[Clang] Expose RequiresNullTerminator in FileManager.

This is needed to fix the reason
0a2be46cfdb698fe (Modules: Invalidate out-of-date PCMs as they're
discovered) and 5b44a4b07fc1d ([modules] Do not cache invalid state for
modules that we attempted to load.) were reverted.

These patches changed Clang to use `isVolatile` when loading modules.
This had the side effect of not using mmap when loading modules, and
thus greatly increased memory usage.

The reason it wasn't using mmap is because `MemoryBuffer` plays some
games with file size when you request null termination, and it has to
disable these when `isVolatile` is set as the size may change by the
time it's mmapped. Clang by default passes
`RequiresNullTerminator = true`, and `shouldUseMmap` ignored if
`RequiresNullTerminator` was even requested.

This patch adds `RequiresNullTerminator` to the `FileManager` interface
so Clang can use it when loading modules, and changes `shouldUseMmap` to
only take volatility into account if `RequiresNullTerminator` is true.
This is fine as both `mmap` and a `read` loop are vulnerable to
modifying the file while reading, but are immune to the rename Clang
does when replacing a module file.

Differential Revision: https://reviews.llvm.org/D77772

1 files changed, 12 insertions, 10 deletions

diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
index ac8af8f..e92e9d5 100644
--- a/clang/lib/Basic/FileManager.cpp
+++ b/clang/lib/Basic/FileManager.cpp
@@ -458,7 +458,8 @@ void FileManager::fillRealPathName(FileEntry *UFE, llvm::StringRef FileName) {
 }
 
 llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
-FileManager::getBufferForFile(const FileEntry *Entry, bool isVolatile) {
+FileManager::getBufferForFile(const FileEntry *Entry, bool isVolatile,
+                              bool RequiresNullTerminator) {
   uint64_t FileSize = Entry->getSize();
   // If there's a high enough chance that the file have changed since we
   // got its size, force a stat before opening it.
@@ -468,28 +469,29 @@ FileManager::getBufferForFile(const FileEntry *Entry, bool isVolatile) {
   StringRef Filename = Entry->getName();
   // If the file is already open, use the open file descriptor.
   if (Entry->File) {
-    auto Result =
-        Entry->File->getBuffer(Filename, FileSize,
-                               /*RequiresNullTerminator=*/true, isVolatile);
+    auto Result = Entry->File->getBuffer(Filename, FileSize,
+                                         RequiresNullTerminator, isVolatile);
     Entry->closeFile();
     return Result;
   }
 
   // Otherwise, open the file.
-  return getBufferForFileImpl(Filename, FileSize, isVolatile);
+  return getBufferForFileImpl(Filename, FileSize, isVolatile,
+                              RequiresNullTerminator);
 }
 
 llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
 FileManager::getBufferForFileImpl(StringRef Filename, int64_t FileSize,
-                                  bool isVolatile) {
+                                  bool isVolatile,
+                                  bool RequiresNullTerminator) {
   if (FileSystemOpts.WorkingDir.empty())
-    return FS->getBufferForFile(Filename, FileSize,
-                                /*RequiresNullTerminator=*/true, isVolatile);
+    return FS->getBufferForFile(Filename, FileSize, RequiresNullTerminator,
+                                isVolatile);
 
   SmallString<128> FilePath(Filename);
   FixupRelativePath(FilePath);
-  return FS->getBufferForFile(FilePath, FileSize,
-                              /*RequiresNullTerminator=*/true, isVolatile);
+  return FS->getBufferForFile(FilePath, FileSize, RequiresNullTerminator,
+                              isVolatile);
 }
 
 /// getStatValue - Get the 'stat' information for the specified path,