[llvm] [CAS] Add OnDiskDataAllocator (PR #161264)

[Paul Kirth via llvm-commits]

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@@ -0,0 +1,94 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// This file declares interface for OnDiskDataAllocator, a file backed data
+/// pool can be used to allocate space to store data packed in a single file. It
+/// is based on MappedFileRegionArena and includes a header in the beginning to
+/// provide metadata.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CAS_ONDISKDATAALLOCATOR_H
+#define LLVM_CAS_ONDISKDATAALLOCATOR_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/CAS/FileOffset.h"
+#include "llvm/Support/Error.h"
+
+namespace llvm::cas {
+
+/// Sink for data. Stores variable length data with 8-byte alignment. Does not
+/// track size of data, which is assumed to known from context, or embedded.
+/// Uses 0-padding but does not guarantee 0-termination.
+class OnDiskDataAllocator {
+public:
+  using ValueProxy = MutableArrayRef<char>;
+
+  /// An iterator-like return value for data insertion. Maybe it should be
+  /// called \c iterator, but it has no increment.
+  class pointer {
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ilovepi wrote:

Maybe `OnDiskPtr`? `OnDiskAddr`?

https://github.com/llvm/llvm-project/pull/161264