Create a CustomEnv class; Add WinFileSystem; Make LegacyFileSystemWra…

…pper private (#7703)

Summary:
This PR does the following:
-> Creates a WinFileSystem class.  This class is the Windows equivalent of the PosixFileSystem and will be used on Windows systems.
-> Introduces a CustomEnv class.  A CustomEnv is an Env that takes a FileSystem as constructor argument.  I believe there will only ever be two implementations of this class (PosixEnv and WinEnv).  There is still a CustomEnvWrapper class that takes an Env and a FileSystem and wraps the Env calls with the input Env but uses the FileSystem for the FileSystem calls
-> Eliminates the public uses of the LegacyFileSystemWrapper.

With this change in place, there are effectively the following patterns of Env:
- "Base Env classes" (PosixEnv, WinEnv).  These classes implement the core Env functions (e.g. Threads) and have a hard-coded input FileSystem.  These classes inherit from CompositeEnv, implement the core Env functions (threads) and delegate the FileSystem-like calls to the input file system.
- Wrapped Composite Env classes (MemEnv).  These classes take in an Env and a FileSystem.  The core env functions are re-directed to the wrapped env.  The file system calls are redirected to the input file system
- Legacy Wrapped Env classes.  These classes take in an Env input (but no FileSystem).  The core env functions are re-directed to the wrapped env.  A "Legacy File System" is created using this env and the file system calls directed to the env itself.

With these changes in place, the PosixEnv becomes a singleton -- there is only ever one created.  Any other use of the PosixEnv is via another wrapped env.  This cleans up some of the issues with the env construction and destruction.

Additionally, there were places in the code that required had an Env when they required a FileSystem.  Many of these places would wrap the Env with a LegacyFileSystemWrapper instead of using the env->GetFileSystem().  These places were changed, thereby removing layers of additional redirection (LegacyFileSystem --> Env --> Env::FileSystem).

Pull Request resolved: facebook/rocksdb#7703

Reviewed By: zhichao-cao

Differential Revision: D25762190

Pulled By: anand1976

fbshipit-source-id: 1a088e97fc916f28ac69c149cd1dcad0ab31704b
Signed-off-by: Changlong Chen <levisonchen@live.cn>

env/file_system.cc

@@ -129,13 +129,4 @@ IOStatus ReadFileToString(FileSystem* fs, const std::string& fname,
   return s;
 }
 
-#ifdef OS_WIN
-std::shared_ptr<FileSystem> FileSystem::Default() {
-  static LegacyFileSystemWrapper default_fs(Env::Default());
-  static std::shared_ptr<LegacyFileSystemWrapper> default_fs_ptr(
-      &default_fs, [](LegacyFileSystemWrapper*) {});
-  return default_fs_ptr;
-}
-#endif
-
 }  // namespace ROCKSDB_NAMESPACE