[VirtualFileSystem] Support directory entries in the YAMLVFSWriter

The current implementation of the JSONWriter does not support writing
out directory entries. Earlier today I added a unit test to illustrate
the problem. When an entry is added to the YAMLVFSWriter and the path is
a directory, it will incorrectly emit the directory as a file, and any
files inside that directory will not be found by the VFS.

It's possible to partially work around the issue by only adding "leaf
nodes" (files) to the YAMLVFSWriter. However, this doesn't work for
representing empty directories. This is a problem for clients of the VFS
that want to iterate over a directory. The directory not being there is
not the same as the directory being empty.

This is not just a hypothetical problem. The FileCollector for example
does not differentiate between file and directory paths. I temporarily
worked around the issue for LLDB by ignoring directories, but I suspect
this will prove problematic sooner rather than later.

This patch fixes the issue by extending the JSONWriter to support
writing out directory entries. We store whether an entry should be
emitted as a file or directory.

Differential revision: https://reviews.llvm.org/D76670

Author: JDevlieghere

Diff for: llvm/include/llvm/Support/VirtualFileSystem.h

@@ -506,10 +506,12 @@ getVFSFromYAML(std::unique_ptr<llvm::MemoryBuffer> Buffer,
 
 struct YAMLVFSEntry {
   template <typename T1, typename T2>
-  YAMLVFSEntry(T1 &&VPath, T2 &&RPath)
-      : VPath(std::forward<T1>(VPath)), RPath(std::forward<T2>(RPath)) {}
+  YAMLVFSEntry(T1 &&VPath, T2 &&RPath, bool IsDirectory = false)
+      : VPath(std::forward<T1>(VPath)), RPath(std::forward<T2>(RPath)),
+        IsDirectory(IsDirectory) {}
   std::string VPath;
   std::string RPath;
+  bool IsDirectory = false;
 };
 
 class VFSFromYamlDirIterImpl;
@@ -771,10 +773,13 @@ class YAMLVFSWriter {
   Optional<bool> UseExternalNames;
   std::string OverlayDir;
 
+  void addEntry(StringRef VirtualPath, StringRef RealPath, bool IsDirectory);
+
 public:
   YAMLVFSWriter() = default;
 
   void addFileMapping(StringRef VirtualPath, StringRef RealPath);
+  void addDirectoryMapping(StringRef VirtualPath, StringRef RealPath);
 
   void setCaseSensitivity(bool CaseSensitive) {
     IsCaseSensitive = CaseSensitive;

Diff for: llvm/lib/Support/VirtualFileSystem.cpp

@@ -1912,11 +1912,21 @@ UniqueID vfs::getNextVirtualUniqueID() {
   return UniqueID(std::numeric_limits<uint64_t>::max(), ID);
 }
 
-void YAMLVFSWriter::addFileMapping(StringRef VirtualPath, StringRef RealPath) {
+void YAMLVFSWriter::addEntry(StringRef VirtualPath, StringRef RealPath,
+                             bool IsDirectory) {
   assert(sys::path::is_absolute(VirtualPath) && "virtual path not absolute");
   assert(sys::path::is_absolute(RealPath) && "real path not absolute");
   assert(!pathHasTraversal(VirtualPath) && "path traversal is not supported");
-  Mappings.emplace_back(VirtualPath, RealPath);
+  Mappings.emplace_back(VirtualPath, RealPath, IsDirectory);
+}
+
+void YAMLVFSWriter::addFileMapping(StringRef VirtualPath, StringRef RealPath) {
+  addEntry(VirtualPath, RealPath, /*IsDirectory=*/false);
+}
+
+void YAMLVFSWriter::addDirectoryMapping(StringRef VirtualPath,
+                                        StringRef RealPath) {
+  addEntry(VirtualPath, RealPath, /*IsDirectory=*/true);
 }
 
 namespace {
@@ -2017,7 +2027,10 @@ void JSONWriter::write(ArrayRef<YAMLVFSEntry> Entries,
 
   if (!Entries.empty()) {
     const YAMLVFSEntry &Entry = Entries.front();
-    startDirectory(path::parent_path(Entry.VPath));
+    bool first_entry_is_directory = Entry.IsDirectory;
+    StringRef Dir =
+        first_entry_is_directory ? Entry.VPath : path::parent_path(Entry.VPath);
+    startDirectory(Dir);
 
