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diff --git a/dom/media/MediaBlockCacheBase.h b/dom/media/MediaBlockCacheBase.h
new file mode 100644
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+++ b/dom/media/MediaBlockCacheBase.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef MEDIA_BLOCK_CACHE_BASE_H_
+#define MEDIA_BLOCK_CACHE_BASE_H_
+
+#include "MediaCache.h"
+#include "mozilla/Span.h"
+
+namespace mozilla {
+
+// Manages block management for the media cache. Data comes in over the network
+// via callbacks on the main thread, however we don't want to write the
+// incoming data to the media cache on the main thread, as this could block
+// causing UI jank.
+//
+// So MediaBlockCacheBase provides an abstraction for a temporary memory buffer
+// or file accessible as an array of blocks, which supports a block move
+// operation, and allows synchronous reading and writing from any thread, with
+// writes being buffered as needed so as not to block.
+//
+// Writes and cache block moves (which require reading) may be deferred to
+// their own non-main thread. This object also ensures that data which has
+// been scheduled to be written, but hasn't actually *been* written, is read
+// as if it had, i.e. pending writes are cached in readable memory until
+// they're flushed to file.
+//
+// To improve efficiency, writes can only be done at block granularity,
+// whereas reads can be done with byte granularity.
+//
+// Note it's also recommended not to read from the media cache from the main
+// thread to prevent jank.
+class MediaBlockCacheBase {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(MediaBlockCacheBase)
+
+  static_assert(
+      MediaCacheStream::BLOCK_SIZE <
+          static_cast<
+              std::remove_const<decltype(MediaCacheStream::BLOCK_SIZE)>::type>(
+              INT32_MAX),
+      "MediaCacheStream::BLOCK_SIZE should fit in 31 bits");
+  static const int32_t BLOCK_SIZE = MediaCacheStream::BLOCK_SIZE;
+
+ protected:
+  virtual ~MediaBlockCacheBase() = default;
+
+ public:
+  // Initialize this cache.
+  virtual nsresult Init() = 0;
+
+  // Erase data and discard pending changes to reset the cache to its pristine
+  // state as it was after Init().
+  virtual void Flush() = 0;
+
+  // Maximum number of blocks expected in this block cache. (But allow overflow
+  // to accomodate incoming traffic before MediaCache can handle it.)
+  virtual size_t GetMaxBlocks(size_t aCacheSizeInKiB) const = 0;
+
+  // Can be called on any thread. This defers to a non-main thread.
+  virtual nsresult WriteBlock(uint32_t aBlockIndex, Span<const uint8_t> aData1,
+                              Span<const uint8_t> aData2) = 0;
+
+  // Synchronously reads data from file. May read from file or memory
+  // depending on whether written blocks have been flushed to file yet.
+  // Not recommended to be called from the main thread, as can cause jank.
+  virtual nsresult Read(int64_t aOffset, uint8_t* aData, int32_t aLength,
+                        int32_t* aBytes) = 0;
+
+  // Moves a block asynchronously. Can be called on any thread.
+  // This defers file I/O to a non-main thread.
+  virtual nsresult MoveBlock(int32_t aSourceBlockIndex,
+                             int32_t aDestBlockIndex) = 0;
+};
+
+}  // End namespace mozilla.
+
+#endif /* MEDIA_BLOCK_CACHE_BASE_H_ */