pw_ring_buffer/public/pw_ring_buffer/prefixed_entry_ring_buffer.h - pigweed/pigweed - Git at Google

 // Copyright 2020 The Pigweed Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
 // use this file except in compliance with the License. You may obtain a copy of
 // the License at
 //
 //     https://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 // WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 // License for the specific language governing permissions and limitations under
 // the License.
 #pragma once

 #include <cstddef>

 #include "pw_containers/intrusive_list.h"
 #include "pw_result/result.h"
 #include "pw_span/span.h"
 #include "pw_status/status.h"

 namespace pw {
 namespace ring_buffer {

 // A circular ring buffer for arbitrary length data entries. Each PushBack()
 // produces a buffer entry. Each entry consists of a preamble followed by an
 // arbitrary length data chunk. The preamble is comprised of an optional user
 // preamble byte and an always present varint. The varint encodes the number of
 // bytes in the data chunk. This is a FIFO queue, with the oldest entries at
 // the 'front' (to be processed by readers) and the newest entries at the 'back'
 // (where the writer pushes to).
 //
 // The ring buffer supports multiple readers, which can be attached/detached
 // from the buffer. Each reader has its own read pointer and can peek and pop
 // the entry at the head. Entries are not bumped out from the buffer until all
 // readers have moved past that entry, or if the buffer is at capacity and space
 // is needed to push a new entry. When making space, the buffer will push slow
 // readers forward to the new oldest entry. Entries are internally wrapped
 // around as needed.
 class PrefixedEntryRingBufferMulti {
  public:
   typedef Status (*ReadOutput)(span<const std::byte>);

   // A reader that provides a single-reader interface into the multi-reader ring
   // buffer it has been attached to via AttachReader(). Readers maintain their
   // read position in the ring buffer as well as the remaining count of entries
   // from that position.
   //
   // If no readers are currently attached, the reader starts at the current
   // write head. If readers are currently attached, the reader is set to the
   // location and entry count of the slowest reader in the set.
   //
   // Readers can peek and pop entries similar to the single-reader interface.
   // When popping entries, although the reader moves forward and drops the
   // entry, the entry is not removed from the ring buffer until all other
   // attached readers have moved past that entry.
   //
   // When the attached ring buffer needs to make space, it may push the reader
   // index forward. Users of this class should consider the possibility of data
   // loss if they read slower than the writer.
   class Reader : public IntrusiveList<Reader>::Item {
    public:
     constexpr Reader() : buffer_(nullptr), read_idx_(0), entry_count_(0) {}

     // TODO(b/235351035): Add locking to the internal functions. Who owns the
     // lock? This class? Does this class need a lock if it's not a multi-reader?
     // (One doesn't exist today but presumably nothing prevents push + pop
     // operations from happening on two different threads).

     // Read the oldest stored data chunk of data from the ring buffer to
     // the provided destination span. The number of bytes read is written
     // to bytes_read
     //
     // Precondition: the buffer data must not be corrupt, otherwise there will
     // be a crash.
     //
     // Return values:
     // OK - Data successfully read from the ring buffer.
     // FAILED_PRECONDITION - Buffer not initialized.
     // OUT_OF_RANGE - No entries in ring buffer to read.
     // RESOURCE_EXHAUSTED - Destination data span was smaller number of
     // bytes than the data size of the data chunk being read.  Available
     // destination bytes were filled, remaining bytes of the data chunk were
     // ignored.
     Status PeekFront(span<std::byte> data, size_t* bytes_read_out) const {
       return buffer_->InternalPeekFront(*this, data, bytes_read_out);
     }

     Status PeekFront(ReadOutput output) const {
       return buffer_->InternalPeekFront(*this, output);
     }

     // Peek the front entry's preamble only to avoid copying data unnecessarily.
     //
     // Precondition: the buffer data must not be corrupt, otherwise there will
     // be a crash.
     Status PeekFrontPreamble(uint32_t& user_preamble_out) const {
       return buffer_->InternalPeekFrontPreamble(*this, user_preamble_out);
     }

     // Same as PeekFront but includes the entry's preamble of optional user
     // value and the varint of the data size.
     // TODO(b/235351847): Move all other APIs to passing bytes_read by
     // reference, as it is required to determine the length populated in the
     // span.
     Status PeekFrontWithPreamble(span<std::byte> data,
                                  uint32_t& user_preamble_out,
                                  size_t& entry_bytes_read_out) const;

     Status PeekFrontWithPreamble(span<std::byte> data,
                                  size_t* bytes_read_out) const {
       return buffer_->InternalPeekFrontWithPreamble(
           *this, data, bytes_read_out);
     }

     Status PeekFrontWithPreamble(ReadOutput output) const {
       return buffer_->InternalPeekFrontWithPreamble(*this, output);
     }

