[llvm] [CAS] Add UnifiedOnDiskCache and OnDiskCAS (PR #114103)

[Justin Bogner via llvm-commits]

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@@ -0,0 +1,172 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CAS_UNIFIEDONDISKCACHE_H
+#define LLVM_CAS_UNIFIEDONDISKCACHE_H
+
+#include "llvm/CAS/BuiltinUnifiedCASDatabases.h"
+#include "llvm/CAS/OnDiskGraphDB.h"
+#include <atomic>
+
+namespace llvm::cas::ondisk {
+
+class OnDiskKeyValueDB;
+
+/// A unified CAS nodes and key-value database, using on-disk storage for both.
+/// It manages storage growth and provides APIs for garbage collection.
+///
+/// High-level properties:
+/// * While \p UnifiedOnDiskCache is open on a directory, by any process, the
+///   storage size in that directory will keep growing unrestricted. For data to
+///   become eligible for garbage-collection there should be no open instances
+///   of \p UnifiedOnDiskCache for that directory, by any process.
+/// * Garbage-collection needs to be triggered explicitly by the client. It can
+///   be triggered on a directory concurrently, at any time and by any process,
+///   without affecting any active readers/writers, in the same process or other
+///   processes.
+///
+/// Usage patterns should be that an instance of \p UnifiedOnDiskCache is open
+/// for a limited period of time, e.g. for the duration of a build operation.
+/// For long-living processes that need periodic access to a
+/// \p UnifiedOnDiskCache, the client should device a scheme where access is
----------------
bogner wrote:

```suggestion
/// \p UnifiedOnDiskCache, the client should devise a scheme where access is
```

https://github.com/llvm/llvm-project/pull/114103