[VFS] Fix inconsistencies between relative paths and fallthrough.

This patch addresses inconsistencies in the way fallthrough is handled
in the RedirectingFileSystem. Rather than trying to change the working
directory of the external filesystem, the RedirectingFileSystem will
canonicalize every path before handing it down. This guarantees that
relative paths are resolved relative to the RedirectingFileSystem's
working directory.

This allows us to have a strictly virtual working directory, and still
fallthrough for absolute paths, but not for relative paths that would
get resolved incorrectly at the lower layer (for example, in case of the
RealFileSystem, because the strictly virtual path does not exist).

Differential revision: https://reviews.llvm.org/D95188

include/llvm/Support/VirtualFileSystem.h

@@ -651,9 +651,12 @@ private:
   friend class VFSFromYamlDirIterImpl;
   friend class RedirectingFileSystemParser;
 
-  bool shouldUseExternalFS() const {
-    return ExternalFSValidWD && IsFallthrough;
-  }
+  bool shouldUseExternalFS() const { return IsFallthrough; }
+
+  /// Canonicalize path by removing ".", "..", "./", components. This is
+  /// a VFS request, do not bother about symlinks in the path components
+  /// but canonicalize in order to perform the correct entry search.
+  std::error_code makeCanonical(SmallVectorImpl<char> &Path) const;
 
   // In a RedirectingFileSystem, keys can be specified in Posix or Windows
   // style (or even a mixture of both), so this comparison helper allows
@@ -672,9 +675,6 @@ private:
   /// The current working directory of the file system.
   std::string WorkingDirectory;
 
-  /// Whether the current working directory is valid for the external FS.
-  bool ExternalFSValidWD = false;
-
   /// The file system to use for external references.
   IntrusiveRefCntPtr<FileSystem> ExternalFS;
 
@@ -722,7 +722,7 @@ private:
 
 public:
   /// Looks up \p Path in \c Roots.
-  ErrorOr<Entry *> lookupPath(const Twine &Path) const;
+  ErrorOr<Entry *> lookupPath(StringRef Path) const;
 
   /// Parses \p Buffer, which is expected to be in YAML format and
   /// returns a virtual file system representing its contents.

lib/Support/VirtualFileSystem.cpp

@@ -1016,7 +1016,6 @@ RedirectingFileSystem::RedirectingFileSystem(IntrusiveRefCntPtr<FileSystem> FS)
     if (auto ExternalWorkingDirectory =
             ExternalFS->getCurrentWorkingDirectory()) {
       WorkingDirectory = *ExternalWorkingDirectory;
-      ExternalFSValidWD = true;
     }
 }
 
@@ -1075,12 +1074,6 @@ RedirectingFileSystem::setCurrentWorkingDirectory(const Twine &Path) {
   if (!exists(Path))
     return errc::no_such_file_or_directory;
 
-  // Always change the external FS but ignore its result.
-  if (ExternalFS) {
-    auto EC = ExternalFS->setCurrentWorkingDirectory(Path);
-    ExternalFSValidWD = !static_cast<bool>(EC);
-  }
-
   SmallString<128> AbsolutePath;
   Path.toVector(AbsolutePath);
   if (std::error_code EC = makeAbsolute(AbsolutePath))
@@ -1089,8 +1082,14 @@ RedirectingFileSystem::setCurrentWorkingDirectory(const Twine &Path) {
   return {};
 }
 
-std::error_code RedirectingFileSystem::isLocal(const Twine &Path,
+std::error_code RedirectingFileSystem::isLocal(const Twine &Path_,
                                                bool &Result) {
+  SmallString<256> Path;
+  Path_.toVector(Path);
+
+  if (std::error_code EC = makeCanonical(Path))
+    return {};
+
   return ExternalFS->isLocal(Path, Result);
 }
 
@@ -1125,14 +1124,21 @@ std::error_code RedirectingFileSystem::makeAbsolute(SmallVectorImpl<char> &Path)
 
 directory_iterator RedirectingFileSystem::dir_begin(const Twine &Dir,
                                                     std::error_code &EC) {
-  ErrorOr<RedirectingFileSystem::Entry *> E = lookupPath(Dir);
+  SmallString<256> Path;
+  Dir.toVector(Path);
+
+  EC = makeCanonical(Path);
+  if (EC)
+    return {};
+
+  ErrorOr<RedirectingFileSystem::Entry *> E = lookupPath(Path);
   if (!E) {
     EC = E.getError();
     if (shouldUseExternalFS() && EC == errc::no_such_file_or_directory)
-      return ExternalFS->dir_begin(Dir, EC);
+      return ExternalFS->dir_begin(Path, EC);
     return {};
   }
-  ErrorOr<Status> S = status(Dir, *E);
+  ErrorOr<Status> S = status(Path, *E);
   if (!S) {
     EC = S.getError();
     return {};
@@ -1145,7 +1151,7 @@ directory_iterator RedirectingFileSystem::dir_begin(const Twine &Dir,
 
   auto *D = cast<RedirectingFileSystem::RedirectingDirectoryEntry>(*E);
   return directory_iterator(std::make_shared<VFSFromYamlDirIterImpl>(
-      Dir, D->contents_begin(), D->contents_end(),
+      Path, D->contents_begin(), D->contents_end(),
       /*IterateExternalFS=*/shouldUseExternalFS(), *ExternalFS, EC));
 }
 