     // Pop and discard the oldest stored data chunk of data from the ring
     // buffer.
     //
     // Precondition: the buffer data must not be corrupt, otherwise there will
     // be a crash.
     //
     // Return values:
     // OK - Data successfully read from the ring buffer.
     // FAILED_PRECONDITION - Buffer not initialized.
     // OUT_OF_RANGE - No entries in ring buffer to pop.
     Status PopFront() { return buffer_->InternalPopFront(*this); }

     // Get the size in bytes of the next chunk, not including preamble, to be
     // read.
     //
     // Precondition: the buffer data must not be corrupt, otherwise there will
     // be a crash.
     size_t FrontEntryDataSizeBytes() const {
       return buffer_->InternalFrontEntryDataSizeBytes(*this);
     }

     // Get the size in bytes of the next chunk, including preamble and data
     // chunk, to be read.
     //
     // Precondition: the buffer data must not be corrupt, otherwise there will
     // be a crash.
     size_t FrontEntryTotalSizeBytes() const {
       return buffer_->InternalFrontEntryTotalSizeBytes(*this);
     }

     // Get the number of variable-length entries currently in the ring buffer.
     //
     // Return value:
     // Entry count.
     size_t EntryCount() const { return entry_count_; }

    private:
     friend PrefixedEntryRingBufferMulti;

     // Internal constructors for the iterator class to create Reader instances
     // at specific positions. Readers constructed through this interface cannot
     // be attached/detached from the multisink.
     constexpr Reader(Reader& reader)
         : Reader(reader.buffer_, reader.read_idx_, reader.entry_count_) {}
     constexpr Reader(PrefixedEntryRingBufferMulti* buffer,
                      size_t read_idx,
                      size_t entry_count)
         : buffer_(buffer), read_idx_(read_idx), entry_count_(entry_count) {}

     PrefixedEntryRingBufferMulti* buffer_;
     size_t read_idx_;
     size_t entry_count_;
   };

   // An entry returned by the iterator containing the byte span of the entry
   // and preamble data (if the ring buffer was configured with a preamble).
   struct Entry {
     span<const std::byte> buffer;
     uint32_t preamble;
   };

   // An iterator that can be used to walk through all entries from a given
   // Reader position, without mutating the underlying buffer. This is useful in
   // crash contexts where all available entries in the buffer must be acquired,
   // even those that have already been consumed by all attached readers.
   class iterator {
    public:
     iterator() : ring_buffer_(nullptr), read_idx_(0), entry_count_(0) {}
     iterator(Reader& reader)
         : ring_buffer_(reader.buffer_),
           read_idx_(0),
           entry_count_(reader.entry_count_) {
       Status dering_result = ring_buffer_->InternalDering(reader);
       PW_DASSERT(dering_result.ok());
     }

     iterator& operator++();
     iterator operator++(int) {
       iterator original = *this;
       ++*this;
       return original;
     }

     // Returns entry at current position.
     const Entry& operator*() const;
     const Entry* operator->() const { return &operator*(); }

     constexpr bool operator==(const iterator& rhs) const {
       return entry_count_ == rhs.entry_count_;
     }

     constexpr bool operator!=(const iterator& rhs) const {
       return entry_count_ != rhs.entry_count_;
     }

     // Returns the status of the last iteration operation. If the iterator
     // fails to read an entry, it will move to iterator::end() and indicate
     // the failure reason here.
     Status status() const { return iteration_status_; }

    private:
     static constexpr Entry kEndEntry = {
         .buffer = span<const std::byte>(),
         .preamble = 0,
     };

     void SkipToEnd(Status status) {
       iteration_status_ = status;
       entry_ = kEndEntry;
       entry_count_ = 0;
     }

     PrefixedEntryRingBufferMulti* ring_buffer_;
     size_t read_idx_;
     size_t entry_count_;

     mutable Entry entry_;
     Status iteration_status_;
   };

   using element_type = const Entry;
   using value_type = std::remove_cv_t<const Entry>;
   using pointer = const Entry;
   using reference = const Entry&;
   using const_iterator = iterator;  // Standard alias for iterable types.

   iterator begin() { return iterator(GetSlowestReaderWritable()); }
   iterator end() { return iterator(); }
   const_iterator cbegin() { return begin(); }
   const_iterator cend() { return end(); }

   // TODO(b/235351861): Consider changing bool to an enum, to explicitly
   // enumerate what this variable means in clients.
   PrefixedEntryRingBufferMulti(bool user_preamble = false)
       : buffer_(nullptr),
         buffer_bytes_(0),
         write_idx_(0),
         user_preamble_(user_preamble) {}

   // Set the raw buffer to be used by the ring buffer.
   //
   // Return values:
   // OK - successfully set the raw buffer.
   // INVALID_ARGUMENT - Argument was nullptr, size zero, or too large.
   Status SetBuffer(span<std::byte> buffer);

