[VFS] Allow multiple RealFileSystem instances with independent CWDs.

Summary:
Previously only one RealFileSystem instance was available, and its working
directory is shared with the process. This doesn't work well for multithreaded
programs that want to work with relative paths - the vfs::FileSystem is assumed
to provide the working directory, but a thread cannot control this exclusively.

The new vfs::createPhysicalFileSystem() factory copies the process's working
directory initially, and then allows it to be independently modified.

This implementation records the working directory path, and glues it to relative
paths to provide the correct absolute path to the sys::fs:: functions.
This will give different results in unusual situations (e.g. the CWD is moved).

The main alternative is the use of openat(), fstatat(), etc to ask the OS to
resolve paths relative to a directory handle which can be kept open. This is
more robust. There are two reasons not to do this initially:
1. these functions are not available on all supported Unixes, and are somewhere
   between difficult and unavailable on Windows. So we need a path-based
   fallback anyway.
2. this would mean also adding support at the llvm::sys::fs level, which is a
   larger project. My clearest idea is an OS-specific `BaseDirectory` object
   that can be optionally passed to functions there. Eventually this could be
   backed by either paths or a fd where openat() is supported.
   This is a large project, and demonstrating here that a path-based fallback
   works is a useful prerequisite.

There is some subtlety to the path-manipulation mechanism:
  - when setting the working directory, both Specified=makeAbsolute(path) and
    Resolved=realpath(path) are recorded. These may differ in the presence of
    symlinks.
  - getCurrentWorkingDirectory() and makeAbsolute() use Specified - this is
    similar to the behavior of $PWD and sys::path::current_path
  - IO operations like openFileForRead use Resolved. This is similar to the
    behavior of an openat() based implementation, that doesn't see changes
    in symlinks.
There may still be combinations of operations and FS states that yield unhelpful
behavior. This is hard to avoid with symlinks and FS abstractions :(

The caching behavior of the current working directory is removed in this patch.
getRealFileSystem() is now specified to link to the process CWD, so the caching
is incorrect.
The user who needed this so far is clangd, which will immediately switch to
createPhysicalFileSystem().

Reviewers: ilya-biryukov, bkramer, labath

Subscribers: ioeric, kadircet, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D56545

llvm-svn: 351050

1 files changed, 73 insertions, 30 deletions

diff --git a/llvm/lib/Support/VirtualFileSystem.cpp b/llvm/lib/Support/VirtualFileSystem.cpp
index 360f8d0..f728a6a 100644
--- a/llvm/lib/Support/VirtualFileSystem.cpp
+++ b/llvm/lib/Support/VirtualFileSystem.cpp
@@ -228,9 +228,28 @@ std::error_code RealFile::close() {
 
 namespace {
 
-/// The file system according to your operating system.
+/// A file system according to your operating system.
+/// This may be linked to the process's working directory, or maintain its own.
+///
+/// Currently, its own working directory is emulated by storing the path and
+/// sending absolute paths to llvm::sys::fs:: functions.
+/// A more principled approach would be to push this down a level, modelling
+/// the working dir as an llvm::sys::fs::WorkingDir or similar.
+/// This would enable the use of openat()-style functions on some platforms.
 class RealFileSystem : public FileSystem {
 public:
+  explicit RealFileSystem(bool LinkCWDToProcess) {
+    if (!LinkCWDToProcess) {
+      SmallString<128> PWD, RealPWD;
+      if (llvm::sys::fs::current_path(PWD))
+        return; // Awful, but nothing to do here.
+      if (auto Err = llvm::sys::fs::real_path(PWD, RealPWD))
+        WD = {PWD, PWD};
+      else
+        WD = {PWD, RealPWD};
+    }
+  }
+
   ErrorOr<Status> status(const Twine &Path) override;
   ErrorOr<std::unique_ptr<File>> openFileForRead(const Twine &Path) override;
   directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override;
@@ -242,15 +261,32 @@ public:
                               SmallVectorImpl<char> &Output) const override;
 
 private:
-  mutable std::mutex CWDMutex;
-  mutable std::string CWDCache;
+  // If this FS has its own working dir, use it to make Path absolute.
+  // The returned twine is safe to use as long as both Storage and Path live.
+  Twine adjustPath(const Twine &Path, SmallVectorImpl<char> &Storage) const {
+    if (!WD)
+      return Path;
+    Path.toVector(Storage);
+    sys::fs::make_absolute(WD->Resolved, Storage);
+    return Storage;
+  }
+
+  struct WorkingDirectory {
+    // The current working directory, without symlinks resolved. (echo $PWD).
+    SmallString<128> Specified;
+    // The current working directory, with links resolved. (readlink .).
+    SmallString<128> Resolved;
+  };
+  Optional<WorkingDirectory> WD;
 };
 
