Modules: Cache PCMs in memory and avoid a use-after-free

Clang's internal build system for implicit modules uses lock files to
ensure that after a process writes a PCM it will read the same one back
in (without contention from other -cc1 commands).  Since PCMs are read
from disk repeatedly while invalidating, building, and importing, the
lock is not released quickly.  Furthermore, the LockFileManager is not
robust in every environment.  Other -cc1 commands can stall until
timeout (after about eight minutes).

This commit changes the lock file from being necessary for correctness
to a (possibly dubious) performance hack.  The remaining benefit is to
reduce duplicate work in competing -cc1 commands which depend on the
same module.  Follow-up commits will change the internal build system to
continue after a timeout, and reduce the timeout.  Perhaps we should
reconsider blocking at all.

This also fixes a use-after-free, when one part of a compilation
validates a PCM and starts using it, and another tries to swap out the
PCM for something new.

The PCMCache is a new type called MemoryBufferCache, which saves memory
buffers based on their filename.  Its ownership is shared by the
CompilerInstance and ModuleManager.

  - The ModuleManager stores PCMs there that it loads from disk, never
    touching the disk if the cache is hot.

  - When modules fail to validate, they're removed from the cache.

  - When a CompilerInstance is spawned to build a new module, each
    already-loaded PCM is assumed to be valid, and is frozen to avoid
    the use-after-free.

  - Any newly-built module is written directly to the cache to avoid the
    round-trip to the filesystem, making lock files unnecessary for
    correctness.

Original patch by Manman Ren; most testcases by Adrian Prantl!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298165 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 80 insertions, 0 deletions

diff --git a/include/clang/Basic/MemoryBufferCache.h b/include/clang/Basic/MemoryBufferCache.h
new file mode 100644
index 0000000000..c79c3c40e4
--- /dev/null
+++ b/include/clang/Basic/MemoryBufferCache.h
@@ -0,0 +1,80 @@
+//===- MemoryBufferCache.h - Cache for loaded memory buffers ----*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_BASIC_MEMORYBUFFERCACHE_H
+#define LLVM_CLANG_BASIC_MEMORYBUFFERCACHE_H
+
+#include "llvm/ADT/IntrusiveRefCntPtr.h"
+#include "llvm/ADT/StringMap.h"
+#include <memory>
+
+namespace llvm {
+class MemoryBuffer;
+} // end namespace llvm
+
+namespace clang {
+
+/// Manage memory buffers across multiple users.
+///
+/// Ensures that multiple users have a consistent view of each buffer.  This is
+/// used by \a CompilerInstance when building PCMs to ensure that each \a
+/// ModuleManager sees the same files.
+///
+/// \a finalizeCurrentBuffers() should be called before creating a new user.
+/// This locks in the current buffers, ensuring that no buffer that has already
+/// been accessed can be purged, preventing use-after-frees.
+class MemoryBufferCache : public llvm::RefCountedBase<MemoryBufferCache> {
+  struct BufferEntry {
+    std::unique_ptr<llvm::MemoryBuffer> Buffer;
+
+    /// Track the timeline of when this was added to the cache.
+    unsigned Index;
+  };
+
+  /// Cache of buffers.
+  llvm::StringMap<BufferEntry> Buffers;
+
+  /// Monotonically increasing index.
+  unsigned NextIndex = 0;
+
+  /// Bumped to prevent "older" buffers from being removed.
+  unsigned FirstRemovableIndex = 0;
+
+public:
+  /// Store the Buffer under the Filename.
+  ///
+  /// \pre There is not already buffer is not already in the cache.
+  /// \return a reference to the buffer as a convenience.
+  llvm::MemoryBuffer &addBuffer(llvm::StringRef Filename,
+                                std::unique_ptr<llvm::MemoryBuffer> Buffer);
+
+  /// Try to remove a buffer from the cache.
+  ///
+  /// \return false on success, iff \c !isBufferFinal().
+  bool tryToRemoveBuffer(llvm::StringRef Filename);
+
+  /// Get a pointer to the buffer if it exists; else nullptr.
+  llvm::MemoryBuffer *lookupBuffer(llvm::StringRef Filename);
+
+  /// Check whether the buffer is final.
+  ///
+  /// \return true iff \a finalizeCurrentBuffers() has been called since the
+  /// buffer was added.  This prevents buffers from being removed.
+  bool isBufferFinal(llvm::StringRef Filename);
+
+  /// Finalize the current buffers in the cache.
+  ///
+  /// Should be called when creating a new user to ensure previous uses aren't
+  /// invalidated.
+  void finalizeCurrentBuffers();
+};
+
+} // end namespace clang
+
+#endif // LLVM_CLANG_BASIC_MEMORYBUFFERCACHE_H