FS accessors

foo.cc

struct FSAccessor {
  virtual Stat stat(const Path & path) = 0;
  virtual StringSet readDirectory(const Path & path) = 0;
  virtual std::string readFile(const Path & path) = 0;
  virtual std::string readLink(const Path & path) = 0;
};

// Accessor to the actual filesystem.
struct RealFSAccessor { ... };

/// Access the contents of a NAR file.
struct NarAccessor { ... };

// Access the contents of a zipfile.
struct ZipFileAccessor {
  ZipFileAccessor(const Path & zipfile);
};

// Access a Git working directory. Disallows access to uncommitted files.
struct GitTreeAccessor { ... };

// A filesystem that has a single file at the root. Useful to replace addTextToStore.
struct SingletonFileAccessor { 
  SingletonFileAccessor(const std::string & contents);
};

// Adaptor accessor that changed the root of the filesystem.
// Useful for operating on a specific directory, e.g.
//   store->addToStore(RootedAccessor(RealFSAccessor(), "/path/to/dir")))
struct RootedAccessor {
  RootedAccessor(FSAccessor & inner);
};
  
// Adaptor that filters out files that don't match some predicate functor.
// E.g.
//   store->addToStore(FilteringAccessor(RootedAccessor(RealFSAccessor(), "/path/to/dir")), 
//     [&](const std::string & name, const Stat & stat) { return name != "blabla"; }));
struct FilteringAccessor {
  FilteringAccessor(FSAccessor & inner, FilterPred pred);
};

// Adaptor that keeps track of all files that were accessed.
struct LoggingAccessor {
  std::set<std::string> accessed;
  LoggingAccessor(FSAccessor & inner);
};

struct Store {
  // Add something to the Nix store. No need for path or filter arguments, we can use adaptors for that.
  Path addToStore(FSAccessor accessor);
}

struct EvalState {
  /* Need to figure out how to use accessors in the evaluator. 
     All path access needs to be relative to some accessor. 
     E.g. if we're evaluating a file from inside a `GitTreeAccessor`, then
     a path literal like `../foo/bar.nix` will need to use the same 
     `GitTreeAccessor` (and it shouldn't be able to escape).
  */
}