[clang] Processing real directories added as virtual ones (#91645)

The `FileManager` might create a virtual directory that can be used
later as a search path. This is the case when we use remapping, as
demonstrated in the suggested LIT test.

We might encounter a 'false cache miss' and add the same directory
twice into `FileManager::SeenDirEntries` if the added record is a real
directory that is present on a disk:
- Once as a virtual directory 
- And once as a real one

This isn't a problem if the added directories have the same name, as in
this case, we will get a cache hit. However, it could lead to
compilation errors if the directory names are different but point to the
same folder. For example, one might use an absolute name and another a
relative one. For instance, the **implicit-module-remap.cpp** LIT test
will fail with the following message:
```
/.../implicit-module-remap.cpp.tmp/test.cpp:1:2: fatal error: module 'a' 
was built in directory '/.../implicit-module-remap.cpp.tmp' but now 
resides in directory '.'
    1 | #include "a.h"
      |  ^
1 error generated.
```

The suggested fix checks if the added virtual directory is present on
the disk and handles it as a real one if that is the case.

Author: ivanmurashko

clang/include/clang/Basic/FileManager.h

@@ -299,6 +299,8 @@ class FileManager : public RefCountedBase<FileManager> {
   getBufferForFileImpl(StringRef Filename, int64_t FileSize, bool isVolatile,
                        bool RequiresNullTerminator) const;
 
+  DirectoryEntry *&getRealDirEntry(const llvm::vfs::Status &Status);
+
 public:
   /// Get the 'stat' information for the given \p Path.
   ///

clang/lib/Basic/FileManager.cpp

@@ -82,6 +82,22 @@ getDirectoryFromFile(FileManager &FileMgr, StringRef Filename,
   return FileMgr.getDirectoryRef(DirName, CacheFailure);
 }
 
+DirectoryEntry *&FileManager::getRealDirEntry(const llvm::vfs::Status &Status) {
+  assert(Status.isDirectory() && "The directory should exist!");
+  // See if we have already opened a directory with the
+  // same inode (this occurs on Unix-like systems when one dir is
+  // symlinked to another, for example) or the same path (on
+  // Windows).
+  DirectoryEntry *&UDE = UniqueRealDirs[Status.getUniqueID()];
+
+  if (!UDE) {
+    // We don't have this directory yet, add it.  We use the string
+    // key from the SeenDirEntries map as the string.
+    UDE = new (DirsAlloc.Allocate()) DirectoryEntry();
+  }
+  return UDE;
+}
+
 /// Add all ancestors of the given path (pointing to either a file or
 /// a directory) as virtual directories.
 void FileManager::addAncestorsAsVirtualDirs(StringRef Path) {
@@ -99,10 +115,21 @@ void FileManager::addAncestorsAsVirtualDirs(StringRef Path) {
   if (NamedDirEnt.second)
     return;
 
-  // Add the virtual directory to the cache.
-  auto *UDE = new (DirsAlloc.Allocate()) DirectoryEntry();
-  NamedDirEnt.second = *UDE;
-  VirtualDirectoryEntries.push_back(UDE);
+  // Check to see if the directory exists.
+  llvm::vfs::Status Status;
+  auto statError =
+      getStatValue(DirName, Status, false, nullptr /*directory lookup*/);
+  if (statError) {
+    // There's no real directory at the given path.
+    // Add the virtual directory to the cache.
+    auto *UDE = new (DirsAlloc.Allocate()) DirectoryEntry();
+    NamedDirEnt.second = *UDE;
+    VirtualDirectoryEntries.push_back(UDE);
+  } else {
+    // There is the real directory
+    DirectoryEntry *&UDE = getRealDirEntry(Status);
+    NamedDirEnt.second = *UDE;
+  }
 
   // Recursively add the other ancestors.
   addAncestorsAsVirtualDirs(DirName);
@@ -162,17 +189,8 @@ FileManager::getDirectoryRef(StringRef DirName, bool CacheFailure) {
     return llvm::errorCodeToError(statError);
   }
 
-  // It exists.  See if we have already opened a directory with the
-  // same inode (this occurs on Unix-like systems when one dir is
-  // symlinked to another, for example) or the same path (on
-  // Windows).
-  DirectoryEntry *&UDE = UniqueRealDirs[Status.getUniqueID()];
-
-  if (!UDE) {
-    // We don't have this directory yet, add it.  We use the string
-    // key from the SeenDirEntries map as the string.
-    UDE = new (DirsAlloc.Allocate()) DirectoryEntry();
-  }
+  // It exists.
+  DirectoryEntry *&UDE = getRealDirEntry(Status);
   NamedDirEnt.second = *UDE;
 
   return DirectoryEntryRef(NamedDirEnt);

clang/test/Modules/implicit-module-remap.cpp

@@ -0,0 +1,21 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -fmodules -fmodule-map-file=module.modulemap -fmodules-cache-path=%t -remap-file "test.cpp;%t/test.cpp"  %t/test.cpp
+
+//--- a.h
+#define FOO
+
+//--- module.modulemap
+module a {
+  header "a.h"
+}
+
+//--- test.cpp
+#include "a.h"
+
+#ifndef FOO
+#error foo
+#endif
+