1 files changed, 126 insertions, 0 deletions
diff --git a/xpcom/io/nsSegmentedBuffer.h b/xpcom/io/nsSegmentedBuffer.h
new file mode 100644
index 0000000000..c654956df9
--- /dev/null
+++ b/xpcom/io/nsSegmentedBuffer.h
@@ -0,0 +1,126 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsSegmentedBuffer_h__
+#define nsSegmentedBuffer_h__
+
+#include <stddef.h>
+#include <functional>
+
+#include "nsCOMPtr.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsTArray.h"
+#include "mozilla/DataMutex.h"
+
+class nsIEventTarget;
+
+class nsSegmentedBuffer {
+ public:
+  nsSegmentedBuffer()
+      : mSegmentSize(0),
+        mSegmentArray(nullptr),
+        mSegmentArrayCount(0),
+        mFirstSegmentIndex(0),
+        mLastSegmentIndex(0) {}
+
+  ~nsSegmentedBuffer() { Empty(); }
+
+  nsresult Init(uint32_t aSegmentSize);
+
+  char* AppendNewSegment();  // pushes at end
+
+  // returns true if no more segments remain:
+  bool DeleteFirstSegment();  // pops from beginning
+
+  // returns true if no more segments remain:
+  bool DeleteLastSegment();  // pops from beginning
+
+  // Call Realloc() on last segment.  This is used to reduce memory
+  // consumption when data is not an exact multiple of segment size.
+  bool ReallocLastSegment(size_t aNewSize);
+
+  void Empty();  // frees all segments
+
+  inline uint32_t GetSegmentCount() {
+    if (mFirstSegmentIndex <= mLastSegmentIndex) {
+      return mLastSegmentIndex - mFirstSegmentIndex;
+    } else {
+      return mSegmentArrayCount + mLastSegmentIndex - mFirstSegmentIndex;
+    }
+  }
+
+  inline uint32_t GetSegmentSize() { return mSegmentSize; }
+
+  inline char* GetSegment(uint32_t aIndex) {
+    NS_ASSERTION(aIndex < GetSegmentCount(), "index out of bounds");
+    int32_t i = ModSegArraySize(mFirstSegmentIndex + (int32_t)aIndex);
+    return mSegmentArray[i];
+  }
+
+ protected:
+  inline int32_t ModSegArraySize(int32_t aIndex) {
+    uint32_t result = aIndex & (mSegmentArrayCount - 1);
+    NS_ASSERTION(result == aIndex % mSegmentArrayCount,
+                 "non-power-of-2 mSegmentArrayCount");
+    return result;
+  }
+
+  inline bool IsFull() {
+    return ModSegArraySize(mLastSegmentIndex + 1) == mFirstSegmentIndex;
+  }
+
+ protected:
+  uint32_t mSegmentSize;
+  char** mSegmentArray;
+  uint32_t mSegmentArrayCount;
+  int32_t mFirstSegmentIndex;
+  int32_t mLastSegmentIndex;
+
+ private:
+  class FreeOMTPointers {
+    NS_INLINE_DECL_THREADSAFE_REFCOUNTING(FreeOMTPointers)
+
+   public:
+    FreeOMTPointers() : mTasks("nsSegmentedBuffer::FreeOMTPointers") {}
+
+    void FreeAll();
+
+    // Adds a task to the array. Returns the size of the array.
+    size_t AddTask(std::function<void()>&& aTask) {
+      auto tasks = mTasks.Lock();
+      tasks->AppendElement(std::move(aTask));
+      return tasks->Length();
+    }
+
+   private:
+    ~FreeOMTPointers() = default;
+
+    mozilla::DataMutex<nsTArray<std::function<void()>>> mTasks;
+  };
+
+  void FreeOMT(void* aPtr);
+  void FreeOMT(std::function<void()>&& aTask);
+
+  nsCOMPtr<nsIEventTarget> mIOThread;
+
+  // This object is created the first time we need to dispatch to another thread
+  // to free segments. It is only freed when the nsSegmentedBufer is destroyed
+  // or when the runnable is finally handled and its refcount goes to 0.
+  RefPtr<FreeOMTPointers> mFreeOMT;
+};
+
+// NS_SEGMENTARRAY_INITIAL_SIZE: This number needs to start out as a
+// power of 2 given how it gets used. We double the segment array
+// when we overflow it, and use that fact that it's a power of 2
+// to compute a fast modulus operation in IsFull.
+//
+// 32 segment array entries can accommodate 128k of data if segments
+// are 4k in size. That seems like a reasonable amount that will avoid
+// needing to grow the segment array.
+#define NS_SEGMENTARRAY_INITIAL_COUNT 32
+
+#endif  // nsSegmentedBuffer_h__