116 lines
4 KiB
C++
116 lines
4 KiB
C++
/*
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* Copyright (C) 2016 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef MESSAGE_BUFFER_H_
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#define MESSAGE_BUFFER_H_
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#include <cstdint>
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#include <memory>
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#include <tuple>
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#include "android-base/macros.h"
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#include "wifilogd/local_utils.h"
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namespace android {
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namespace wifilogd {
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// A fixed-size buffer, which provides FIFO access to read and write
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// a sequence of messages.
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class MessageBuffer {
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public:
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// A wrapper which guarantees that a MessageBuffer will be rewound,
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// when the program exits the wrapper's scope. The user must ensure that
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// |buffer| does not expire before the ScopedRewinder.
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class ScopedRewinder {
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public:
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explicit ScopedRewinder(NONNULL MessageBuffer* buffer) : buffer_(buffer) {}
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~ScopedRewinder() { buffer_->Rewind(); }
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private:
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MessageBuffer* const buffer_;
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};
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// Constructs the buffer. |size| must be greater than GetHeaderSize().
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explicit MessageBuffer(size_t size);
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// Appends a single message to the buffer. |data_len| must be >=1. Returns
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// true if the message was added to the buffer.
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bool Append(NONNULL const uint8_t* data, uint16_t data_len);
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// Returns true if the buffer is large enough to hold |length| bytes of user
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// data, when the buffer is empty.
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bool CanFitEver(uint16_t length) const;
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// Returns true if the buffer currently has enough free space to hold |length|
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// bytes of user data.
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bool CanFitNow(uint16_t length) const;
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// Clears the buffer. An immediately following read operation will return an
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// empty message. An immediately following write operation will write to the
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// head of the buffer. Clearing may be lazy (i.e., underlying storage is not
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// necessarily zeroed).
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void Clear();
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// Returns the first unread message in the buffer. If there is no such
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// message, returns {nullptr, 0}. MessageBuffer retains ownership of the
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// message's storage.
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std::tuple<const uint8_t*, size_t> ConsumeNextMessage();
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// Returns the size of MessageBuffer's per-message header.
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static constexpr size_t GetHeaderSize() { return sizeof(LengthHeader); }
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// Returns the total available free space in the buffer. This may be
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// larger than the usable space, due to overheads.
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size_t GetFreeSize() const { return capacity_ - write_pos_; }
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// Resets the read pointer to the start of the buffer. An immediately
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// following read will return the first message in the buffer. An immediately
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// following write, however, will be placed at the same position as if
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// Rewind() had not been called.
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void Rewind();
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private:
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struct LengthHeader {
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uint16_t payload_len;
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};
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// Prepares a header, and writes that header into the buffer.
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void AppendHeader(uint16_t message_len);
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// Writes arbitrary data into the buffer.
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void AppendRawBytes(NONNULL const void* data_start, size_t data_len);
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// Returns the total number of bytes available for reading. This number
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// includes headers.
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size_t GetReadableSize() const { return write_pos_ - read_pos_; }
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const std::unique_ptr<uint8_t[]> data_;
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const size_t capacity_;
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size_t read_pos_;
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size_t write_pos_;
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// MessageBuffer is a value type, so it would be semantically reasonable to
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// support copy and assign. Performance-wise, though, we should avoid
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// copies. Remove the copy constructor and the assignment operator, to
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// ensure that we don't accidentally make copies.
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DISALLOW_COPY_AND_ASSIGN(MessageBuffer);
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};
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} // namespace wifilogd
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} // namespace android
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#endif // MESSAGE_BUFFER_H_
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