1 files changed, 78 insertions, 0 deletions
diff --git a/xpcom/io/FixedBufferOutputStream.h b/xpcom/io/FixedBufferOutputStream.h
new file mode 100644
index 0000000000..db7fba3c3a
--- /dev/null
+++ b/xpcom/io/FixedBufferOutputStream.h
@@ -0,0 +1,78 @@
+/* -*- 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 mozilla_FixedBufferOutputStream_h
+#define mozilla_FixedBufferOutputStream_h
+
+#include <cstddef>
+#include "mozilla/fallible.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Span.h"
+#include "mozilla/UniquePtr.h"
+#include "nsIOutputStream.h"
+#include "nsStringFwd.h"
+
+template <class T>
+class RefPtr;
+
+namespace mozilla {
+
+class StreamBufferSink;
+
+// An output stream so you can read your potentially-async input stream into
+// a contiguous buffer using NS_AsyncCopy. Back when streams were more
+// synchronous and people didn't know blocking I/O was bad, if you wanted to
+// read a stream into a flat buffer, you could use NS_ReadInputStreamToString
+// or NS_ReadInputStreamToBuffer.  But those don't work with async streams.
+// This can be used to replace hand-rolled Read/AsyncWait() loops.  Because you
+// specify the expected size up front, the buffer is pre-allocated so wasteful
+// reallocations can be avoided.
+class FixedBufferOutputStream final : public nsIOutputStream {
+  template <typename T, typename... Args>
+  friend RefPtr<T> MakeRefPtr(Args&&... aArgs);
+
+ public:
+  // Factory method to get a FixedBufferOutputStream by allocating a char buffer
+  // with the given length.
+  static RefPtr<FixedBufferOutputStream> Create(size_t aLength);
+
+  // Factory method to get a FixedBufferOutputStream by allocating a char buffer
+  // with the given length, fallible version.
+  static RefPtr<FixedBufferOutputStream> Create(size_t aLength,
+                                                const mozilla::fallible_t&);
+
+  // Factory method to get a FixedBufferOutputStream from a preallocated char
+  // buffer. The output stream doesn't take ownership of the char buffer, so the
+  // char buffer must outlive the output stream (to avoid a use-after-free).
+  static RefPtr<FixedBufferOutputStream> Create(mozilla::Span<char> aBuffer);
+
+  // Factory method to get a FixedBufferOutputStream from an arbitrary
+  // StreamBufferSink.
+  static RefPtr<FixedBufferOutputStream> Create(
+      UniquePtr<StreamBufferSink>&& aSink);
+
+  nsDependentCSubstring WrittenData();
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOUTPUTSTREAM
+
+ private:
+  explicit FixedBufferOutputStream(UniquePtr<StreamBufferSink>&& aSink);
+
+  ~FixedBufferOutputStream() = default;
+
+  const UniquePtr<StreamBufferSink> mSink;
+
+  Mutex mMutex;
+
+  size_t mOffset MOZ_GUARDED_BY(mMutex);
+  bool mWriting MOZ_GUARDED_BY(mMutex);
+  bool mClosed MOZ_GUARDED_BY(mMutex);
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_FixedBufferOutputStream_h