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// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef COMPONENTS_TRACING_CORE_SCATTERED_STREAM_WRITER_H_
#define COMPONENTS_TRACING_CORE_SCATTERED_STREAM_WRITER_H_
#include <stdint.h>
#include "base/logging.h"
#include "base/macros.h"
#include "components/tracing/tracing_export.h"
namespace tracing {
namespace v2 {
struct ContiguousMemoryRange {
uint8_t* begin;
uint8_t* end; // STL style: one byte past the end of the buffer.
inline bool is_valid() const { return begin != nullptr; }
inline void reset() { begin = nullptr; }
inline size_t size() { return static_cast<size_t>(end - begin); }
};
// This class deals with the following problem: append-only proto messages want
// to write a stream of bytes, without caring about the implementation of the
// underlying buffer (which concretely will be either the trace ring buffer
// or a heap-allocated buffer). The main deal is: proto messages don't know in
// advance what their size will be.
// Due to the tracing buffer being split into fixed-size chunks, on some
// occasions, these writes need to be spread over two (or more) non-contiguous
// chunks of memory. Similarly, when the buffer is backed by the heap, we want
// to avoid realloc() calls, as they might cause a full copy of the contents
// of the buffer.
// The purpose of this class is to abtract away the non-contiguous write logic.
// This class knows how to deal with writes as long as they fall in the same
// ContiguousMemoryRange and defers the chunk-chaining logic to the Delegate.
class TRACING_EXPORT ScatteredStreamWriter {
public:
class TRACING_EXPORT Delegate {
public:
virtual ~Delegate();
virtual ContiguousMemoryRange GetNewBuffer() = 0;
};
explicit ScatteredStreamWriter(Delegate* delegate);
~ScatteredStreamWriter();
void WriteByte(uint8_t value);
void WriteBytes(const uint8_t* src, size_t size);
// Reserves a fixed amount of bytes to be backfilled later. The reserved range
// is guaranteed to be contiguous and not span across chunks.
ContiguousMemoryRange ReserveBytes(size_t size);
// Fast (but unsafe) version of the above. The caller must have previously
// checked that there are at least |size| contiguos bytes available.
// Returns only the start pointer of the reservation.
uint8_t* ReserveBytesUnsafe(size_t size) {
uint8_t* begin = write_ptr_;
write_ptr_ += size;
DCHECK_LE(write_ptr_, cur_range_.end);
return begin;
}
// Resets the buffer boundaries and the write pointer to the given |range|.
// Subsequent WriteByte(s) will write into |range|.
void Reset(ContiguousMemoryRange range);
// Number of contiguous free bytes in |cur_range_| that can be written without
// requesting a new buffer.
size_t bytes_available() const {
return static_cast<size_t>(cur_range_.end - write_ptr_);
}
uint8_t* write_ptr() const { return write_ptr_; }
private:
void Extend();
Delegate* const delegate_;
ContiguousMemoryRange cur_range_;
uint8_t* write_ptr_;
DISALLOW_COPY_AND_ASSIGN(ScatteredStreamWriter);
};
} // namespace v2
} // namespace tracing
#endif // COMPONENTS_TRACING_CORE_SCATTERED_STREAM_WRITER_H_
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