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

[Ivan Murashko via cfe-commits]

https://github.com/ivanmurashko created https://github.com/llvm/llvm-project/pull/91645

The `FileManager` might create a virtual directory that can be used later as a search path. If the added record is a real directory that is present on a disk, we might encounter a false cache miss and add the same directory twice:
- Once as a virtual directory 
- And once as a real one

This isn't a problem if the added directories have the same name. However, it could lead to compilation errors if the directory names are different. 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.

>From fc1b3832d8413594fb21ad5315570efa2c406be7 Mon Sep 17 00:00:00 2001
From: Ivan Murashko <ivanmurashko at meta.com>
Date: Thu, 9 May 2024 17:22:53 +0100
Subject: [PATCH] [clang] Processing for real directories added as virtual

Sometimes, FileManager might create a virtual directory that can
be used later as a search path. If there is a real directory present
on a disk, we might get a false cache miss and add the same directory
twice: once as a virtual directory and another time as a real one.
This could lead to compilation errors. For instance, the suggested LIT
test will fail with the following message:
```
fatal error: module 'a' was built in directory '...'
but now resides in directory '.'
```

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.
---
 clang/include/clang/Basic/FileManager.h      |  2 +
 clang/lib/Basic/FileManager.cpp              | 48 ++++++++++++++------
 clang/test/Modules/implicit-module-remap.cpp | 21 +++++++++
 3 files changed, 56 insertions(+), 15 deletions(-)
 create mode 100644 clang/test/Modules/implicit-module-remap.cpp

diff --git a/clang/include/clang/Basic/FileManager.h b/clang/include/clang/Basic/FileManager.h
index 8b4206e52cd48..e1f33d57a8980 100644
--- a/clang/include/clang/Basic/FileManager.h
+++ b/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.
   ///
diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
index 143c04309d075..1dc51deb82987 100644
--- a/clang/lib/Basic/FileManager.cpp
+++ b/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);
diff --git a/clang/test/Modules/implicit-module-remap.cpp b/clang/test/Modules/implicit-module-remap.cpp
new file mode 100644
index 0000000000000..47927b9694016
--- /dev/null
+++ b/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
+