directory twice.
    // TODO: During benchmarking, test recursive vs normal mutex performance.
    std::unique_lock<std::recursive_mutex> lock;
    std::shared_ptr<File> file;

  public:
    Handle(std::shared_ptr<File> file) : file(file), lock(file->mutex) {}
    Handle(std::shared_ptr<File> file, std::defer_lock_t)
      : file(file), lock(file->mutex, std::defer_lock) {}
    bool trylock() { return lock.try_lock(); }
    size_t getSize() { return file->getSize(); }
    mode_t& mode() { return file->mode; }
    time_t& ctime() { return file->ctime; }
    time_t& mtime() { return file->mtime; }
    time_t& atime() { return file->atime; }

    // Note: parent.lock() creates a new shared_ptr to the same Directory
    // specified by the parent weak_ptr.
    std::shared_ptr<File> getParent() { return file->parent.lock(); }
    void setParent(std::shared_ptr<File> parent) { file->parent = parent; }
  };

  Handle locked() { return Handle(shared_from_this()); }

  std::optional<Handle> maybeLocked() {
    auto handle = Handle(shared_from_this(), std::defer_lock);
    if (handle.trylock()) {
      return Handle(shared_from_this());
    } else {
      return {};
    }
  }

protected:
  File(FileKind kind, mode_t mode) : kind(kind), mode(mode) {}
  // A mutex is needed for multiple accesses to the same file.
  std::recursive_mutex mutex;

  virtual size_t getSize() = 0;

  mode_t mode = 0; // User and group mode bits for access permission.

  time_t ctime = 0; // Time when the file node was last modified.
  time_t mtime = 0; // Time when the file content was last modified.
  time_t atime = 0; // Time when the content was last accessed.

  FileKind kind;

  // Reference to parent of current file node. This can be used to
  // traverse up the directory tree. A weak_ptr ensures that the ref
  // count is not incremented. This also ensures that there are no cyclic
  // dependencies where the parent and child have shared_ptrs that reference
  // each other. This prevents the case in which an uncollectable cycle occurs.
  std::weak_ptr<File> parent;
};

class DataFile : public File {

  virtual __wasi_errno_t read(uint8_t* buf, size_t len, off_t offset) = 0;
  virtual __wasi_errno_t
  write(const uint8_t* buf, size_t len, off_t offset) = 0;

public:
  static constexpr FileKind expectedKind = File::DataFileKind;
  DataFile(mode_t mode) : File(File::DataFileKind, mode) {}
  virtual ~DataFile() = default;

  class Handle : public File::Handle {

    std::shared_ptr<DataFile> getFile() { return file->cast<DataFile>(); }

  public:
    Handle(std::shared_ptr<File> dataFile) : File::Handle(dataFile) {}
    Handle(Handle&&) = default;

    __wasi_errno_t read(uint8_t* buf, size_t len, off_t offset) {
      return getFile()->read(buf, len, offset);
    }
    __wasi_errno_t write(const uint8_t* buf, size_t len, off_t offset) {
      return getFile()->write(buf, len, offset);
    }
  };

  Handle locked() { return Handle(shared_from_this()); }
};

class Directory : public File {
protected:
  // TODO: maybe change to vector?
  std::map<std::string, std::shared_ptr<File>> entries;
  // 4096 bytes is the size of a block in ext4.
  // This value was also copied from the existing file system.
  size_t getSize() override { return 4096; }

  // This specifies which backend a directory is associated with. By default,
  // files and sub-directories added to this directory's entries will be created
  // through this same backend unless an alternative is specified.
  backend_t backend;

public:
  static constexpr FileKind expectedKind = File::DirectoryKind;
  Directory(mode_t mode, backend_t backend)
    : File(File::DirectoryKind, mode), backend(backend) {}

  struct Entry {
    std::string name;
    std::shared_ptr<File> file;
  };

  class Handle : public File::Handle {
    std::shared_ptr<Directory> getDir() { return file->cast<Directory>(); }

  public:
    Handle(std::shared_ptr<File> directory) : File::Handle(directory) {}
    Handle(std::shared_ptr<File> directory, std::defer_lock_t)
      : File::Handle(directory, std::defer_lock) {}

    std::shared_ptr<File> getEntry(std::string pathName);

    void setEntry(std::string pathName, std::shared_ptr<File> inserted) {
      // Hold the lock over both functions to cover the case in which two
      // directories attempt to add the file.
      auto lockedInserted = inserted->locked();
      getDir()->entries[pathName] = inserted;
      // Simultaneously, set the parent of the inserted node to be this Dir.
      // inserted must be locked because we have to go through Handle.
      // TODO: When rename is implemented, ensure that the source directory has
      // been removed as a parent.
      // https://github.com/emscripten-core/emscripten/pull/15410#discussion_r742171264
      assert(!lockedInserted.getParent());
      lockedInserted.setParent(file);
    }

    void unlinkEntry(std::string pathName) {
      // The file lock must be held for both operations. Removing the child file
      // from the parent's entries and removing the parent pointer from the
      // child should be atomic. The state should not be mutated in between.
      auto unlinked = getDir()->entries[pathName]->locked();
      unlinked.setParent({});
      getDir()->entries.erase(pathName);
    }

    // Used to obtain name of child File in the directory entries vector.
    std::string getName(std::shared_ptr<File> target) {
      for (const auto& [key, value] : getDir()->entries) {
        if (value == target) {
          return key;
        }
      }

      return "";
    }

    int getNumEntries() { return getDir()->entries.size(); }

    // Return a vector of the key-value pairs in entries.
    std::vector<Directory::Entry> getEntries() {
      std::vector<Directory::Entry> entries;
      for (const auto& [key, value] : getDir()->entries) {
        entries.push_back({key, value});
      }
      return entries;
    }

    backend_t getBackend() { return getDir()->backend; }

#ifdef WASMFS_DEBUG
    void printKeys() {
      for (auto keyPair : getDir()->entries) {
        emscripten_console_log(keyPair.first.c_str());
      }
    }
#endif
  };

  Handle locked() { return Handle(shared_from_this()); }

  std::optional<Handle> maybeLocked() {
    auto handle = Handle(shared_from_this(), std::defer_lock);
    if (handle.trylock()) {
      return Handle(shared_from_this());
    } else {
      return {};
    }
  }
};

// Obtains parent directory of a given pathname.
// Will return a nullptr if the parent is not a directory.
// Will error if the forbiddenAncestor is encountered while processing.
// If the forbiddenAncestor is encountered, err will be set to EINVAL and
// nullptr will be returned.
std::shared_ptr<Directory>
getDir(std::vector<std::string>::iterator begin,
       std::vector<std::string>::iterator end,
       long& err,
       std::shared_ptr<File> forbiddenAncestor = nullptr);

// Return a vector of the '/'-delimited components of a path. The first element
// will be "/" iff the path is an absolute path.
std::vector<std::string> splitPath(char* pathname);

} // namespace wasmfs