   // Determines if the ring buffer has corrupted entries.
   //
   // Precondition: At least one reader must be attached to the ring buffer.
   // Return values:
   // OK - No corruption was detected.
   // DATA_LOSS - Corruption was detected.
   Status CheckForCorruption() {
     iterator it = begin();
     for (; it != end(); ++it) {
     }
     return it.status();
   }

   // Attach reader to the ring buffer. Readers can only be attached to one
   // ring buffer at a time.
   //
   // Return values:
   // OK - Successfully configured reader for ring buffer.
   // INVALID_ARGUMENT - Argument was already attached to another ring buffer.
   Status AttachReader(Reader& reader);

   // Detach reader from the ring buffer. Readers can only be detached if they
   // were previously attached.
   //
   // Return values:
   // OK - Successfully removed reader for ring buffer.
   // INVALID_ARGUMENT - Argument was not previously attached to this ring
   // buffer.
   Status DetachReader(Reader& reader);

   // Removes all data from the ring buffer.
   void Clear();

   // Write a chunk of data to the ring buffer. If available space is less than
   // size of data chunk to be written then silently pop and discard oldest
   // stored data chunks until space is available.
   //
   // Preamble argument is a caller-provided value prepended to the front of the
   // entry. It is only used if user_preamble was set at class construction
   // time. It is varint-encoded before insertion into the buffer.
   //
   // Return values:
   // OK - Data successfully written to the ring buffer.
   // FAILED_PRECONDITION - Buffer not initialized.
   // OUT_OF_RANGE - Size of data is greater than buffer size.
   Status PushBack(span<const std::byte> data, uint32_t user_preamble_data = 0) {
     return InternalPushBack(data, user_preamble_data, true);
   }

   // [Deprecated] An implementation of PushBack that accepts a single-byte as
   // preamble data. Clients should migrate to passing uint32_t preamble data.
   Status PushBack(span<const std::byte> data, std::byte user_preamble_data) {
     return PushBack(data, static_cast<uint32_t>(user_preamble_data));
   }

   // Write a chunk of data to the ring buffer if there is space available.
   //
   // Preamble argument is a caller-provided value prepended to the front of the
   // entry. It is only used if user_preamble was set at class construction
   // time. It is varint-encoded before insertion into the buffer.
   //
   // Precondition: the buffer data must not be corrupt, otherwise there will
   // be a crash.
   //
   // Return values:
   // OK - Data successfully written to the ring buffer.
   // INVALID_ARGUMENT - Size of data to write is zero bytes
   // FAILED_PRECONDITION - Buffer not initialized.
   // OUT_OF_RANGE - Size of data is greater than buffer size.
   // RESOURCE_EXHAUSTED - The ring buffer doesn't have space for the data
   // without popping off existing elements.
   Status TryPushBack(span<const std::byte> data,
                      uint32_t user_preamble_data = 0) {
     return InternalPushBack(data, user_preamble_data, false);
   }

   // [Deprecated] An implementation of TryPushBack that accepts a single-byte as
   // preamble data. Clients should migrate to passing uint32_t preamble data.
   Status TryPushBack(span<const std::byte> data, std::byte user_preamble_data) {
     return TryPushBack(data, static_cast<uint32_t>(user_preamble_data));
   }

   // Get the size in bytes of all the current entries in the ring buffer,
   // including preamble and data chunk.
   size_t TotalUsedBytes() const { return buffer_bytes_ - RawAvailableBytes(); }

   // Returns total size of ring buffer in bytes.
   size_t TotalSizeBytes() const { return buffer_bytes_; }

   // Dering the buffer by reordering entries internally in the buffer by
   // rotating to have the oldest entry is at the lowest address/index with
   // newest entry at the highest address. If no readers are attached, the buffer
   // is deringed at the current write index.
   //
   // Return values:
   // OK - Buffer data successfully deringed.
   // FAILED_PRECONDITION - Buffer not initialized.
   Status Dering();

  private:
   // Read the oldest stored data chunk of data from the ring buffer to
   // the provided destination span. The number of bytes read is written to
   // `bytes_read_out`.
   //
   // Precondition: the buffer data must not be corrupt, otherwise there will
   // be a crash.
   //
   // Return values:
   // OK - Data successfully read from the ring buffer.
   // FAILED_PRECONDITION - Buffer not initialized.
   // OUT_OF_RANGE - No entries in ring buffer to read.
   // RESOURCE_EXHAUSTED - Destination data span was smaller number of bytes
   // than the data size of the data chunk being read.  Available destination
   // bytes were filled, remaining bytes of the data chunk were ignored.
   Status InternalPeekFront(const Reader& reader,
                            span<std::byte> data,
                            size_t* bytes_read_out) const;
   Status InternalPeekFront(const Reader& reader, ReadOutput output) const;