 } // namespace
 
 ErrorOr<Status> RealFileSystem::status(const Twine &Path) {
+  SmallString<256> Storage;
   sys::fs::file_status RealStatus;
-  if (std::error_code EC = sys::fs::status(Path, RealStatus))
+  if (std::error_code EC =
+          sys::fs::status(adjustPath(Path, Storage), RealStatus))
     return EC;
   return Status::copyWithNewName(RealStatus, Path.str());
 }
@@ -258,56 +294,61 @@ ErrorOr<Status> RealFileSystem::status(const Twine &Path) {
 ErrorOr<std::unique_ptr<File>>
 RealFileSystem::openFileForRead(const Twine &Name) {
   int FD;
-  SmallString<256> RealName;
-  if (std::error_code EC =
-          sys::fs::openFileForRead(Name, FD, sys::fs::OF_None, &RealName))
+  SmallString<256> RealName, Storage;
+  if (std::error_code EC = sys::fs::openFileForRead(
+          adjustPath(Name, Storage), FD, sys::fs::OF_None, &RealName))
     return EC;
   return std::unique_ptr<File>(new RealFile(FD, Name.str(), RealName.str()));
 }
 
 llvm::ErrorOr<std::string> RealFileSystem::getCurrentWorkingDirectory() const {
-  std::lock_guard<std::mutex> Lock(CWDMutex);
-  if (!CWDCache.empty())
-    return CWDCache;
-  SmallString<256> Dir;
+  if (WD)
+    return WD->Specified.str();
+
+  SmallString<128> Dir;
   if (std::error_code EC = llvm::sys::fs::current_path(Dir))
     return EC;
-  CWDCache = Dir.str();
-  return CWDCache;
+  return Dir.str();
 }
 
 std::error_code RealFileSystem::setCurrentWorkingDirectory(const Twine &Path) {
-  // FIXME: chdir is thread hostile; on the other hand, creating the same
-  // behavior as chdir is complex: chdir resolves the path once, thus
-  // guaranteeing that all subsequent relative path operations work
-  // on the same path the original chdir resulted in. This makes a
-  // difference for example on network filesystems, where symlinks might be
-  // switched during runtime of the tool. Fixing this depends on having a
-  // file system abstraction that allows openat() style interactions.
-  if (auto EC = llvm::sys::fs::set_current_path(Path))
-    return EC;
-
-  // Invalidate cache.
-  std::lock_guard<std::mutex> Lock(CWDMutex);
-  CWDCache.clear();
+  if (!WD)
+    return llvm::sys::fs::set_current_path(Path);
+
+  SmallString<128> Absolute, Resolved, Storage;
+  adjustPath(Path, Storage).toVector(Absolute);
+  bool IsDir;
+  if (auto Err = llvm::sys::fs::is_directory(Absolute, IsDir))
+    return Err;
+  if (!IsDir)
+    return std::make_error_code(std::errc::not_a_directory);
+  if (auto Err = llvm::sys::fs::real_path(Absolute, Resolved))
+    return Err;
+  WD = {Absolute, Resolved};
   return std::error_code();
 }
 
 std::error_code RealFileSystem::isLocal(const Twine &Path, bool &Result) {
-  return llvm::sys::fs::is_local(Path, Result);
+  SmallString<256> Storage;
+  return llvm::sys::fs::is_local(adjustPath(Path, Storage), Result);
 }
 
 std::error_code
 RealFileSystem::getRealPath(const Twine &Path,
                             SmallVectorImpl<char> &Output) const {
-  return llvm::sys::fs::real_path(Path, Output);
+  SmallString<256> Storage;
+  return llvm::sys::fs::real_path(adjustPath(Path, Storage), Output);
 }
 
 IntrusiveRefCntPtr<FileSystem> vfs::getRealFileSystem() {
-  static IntrusiveRefCntPtr<FileSystem> FS = new RealFileSystem();
+  static IntrusiveRefCntPtr<FileSystem> FS(new RealFileSystem(true));
   return FS;
 }
 
+std::unique_ptr<FileSystem> vfs::createPhysicalFileSystem() {
+  return llvm::make_unique<RealFileSystem>(false);
+}
+
 namespace {
 
 class RealFSDirIter : public llvm::vfs::detail::DirIterImpl {
@@ -333,7 +374,9 @@ public:
 
 directory_iterator RealFileSystem::dir_begin(const Twine &Dir,
                                              std::error_code &EC) {
-  return directory_iterator(std::make_shared<RealFSDirIter>(Dir, EC));
+  SmallString<128> Storage;
+  return directory_iterator(
+      std::make_shared<RealFSDirIter>(adjustPath(Dir, Storage), EC));
 }
 
 //===-----------------------------------------------------------------------===/