     StringRef RPath = Entry.RPath;
     if (UseOverlayRelative) {
@@ -2027,13 +2040,18 @@ void JSONWriter::write(ArrayRef<YAMLVFSEntry> Entries,
       RPath = RPath.slice(OverlayDirLen, RPath.size());
     }
 
-    writeEntry(path::filename(Entry.VPath), RPath);
+    if (!first_entry_is_directory)
+      writeEntry(path::filename(Entry.VPath), RPath);
 
     for (const auto &Entry : Entries.slice(1)) {
-      StringRef Dir = path::parent_path(Entry.VPath);
-      if (Dir == DirStack.back())
-        OS << ",\n";
-      else {
+      StringRef Dir =
+          Entry.IsDirectory ? Entry.VPath : path::parent_path(Entry.VPath);
+      if (Dir == DirStack.back()) {
+        if (!first_entry_is_directory) {
+          OS << ",\n";
+          first_entry_is_directory = false;
+        }
+      } else {
         while (!DirStack.empty() && !containedIn(DirStack.back(), Dir)) {
           OS << "\n";
           endDirectory();

Diff for: llvm/unittests/Support/VirtualFileSystemTest.cpp

@@ -2200,20 +2200,14 @@ TEST_F(VFSFromYAMLTest, YAMLVFSWriterTest) {
   ScopedDir _g(TestDirectory + "/g");
   ScopedFile _h(TestDirectory + "/h", "");
 
-  // This test exposes a bug/shortcoming in the YAMLVFSWriter. Below we call
-  // addFileMapping for _a and _e, which causes _ab and _ef not to exists in
-  // the deserialized file system, because _a and _e got emitted as regular
-  // files. The counter example is _c, if we only call addFileMapping for _cd,
-  // things work as expected.
-
   vfs::YAMLVFSWriter VFSWriter;
-  VFSWriter.addFileMapping(_a.Path, "//root/a");
+  VFSWriter.addDirectoryMapping(_a.Path, "//root/a");
   VFSWriter.addFileMapping(_ab.Path, "//root/a/b");
   VFSWriter.addFileMapping(_cd.Path, "//root/c/d");
-  VFSWriter.addFileMapping(_e.Path, "//root/e");
-  VFSWriter.addFileMapping(_ef.Path, "//root/e/f");
+  VFSWriter.addDirectoryMapping(_e.Path, "//root/e");
+  VFSWriter.addDirectoryMapping(_ef.Path, "//root/e/f");
   VFSWriter.addFileMapping(_g.Path, "//root/g");
-  VFSWriter.addFileMapping(_h.Path, "//root/h");
+  VFSWriter.addDirectoryMapping(_h.Path, "//root/h");
 
   std::string Buffer;
   raw_string_ostream OS(Buffer);
@@ -2236,11 +2230,11 @@ TEST_F(VFSFromYAMLTest, YAMLVFSWriterTest) {
   ASSERT_TRUE(FS.get() != nullptr);
 
   EXPECT_TRUE(FS->exists(_a.Path));
-  EXPECT_FALSE(FS->exists(_ab.Path)); // FIXME: See explanation above.
+  EXPECT_TRUE(FS->exists(_ab.Path));
   EXPECT_TRUE(FS->exists(_c.Path));
   EXPECT_TRUE(FS->exists(_cd.Path));
   EXPECT_TRUE(FS->exists(_e.Path));
-  EXPECT_FALSE(FS->exists(_ef.Path)); // FIXME: See explanation above.
+  EXPECT_TRUE(FS->exists(_ef.Path));
   EXPECT_TRUE(FS->exists(_g.Path));
   EXPECT_TRUE(FS->exists(_h.Path));
 }