   Status InternalPeekFrontPreamble(const Reader& reader,
                                    uint32_t& user_preamble_out) const;
   // Same as Read but includes the entry's preamble of optional user value and
   // the varint of the data size
   Status InternalPeekFrontWithPreamble(const Reader& reader,
                                        span<std::byte> data,
                                        size_t* bytes_read_out) const;
   Status InternalPeekFrontWithPreamble(const Reader& reader,
                                        ReadOutput output) const;

   // Pop and discard the oldest stored data chunk of data from the ring buffer.
   //
   // Precondition: the buffer data must not be corrupt, otherwise there will
   // be a crash.
   //
   // Return values:
   // OK - Data successfully read from the ring buffer.
   // FAILED_PRECONDITION - Buffer not initialized.
   // OUT_OF_RANGE - No entries in ring buffer to pop.
   Status InternalPopFront(Reader& reader);

   // Get the size in bytes of the next chunk, not including preamble, to be
   // read.
   size_t InternalFrontEntryDataSizeBytes(const Reader& reader) const;

   // Get the size in bytes of the next chunk, including preamble and data
   // chunk, to be read.
   size_t InternalFrontEntryTotalSizeBytes(const Reader& reader) const;

   // Internal version of Read used by all the public interface versions. T
   // should be of type ReadOutput.
   template <typename T>
   Status InternalRead(const Reader& reader,
                       T read_output,
                       bool include_preamble_in_output,
                       uint32_t* user_preamble_out = nullptr) const;

   // Dering the buffer by reordering entries internally in the buffer by
   // rotating to have the oldest entry is at the lowest address/index with
   // newest entry at the highest address. If no readers are attached, the buffer
   // is deringed at the current write index.
   //
   // Return values:
   // OK - Buffer data successfully deringed.
   // FAILED_PRECONDITION - Buffer not initialized.
   Status InternalDering(Reader& reader);

   struct EntryInfo {
     size_t preamble_bytes;
     uint32_t user_preamble;
     size_t data_bytes;
   };

   // Push back implementation, which optionally discards front elements to fit
   // the incoming element.
   Status InternalPushBack(span<const std::byte> data,
                           uint32_t user_preamble_data,
                           bool pop_front_if_needed);

   // Internal function to pop all of the slowest readers. This function may pop
   // multiple readers if multiple are slow.
   //
   // Precondition: This function requires that at least one reader is attached
   // and has at least one entry to pop. There will be a crash if data is
   // corrupted.
   void InternalPopFrontAll();

   // Returns a the slowest reader in the list.
   //
   // Precondition: This function requires that at least one reader is attached.
   const Reader& GetSlowestReader() const;
   Reader& GetSlowestReaderWritable() {
     return const_cast<Reader&>(GetSlowestReader());
   }

   // Get info struct with the size of the preamble and data chunk for the next
   // entry to be read. Calls RawFrontEntryInfo and asserts on failure.
   //
   // Precondition: the buffer data must not be corrupt, otherwise there will
   // be a crash.
   EntryInfo FrontEntryInfo(const Reader& reader) const;

   // Get info struct with the size of the preamble and data chunk for the next
   // entry to be read.
   //
   // Returns:
   // OK - EntryInfo containing the next entry metadata.
   // DATA_LOSS - Failed to read the metadata at this location.
   Result<EntryInfo> RawFrontEntryInfo(size_t source_idx) const;

   // Get the raw number of available bytes free in the ring buffer. This is
   // not available bytes for data, since there is a variable size preamble for
   // each entry.
   size_t RawAvailableBytes() const;

   // Do the basic write of the specified number of bytes starting at the last
   // write index of the ring buffer to the destination, handing any wrap-around
   // of the ring buffer. This is basic, raw operation with no safety checks.
   void RawWrite(span<const std::byte> source);

   // Do the basic read of the specified number of bytes starting at the given
   // index of the ring buffer to the destination, handing any wrap-around of
   // the ring buffer. This is basic, raw operation with no safety checks.
   void RawRead(std::byte* destination,
                size_t source_idx,
                size_t length_bytes) const;

   size_t IncrementIndex(size_t index, size_t count) const;

   std::byte* buffer_;
   size_t buffer_bytes_;

   size_t write_idx_;
   const bool user_preamble_;

   // List of attached readers.
   IntrusiveList<Reader> readers_;

   // Maximum bufer size allowed. Restricted to this to allow index aliasing to
   // not overflow.
   static constexpr size_t kMaxBufferBytes =
       std::numeric_limits<size_t>::max() / 2;
 };

 class PrefixedEntryRingBuffer : public PrefixedEntryRingBufferMulti,
                                 public PrefixedEntryRingBufferMulti::Reader {
  public:
   PrefixedEntryRingBuffer(bool user_preamble = false)
       : PrefixedEntryRingBufferMulti(user_preamble) {
     AttachReader(*this)
         .IgnoreError();  // TODO(b/242598609): Handle Status properly
   }
 };

