[clang] Fix handling of adding a file with the same name as an existing dir to VFS (#94461)

When trying to add a file to clang's VFS via `addFile` and a directory
of the same name already exists, we run into a [out-of-bound
access](https://github.com/llvm/llvm-project/blob/145815c180fc82c5a55bf568d01d98d250490a55/llvm/lib/Support/Path.cpp#L244).

The problem is that the file name is [recognised as existing path](
https://github.com/llvm/llvm-project/blob/145815c180fc82c5a55bf568d01d98d250490a55/llvm/lib/Support/VirtualFileSystem.cpp#L896)
and thus continues to process the next part of the path which doesn't
exist.

This patch adds a check if we have reached the last part of the filename
and return false in that case.
This we reject to add a file if a directory of the same name already
exists.

This is in sync with [this
check](https://github.com/llvm/llvm-project/blob/145815c180fc82c5a55bf568d01d98d250490a55/llvm/lib/Support/VirtualFileSystem.cpp#L903)
that rejects adding a path if a file of the same name already exists.

Author: jensmassberg

llvm/lib/Support/VirtualFileSystem.cpp

@@ -867,21 +867,16 @@ bool InMemoryFileSystem::addFile(const Twine &P, time_t ModificationTime,
   // Any intermediate directories we create should be accessible by
   // the owner, even if Perms says otherwise for the final path.
   const auto NewDirectoryPerms = ResolvedPerms | sys::fs::owner_all;
+
+  StringRef Name = *I;
   while (true) {
-    StringRef Name = *I;
-    detail::InMemoryNode *Node = Dir->getChild(Name);
+    Name = *I;
     ++I;
+    if (I == E)
+      break;
+    detail::InMemoryNode *Node = Dir->getChild(Name);
     if (!Node) {
-      if (I == E) {
-        // End of the path.
-        Dir->addChild(
-            Name, MakeNode({Dir->getUniqueID(), Path, Name, ModificationTime,
-                            std::move(Buffer), ResolvedUser, ResolvedGroup,
-                            ResolvedType, ResolvedPerms}));
-        return true;
-      }
-
-      // Create a new directory. Use the path up to here.
+      // This isn't the last element, so we create a new directory.
       Status Stat(
           StringRef(Path.str().begin(), Name.end() - Path.str().begin()),
           getDirectoryID(Dir->getUniqueID(), Name),
@@ -891,27 +886,33 @@ bool InMemoryFileSystem::addFile(const Twine &P, time_t ModificationTime,
           Name, std::make_unique<detail::InMemoryDirectory>(std::move(Stat))));
       continue;
     }
+    // Creating file under another file.
+    if (!isa<detail::InMemoryDirectory>(Node))
+      return false;
+    Dir = cast<detail::InMemoryDirectory>(Node);
+  }
+  detail::InMemoryNode *Node = Dir->getChild(Name);
+  if (!Node) {
+    Dir->addChild(Name,
+                  MakeNode({Dir->getUniqueID(), Path, Name, ModificationTime,
+                            std::move(Buffer), ResolvedUser, ResolvedGroup,
+                            ResolvedType, ResolvedPerms}));
+    return true;
+  }
+  if (isa<detail::InMemoryDirectory>(Node))
+    return ResolvedType == sys::fs::file_type::directory_file;
 
-    if (auto *NewDir = dyn_cast<detail::InMemoryDirectory>(Node)) {
-      Dir = NewDir;
-    } else {
-      assert((isa<detail::InMemoryFile>(Node) ||
-              isa<detail::InMemoryHardLink>(Node)) &&
-             "Must be either file, hardlink or directory!");
-
-      // Trying to insert a directory in place of a file.
-      if (I != E)
-        return false;
+  assert((isa<detail::InMemoryFile>(Node) ||
+          isa<detail::InMemoryHardLink>(Node)) &&
+         "Must be either file, hardlink or directory!");
 
-      // Return false only if the new file is different from the existing one.
-      if (auto Link = dyn_cast<detail::InMemoryHardLink>(Node)) {
-        return Link->getResolvedFile().getBuffer()->getBuffer() ==
-               Buffer->getBuffer();
-      }
-      return cast<detail::InMemoryFile>(Node)->getBuffer()->getBuffer() ==
-             Buffer->getBuffer();
-    }
+  // Return false only if the new file is different from the existing one.
+  if (auto *Link = dyn_cast<detail::InMemoryHardLink>(Node)) {
+    return Link->getResolvedFile().getBuffer()->getBuffer() ==
+           Buffer->getBuffer();
   }
+  return cast<detail::InMemoryFile>(Node)->getBuffer()->getBuffer() ==
+         Buffer->getBuffer();
 }
 
 bool InMemoryFileSystem::addFile(const Twine &P, time_t ModificationTime,

llvm/unittests/Support/VirtualFileSystemTest.cpp

@@ -1138,6 +1138,11 @@ TEST_F(InMemoryFileSystemTest, DuplicatedFile) {
   ASSERT_FALSE(FS.addFile("/a/b", 0, MemoryBuffer::getMemBuffer("a")));
   ASSERT_TRUE(FS.addFile("/a", 0, MemoryBuffer::getMemBuffer("a")));
   ASSERT_FALSE(FS.addFile("/a", 0, MemoryBuffer::getMemBuffer("b")));
+  ASSERT_TRUE(FS.addFile("/b/c/d", 0, MemoryBuffer::getMemBuffer("a")));
+  ASSERT_FALSE(FS.addFile("/b/c", 0, MemoryBuffer::getMemBuffer("a")));
+  ASSERT_TRUE(FS.addFile(
+      "/b/c", 0, MemoryBuffer::getMemBuffer(""), /*User=*/std::nullopt,
+      /*Group=*/std::nullopt, sys::fs::file_type::directory_file));
 }
 
 TEST_F(InMemoryFileSystemTest, DirectoryIteration) {