@@ -1739,22 +1745,22 @@ std::unique_ptr<RedirectingFileSystem> RedirectingFileSystem::create(
   return FS;
 }
 
-ErrorOr<RedirectingFileSystem::Entry *>
-RedirectingFileSystem::lookupPath(const Twine &Path_) const {
-  SmallString<256> Path;
-  Path_.toVector(Path);
-
-  // Handle relative paths
+std::error_code
+RedirectingFileSystem::makeCanonical(SmallVectorImpl<char> &Path) const {
   if (std::error_code EC = makeAbsolute(Path))
     return EC;
 
-  // Canonicalize path by removing ".", "..", "./", components. This is
-  // a VFS request, do not bother about symlinks in the path components
-  // but canonicalize in order to perform the correct entry search.
-  Path = canonicalize(Path);
-  if (Path.empty())
+  llvm::SmallString<256> CanonicalPath =
+      canonicalize(StringRef(Path.data(), Path.size()));
+  if (CanonicalPath.empty())
     return make_error_code(llvm::errc::invalid_argument);
 
+  Path.assign(CanonicalPath.begin(), CanonicalPath.end());
+  return {};
+}
+
+ErrorOr<RedirectingFileSystem::Entry *>
+RedirectingFileSystem::lookupPath(StringRef Path) const {
   sys::path::const_iterator Start = sys::path::begin(Path);
   sys::path::const_iterator End = sys::path::end(Path);
   for (const auto &Root : Roots) {
@@ -1829,7 +1835,13 @@ ErrorOr<Status> RedirectingFileSystem::status(const Twine &Path,
   }
 }
 
-ErrorOr<Status> RedirectingFileSystem::status(const Twine &Path) {
+ErrorOr<Status> RedirectingFileSystem::status(const Twine &Path_) {
+  SmallString<256> Path;
+  Path_.toVector(Path);
+
+  if (std::error_code EC = makeCanonical(Path))
+    return EC;
+
   ErrorOr<RedirectingFileSystem::Entry *> Result = lookupPath(Path);
   if (!Result) {
     if (shouldUseExternalFS() &&
@@ -1867,7 +1879,13 @@ public:
 } // namespace
 
 ErrorOr<std::unique_ptr<File>>
-RedirectingFileSystem::openFileForRead(const Twine &Path) {
+RedirectingFileSystem::openFileForRead(const Twine &Path_) {
+  SmallString<256> Path;
+  Path_.toVector(Path);
+
+  if (std::error_code EC = makeCanonical(Path))
+    return EC;
+
   ErrorOr<RedirectingFileSystem::Entry *> E = lookupPath(Path);
   if (!E) {
     if (shouldUseExternalFS() &&
@@ -1897,8 +1915,14 @@ RedirectingFileSystem::openFileForRead(const Twine &Path) {
 }
 
 std::error_code
-RedirectingFileSystem::getRealPath(const Twine &Path,
+RedirectingFileSystem::getRealPath(const Twine &Path_,
                                    SmallVectorImpl<char> &Output) const {
+  SmallString<256> Path;
+  Path_.toVector(Path);
+
+  if (std::error_code EC = makeCanonical(Path))
+    return EC;
+
   ErrorOr<RedirectingFileSystem::Entry *> Result = lookupPath(Path);
   if (!Result) {
     if (shouldUseExternalFS() &&

unittests/Support/VirtualFileSystemTest.cpp

@@ -2135,7 +2135,41 @@ TEST_F(VFSFromYAMLTest, WorkingDirectoryFallthroughInvalid) {
   EXPECT_TRUE(Status->exists());
 
   Status = FS->status("foo/a");
-  ASSERT_TRUE(Status.getError());
+  ASSERT_FALSE(Status.getError());
+  EXPECT_TRUE(Status->exists());
+}
+
+TEST_F(VFSFromYAMLTest, VirtualWorkingDirectory) {
+  IntrusiveRefCntPtr<ErrorDummyFileSystem> Lower(new ErrorDummyFileSystem());
+  Lower->addDirectory("//root/");
+  Lower->addDirectory("//root/foo");
+  Lower->addRegularFile("//root/foo/a");
+  Lower->addRegularFile("//root/foo/b");
+  Lower->addRegularFile("//root/c");
+  IntrusiveRefCntPtr<vfs::FileSystem> FS = getFromYAMLString(
+      "{ 'use-external-names': false,\n"
+      "  'roots': [\n"
+      "{\n"
+      "  'type': 'directory',\n"
+      "  'name': '//root/bar',\n"
+      "  'contents': [ {\n"
+      "                  'type': 'file',\n"
+      "                  'name': 'a',\n"
+      "                  'external-contents': '//root/foo/a'\n"
+      "                }\n"
+      "              ]\n"
+      "}\n"
+      "]\n"
+      "}",
+      Lower);
+  ASSERT_TRUE(FS.get() != nullptr);
+  std::error_code EC = FS->setCurrentWorkingDirectory("//root/bar");
+  ASSERT_FALSE(EC);
+  ASSERT_TRUE(FS.get() != nullptr);
+
+  llvm::ErrorOr<vfs::Status> Status = FS->status("a");
+  ASSERT_FALSE(Status.getError());
+  EXPECT_TRUE(Status->exists());
 }
 
 TEST_F(VFSFromYAMLTest, YAMLVFSWriterTest) {