 }  // namespace ring_buffer
 }  // namespace pw
	// Copyright 2020 The Pigweed Authors
	//
	// Licensed under the Apache License, Version 2.0 (the "License"); you may not
	// use this file except in compliance with the License. You may obtain a copy of
	// the License at
	//
	// https://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
	// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
	// License for the specific language governing permissions and limitations under
	// the License.
	#pragma once

	#include <cstddef>

	#include "pw_containers/intrusive_list.h"
	#include "pw_result/result.h"
	#include "pw_span/span.h"
	#include "pw_status/status.h"

	namespace pw {
	namespace ring_buffer {

	// A circular ring buffer for arbitrary length data entries. Each PushBack()
	// produces a buffer entry. Each entry consists of a preamble followed by an
	// arbitrary length data chunk. The preamble is comprised of an optional user
	// preamble byte and an always present varint. The varint encodes the number of
	// bytes in the data chunk. This is a FIFO queue, with the oldest entries at
	// the 'front' (to be processed by readers) and the newest entries at the 'back'
	// (where the writer pushes to).
	//
	// The ring buffer supports multiple readers, which can be attached/detached
	// from the buffer. Each reader has its own read pointer and can peek and pop
	// the entry at the head. Entries are not bumped out from the buffer until all
	// readers have moved past that entry, or if the buffer is at capacity and space
	// is needed to push a new entry. When making space, the buffer will push slow
	// readers forward to the new oldest entry. Entries are internally wrapped
	// around as needed.
	class PrefixedEntryRingBufferMulti {
	public:
	typedef Status (*ReadOutput)(span<const std::byte>);

	// A reader that provides a single-reader interface into the multi-reader ring
	// buffer it has been attached to via AttachReader(). Readers maintain their
	// read position in the ring buffer as well as the remaining count of entries
	// from that position.
	//
	// If no readers are currently attached, the reader starts at the current
	// write head. If readers are currently attached, the reader is set to the
	// location and entry count of the slowest reader in the set.
	//
	// Readers can peek and pop entries similar to the single-reader interface.
	// When popping entries, although the reader moves forward and drops the
	// entry, the entry is not removed from the ring buffer until all other
	// attached readers have moved past that entry.
	//
	// When the attached ring buffer needs to make space, it may push the reader
	// index forward. Users of this class should consider the possibility of data
	// loss if they read slower than the writer.
	class Reader : public IntrusiveList<Reader>::Item {
	public:
	constexpr Reader() : buffer_(nullptr), read_idx_(0), entry_count_(0) {}

	// TODO(b/235351035): Add locking to the internal functions. Who owns the
	// lock? This class? Does this class need a lock if it's not a multi-reader?
	// (One doesn't exist today but presumably nothing prevents push + pop
	// operations from happening on two different threads).

	// Read the oldest stored data chunk of data from the ring buffer to
	// the provided destination span. The number of bytes read is written
	// to bytes_read
	//
	// Precondition: the buffer data must not be corrupt, otherwise there will
	// be a crash.
	//
	// Return values:
	// OK - Data successfully read from the ring buffer.
	// FAILED_PRECONDITION - Buffer not initialized.
	// OUT_OF_RANGE - No entries in ring buffer to read.
	// RESOURCE_EXHAUSTED - Destination data span was smaller number of
	// bytes than the data size of the data chunk being read. Available
	// destination bytes were filled, remaining bytes of the data chunk were
	// ignored.
	Status PeekFront(span<std::byte> data, size_t* bytes_read_out) const {
	return buffer_->InternalPeekFront(*this, data, bytes_read_out);
	}

	Status PeekFront(ReadOutput output) const {
	return buffer_->InternalPeekFront(*this, output);
	}

	// Peek the front entry's preamble only to avoid copying data unnecessarily.
	//
	// Precondition: the buffer data must not be corrupt, otherwise there will
	// be a crash.
	Status PeekFrontPreamble(uint32_t& user_preamble_out) const {
	return buffer_->InternalPeekFrontPreamble(*this, user_preamble_out);
	}

	// Same as PeekFront but includes the entry's preamble of optional user
	// value and the varint of the data size.
	// TODO(b/235351847): Move all other APIs to passing bytes_read by
	// reference, as it is required to determine the length populated in the
	// span.
	Status PeekFrontWithPreamble(span<std::byte> data,
	uint32_t& user_preamble_out,
	size_t& entry_bytes_read_out) const;

	Status PeekFrontWithPreamble(span<std::byte> data,
	size_t* bytes_read_out) const {
	return buffer_->InternalPeekFrontWithPreamble(
	*this, data, bytes_read_out);
	}

	Status PeekFrontWithPreamble(ReadOutput output) const {
	return buffer_->InternalPeekFrontWithPreamble(*this, output);
	}

	// Pop and discard the oldest stored data chunk of data from the ring
	// buffer.
	//
	// Precondition: the buffer data must not be corrupt, otherwise there will
	// be a crash.
	//
	// Return values:
	// OK - Data successfully read from the ring buffer.
	// FAILED_PRECONDITION - Buffer not initialized.
	// OUT_OF_RANGE - No entries in ring buffer to pop.
	Status PopFront() { return buffer_->InternalPopFront(*this); }

	// Get the size in bytes of the next chunk, not including preamble, to be
	// read.
	//
	// Precondition: the buffer data must not be corrupt, otherwise there will
	// be a crash.
	size_t FrontEntryDataSizeBytes() const {
	return buffer_->InternalFrontEntryDataSizeBytes(*this);
	}

	// Get the size in bytes of the next chunk, including preamble and data
	// chunk, to be read.
	//
	// Precondition: the buffer data must not be corrupt, otherwise there will
	// be a crash.
	size_t FrontEntryTotalSizeBytes() const {
	return buffer_->InternalFrontEntryTotalSizeBytes(*this);
	}

	// Get the number of variable-length entries currently in the ring buffer.
	//
	// Return value:
	// Entry count.
	size_t EntryCount() const { return entry_count_; }

	private:
	friend PrefixedEntryRingBufferMulti;

	// Internal constructors for the iterator class to create Reader instances
	// at specific positions. Readers constructed through this interface cannot
	// be attached/detached from the multisink.
	constexpr Reader(Reader& reader)
	: Reader(reader.buffer_, reader.read_idx_, reader.entry_count_) {}
	constexpr Reader(PrefixedEntryRingBufferMulti* buffer,
	size_t read_idx,
	size_t entry_count)
	: buffer_(buffer), read_idx_(read_idx), entry_count_(entry_count) {}

	PrefixedEntryRingBufferMulti* buffer_;
	size_t read_idx_;
	size_t entry_count_;
	};

	// An entry returned by the iterator containing the byte span of the entry
	// and preamble data (if the ring buffer was configured with a preamble).
	struct Entry {
	span<const std::byte> buffer;
	uint32_t preamble;
	};

	// An iterator that can be used to walk through all entries from a given
	// Reader position, without mutating the underlying buffer. This is useful in
	// crash contexts where all available entries in the buffer must be acquired,
	// even those that have already been consumed by all attached readers.
	class iterator {
	public:
	iterator() : ring_buffer_(nullptr), read_idx_(0), entry_count_(0) {}
	iterator(Reader& reader)
	: ring_buffer_(reader.buffer_),
	read_idx_(0),
	entry_count_(reader.entry_count_) {
	Status dering_result = ring_buffer_->InternalDering(reader);
	PW_DASSERT(dering_result.ok());
	}

	iterator& operator++();
	iterator operator++(int) {
	iterator original = *this;
	++*this;
	return original;
	}

	// Returns entry at current position.
	const Entry& operator*() const;
	const Entry* operator->() const { return &operator*(); }

	constexpr bool operator==(const iterator& rhs) const {
	return entry_count_ == rhs.entry_count_;
	}

	constexpr bool operator!=(const iterator& rhs) const {
	return entry_count_ != rhs.entry_count_;
	}

	// Returns the status of the last iteration operation. If the iterator
	// fails to read an entry, it will move to iterator::end() and indicate
	// the failure reason here.
	Status status() const { return iteration_status_; }

	private:
	static constexpr Entry kEndEntry = {
	.buffer = span<const std::byte>(),
	.preamble = 0,
	};

	void SkipToEnd(Status status) {
	iteration_status_ = status;
	entry_ = kEndEntry;
	entry_count_ = 0;
	}

	PrefixedEntryRingBufferMulti* ring_buffer_;
	size_t read_idx_;
	size_t entry_count_;

	mutable Entry entry_;
	Status iteration_status_;
	};

	using element_type = const Entry;
	using value_type = std::remove_cv_t<const Entry>;
	using pointer = const Entry;
	using reference = const Entry&;
	using const_iterator = iterator; // Standard alias for iterable types.

	iterator begin() { return iterator(GetSlowestReaderWritable()); }
	iterator end() { return iterator(); }
	const_iterator cbegin() { return begin(); }
	const_iterator cend() { return end(); }

	// TODO(b/235351861): Consider changing bool to an enum, to explicitly
	// enumerate what this variable means in clients.
	PrefixedEntryRingBufferMulti(bool user_preamble = false)
	: buffer_(nullptr),
	buffer_bytes_(0),
	write_idx_(0),
	user_preamble_(user_preamble) {}

	// Set the raw buffer to be used by the ring buffer.
	//
	// Return values:
	// OK - successfully set the raw buffer.
	// INVALID_ARGUMENT - Argument was nullptr, size zero, or too large.
	Status SetBuffer(span<std::byte> buffer);

	// Determines if the ring buffer has corrupted entries.
	//
	// Precondition: At least one reader must be attached to the ring buffer.
	// Return values:
	// OK - No corruption was detected.
	// DATA_LOSS - Corruption was detected.
	Status CheckForCorruption() {
	iterator it = begin();
	for (; it != end(); ++it) {
	}
	return it.status();
	}

	// Attach reader to the ring buffer. Readers can only be attached to one
	// ring buffer at a time.
	//
	// Return values:
	// OK - Successfully configured reader for ring buffer.
	// INVALID_ARGUMENT - Argument was already attached to another ring buffer.
	Status AttachReader(Reader& reader);

	// Detach reader from the ring buffer. Readers can only be detached if they
	// were previously attached.
	//
	// Return values:
	// OK - Successfully removed reader for ring buffer.
	// INVALID_ARGUMENT - Argument was not previously attached to this ring
	// buffer.
	Status DetachReader(Reader& reader);

	// Removes all data from the ring buffer.
	void Clear();

	// Write a chunk of data to the ring buffer. If available space is less than
	// size of data chunk to be written then silently pop and discard oldest
	// stored data chunks until space is available.
	//
	// Preamble argument is a caller-provided value prepended to the front of the
	// entry. It is only used if user_preamble was set at class construction
	// time. It is varint-encoded before insertion into the buffer.
	//
	// Return values:
	// OK - Data successfully written to the ring buffer.
	// FAILED_PRECONDITION - Buffer not initialized.
	// OUT_OF_RANGE - Size of data is greater than buffer size.
	Status PushBack(span<const std::byte> data, uint32_t user_preamble_data = 0) {
	return InternalPushBack(data, user_preamble_data, true);
	}

	// [Deprecated] An implementation of PushBack that accepts a single-byte as
	// preamble data. Clients should migrate to passing uint32_t preamble data.
	Status PushBack(span<const std::byte> data, std::byte user_preamble_data) {
	return PushBack(data, static_cast<uint32_t>(user_preamble_data));
	}

	// Write a chunk of data to the ring buffer if there is space available.
	//
	// Preamble argument is a caller-provided value prepended to the front of the
	// entry. It is only used if user_preamble was set at class construction
	// time. It is varint-encoded before insertion into the buffer.
	//
	// Precondition: the buffer data must not be corrupt, otherwise there will
	// be a crash.
	//
	// Return values:
	// OK - Data successfully written to the ring buffer.
	// INVALID_ARGUMENT - Size of data to write is zero bytes
	// FAILED_PRECONDITION - Buffer not initialized.
	// OUT_OF_RANGE - Size of data is greater than buffer size.
	// RESOURCE_EXHAUSTED - The ring buffer doesn't have space for the data
	// without popping off existing elements.
	Status TryPushBack(span<const std::byte> data,
	uint32_t user_preamble_data = 0) {
	return InternalPushBack(data, user_preamble_data, false);
	}

	// [Deprecated] An implementation of TryPushBack that accepts a single-byte as
	// preamble data. Clients should migrate to passing uint32_t preamble data.
	Status TryPushBack(span<const std::byte> data, std::byte user_preamble_data) {
	return TryPushBack(data, static_cast<uint32_t>(user_preamble_data));
	}

	// Get the size in bytes of all the current entries in the ring buffer,
	// including preamble and data chunk.
	size_t TotalUsedBytes() const { return buffer_bytes_ - RawAvailableBytes(); }

	// Returns total size of ring buffer in bytes.
	size_t TotalSizeBytes() const { return buffer_bytes_; }

	// Dering the buffer by reordering entries internally in the buffer by
	// rotating to have the oldest entry is at the lowest address/index with
	// newest entry at the highest address. If no readers are attached, the buffer
	// is deringed at the current write index.
	//
	// Return values:
	// OK - Buffer data successfully deringed.
	// FAILED_PRECONDITION - Buffer not initialized.
	Status Dering();

	private:
	// Read the oldest stored data chunk of data from the ring buffer to
	// the provided destination span. The number of bytes read is written to
	// `bytes_read_out`.
	//
	// Precondition: the buffer data must not be corrupt, otherwise there will
	// be a crash.
	//
	// Return values:
	// OK - Data successfully read from the ring buffer.
	// FAILED_PRECONDITION - Buffer not initialized.
	// OUT_OF_RANGE - No entries in ring buffer to read.
	// RESOURCE_EXHAUSTED - Destination data span was smaller number of bytes
	// than the data size of the data chunk being read. Available destination
	// bytes were filled, remaining bytes of the data chunk were ignored.
	Status InternalPeekFront(const Reader& reader,
	span<std::byte> data,
	size_t* bytes_read_out) const;
	Status InternalPeekFront(const Reader& reader, ReadOutput output) const;

	Status InternalPeekFrontPreamble(const Reader& reader,
	uint32_t& user_preamble_out) const;
	// Same as Read but includes the entry's preamble of optional user value and
	// the varint of the data size
	Status InternalPeekFrontWithPreamble(const Reader& reader,
	span<std::byte> data,
	size_t* bytes_read_out) const;
	Status InternalPeekFrontWithPreamble(const Reader& reader,
	ReadOutput output) const;

	// Pop and discard the oldest stored data chunk of data from the ring buffer.
	//
	// Precondition: the buffer data must not be corrupt, otherwise there will
	// be a crash.
	//
	// Return values:
	// OK - Data successfully read from the ring buffer.
	// FAILED_PRECONDITION - Buffer not initialized.
	// OUT_OF_RANGE - No entries in ring buffer to pop.
	Status InternalPopFront(Reader& reader);

	// Get the size in bytes of the next chunk, not including preamble, to be
	// read.
	size_t InternalFrontEntryDataSizeBytes(const Reader& reader) const;

	// Get the size in bytes of the next chunk, including preamble and data
	// chunk, to be read.
	size_t InternalFrontEntryTotalSizeBytes(const Reader& reader) const;

	// Internal version of Read used by all the public interface versions. T
	// should be of type ReadOutput.
	template <typename T>
	Status InternalRead(const Reader& reader,
	T read_output,
	bool include_preamble_in_output,
	uint32_t* user_preamble_out = nullptr) const;

	// Dering the buffer by reordering entries internally in the buffer by
	// rotating to have the oldest entry is at the lowest address/index with
	// newest entry at the highest address. If no readers are attached, the buffer
	// is deringed at the current write index.
	//
	// Return values:
	// OK - Buffer data successfully deringed.
	// FAILED_PRECONDITION - Buffer not initialized.
	Status InternalDering(Reader& reader);

	struct EntryInfo {
	size_t preamble_bytes;
	uint32_t user_preamble;
	size_t data_bytes;
	};

	// Push back implementation, which optionally discards front elements to fit
	// the incoming element.
	Status InternalPushBack(span<const std::byte> data,
	uint32_t user_preamble_data,
	bool pop_front_if_needed);

	// Internal function to pop all of the slowest readers. This function may pop
	// multiple readers if multiple are slow.
	//
	// Precondition: This function requires that at least one reader is attached
	// and has at least one entry to pop. There will be a crash if data is
	// corrupted.
	void InternalPopFrontAll();

	// Returns a the slowest reader in the list.
	//
	// Precondition: This function requires that at least one reader is attached.
	const Reader& GetSlowestReader() const;
	Reader& GetSlowestReaderWritable() {
	return const_cast<Reader&>(GetSlowestReader());
	}

	// Get info struct with the size of the preamble and data chunk for the next
	// entry to be read. Calls RawFrontEntryInfo and asserts on failure.
	//
	// Precondition: the buffer data must not be corrupt, otherwise there will
	// be a crash.
	EntryInfo FrontEntryInfo(const Reader& reader) const;

	// Get info struct with the size of the preamble and data chunk for the next
	// entry to be read.
	//
	// Returns:
	// OK - EntryInfo containing the next entry metadata.
	// DATA_LOSS - Failed to read the metadata at this location.
	Result<EntryInfo> RawFrontEntryInfo(size_t source_idx) const;

	// Get the raw number of available bytes free in the ring buffer. This is
	// not available bytes for data, since there is a variable size preamble for
	// each entry.
	size_t RawAvailableBytes() const;

	// Do the basic write of the specified number of bytes starting at the last
	// write index of the ring buffer to the destination, handing any wrap-around
	// of the ring buffer. This is basic, raw operation with no safety checks.
	void RawWrite(span<const std::byte> source);

	// Do the basic read of the specified number of bytes starting at the given
	// index of the ring buffer to the destination, handing any wrap-around of
	// the ring buffer. This is basic, raw operation with no safety checks.
	void RawRead(std::byte* destination,
	size_t source_idx,
	size_t length_bytes) const;

	size_t IncrementIndex(size_t index, size_t count) const;

	std::byte* buffer_;
	size_t buffer_bytes_;

	size_t write_idx_;
	const bool user_preamble_;

	// List of attached readers.
	IntrusiveList<Reader> readers_;

	// Maximum bufer size allowed. Restricted to this to allow index aliasing to
	// not overflow.
	static constexpr size_t kMaxBufferBytes =
	std::numeric_limits<size_t>::max() / 2;
	};

	class PrefixedEntryRingBuffer : public PrefixedEntryRingBufferMulti,
	public PrefixedEntryRingBufferMulti::Reader {
	public:
	PrefixedEntryRingBuffer(bool user_preamble = false)
	: PrefixedEntryRingBufferMulti(user_preamble) {
	AttachReader(*this)
	.IgnoreError(); // TODO(b/242598609): Handle Status properly
	}
	};

	} // namespace ring_buffer
	} // namespace pw