Adds a new class HttpCache::Writers for multiple cache transactions reading from the network.

net/http/http_cache_writers.h

+// Copyright (c) 2017 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 NET_HTTP_HTTP_CACHE_WRITERS_H_
+#define NET_HTTP_HTTP_CACHE_WRITERS_H_
+#include <list>
+#include <memory>
+#include "base/memory/weak_ptr.h"
+#include "net/base/completion_callback.h"
+#include "net/http/http_cache.h"
+
+// If multiple HttpCache::Transactions are accessing the same cache entry
+// simultaneously, their access to the data read from network is synchronized
+// by HttpCache::Writers. This enables each of those transactions to drive
+// reading the response body from the network ensuring a slow consumer does not
+// starve other consumers of the same resource.
+
+namespace net {
+
+// Writer represents the set of all HttpCache::Transactions that are
+// reading from the network using the same network transaction and writing to
+// the same cache entry. It is owned by the ActiveEntry.
+class HttpCache::Writers {
+ public:
+  // |*entry| and |*cache| should always be valid when accessed from Writers.
+  Writers(HttpCache* cache, ActiveEntry* entry);
+  ~Writers();
+
+  // Invokes Read on network transaction if a read is not already in progress.
+  // In case a read is already in progress then this transaction is added to
+  // a waiting queue and ERR_IO_PENDING is returned. If ERR_IO_PENDING is
+  // returned, the result of the read will be later communicated to the consumer
+  // via the |callback|.
+  int Read(scoped_refptr<IOBuffer> buf,
+           int buf_len,
+           const CompletionCallback& callback,
+           Transaction* transaction);
+
+  // Invoked when StopCaching is called on a member transaction.
+  // It stops caching only if there are no other transactions. Returns true if
+  // caching can be stopped.
+  bool StopCaching(Transaction* transaction);
+
+  // Membership functions.
+
+  // Adds an HttpCache::Transaction to Writers. Should only be invoked if
+  // CanAddTransaction() returns true. If |network_transaction| is non-null, it
+  // will be assigned to |network_transaction_|. The first transaction added
+  // should definitely pass a non-null |network_transaction|. |transaction| can
+  // be destroyed at any point and it should invoke RemoveTransaction() during
+  // its destruction.
+  void AddTransaction(Transaction* transaction,
+                      HttpTransaction* network_transaction);
+
+  // Removes a transaction.
+  void RemoveTransaction(Transaction* transaction);
+
+  // Removes all transactions.
+  void RemoveAllTransactions();
+
+  // Invoked when there is a change in a member transaction's priority or a
+  // member transaction is removed.
+  void PriorityChanged();
+
+  // Returns true if this object is empty.
+  bool IsEmpty() const { return all_writers_.empty(); }
+
+  // Returns true is |transaction| is part of writers.
+  bool IsPresent(Transaction* transaction) const {
+    return all_writers_.count(transaction) > 0;
+  }
+
+  // When response is completely written, any idle writers are moved to
+  // entry_->readers.
+  void MoveIdleWritersToReaders();
+
+  // Makes writing an exclusine operation implying that Writers can contain at
+  // most one transaction for the complete reading/writing of the response body.
+  void SetExclusive() { is_exclusive_ = true; }
+
+  // Returns true if more writers can be added for shared writing.
+  bool CanAddWriters() const;
+
+  HttpTransaction* network_transaction() { return network_transaction_.get(); }
+
+  // Invoked from HttpCache when it is notified of a transaction failing to
+  // write. |error| indicates network read error code or cache write error.
+  void ProcessFailure(Transaction* transaction, int error);
+
+  // Invoked to mark an entry as truncated. Should be invoked when reading is
+  // not currently in progress.

[Randy Smith (Not in Mondays), 2017/05/18 01:02:54]

Is it possible for a consumer to know this?  Naively, I'd expect consumers would
only know about their interactions with the SharedWriters class.

+  void TruncateEntry();
+
+  LoadState GetWriterLoadState();
+
+  // For testing.
+
+  int CountTransactionsForTesting() const { return all_writers_.size(); }
+  bool IsTruncatedForTesting() const { return truncated_; }
+
+ private:
+  enum class State {
+    NONE,
+    NETWORK_READ,
+    NETWORK_READ_COMPLETE,
+    CACHE_WRITE_DATA,
+    CACHE_WRITE_DATA_COMPLETE,
+    CACHE_WRITE_TRUNCATED_RESPONSE,
+    CACHE_WRITE_TRUNCATED_RESPONSE_COMPLETE,
+  };
+
+  // These transactions are waiting on Read. After the active transaction
+  // completes writing the data to the cache, their buffer would be filled with
+  // the data and their callback will be invoked.
+  struct WaitingForRead {
+    Transaction* transaction;
+    scoped_refptr<IOBuffer> read_buf;
+    int read_buf_len;
+    int write_len;
+    const CompletionCallback callback;
+    WaitingForRead(Transaction* transaction,
+                   scoped_refptr<IOBuffer> read_buf,
+                   int len,
+                   const CompletionCallback& consumer_callback);
+    ~WaitingForRead();
+    WaitingForRead(const WaitingForRead&);
+  };
+  using WaitingForReadList = std::list<WaitingForRead>;
+
+  // Runs the state transition loop. Resets and calls |callback_| on exit,
+  // unless the return value is ERR_IO_PENDING.
+  int DoLoop(int result);
+
+  // State machine functions.
+  int DoNetworkRead();
+  int DoNetworkReadComplete(int result);
+  int DoCacheWriteData(int num_bytes);
+  int DoCacheWriteDataComplete(int result);
+  int DoCacheWriteTruncatedResponse();
+  int DoCacheWriteTruncatedResponseComplete(int result);
+
+  // Helper functions for callback.
+
+  void OnNetworkReadFailure(int result);
+  void OnCacheWriteFailure();
+  void OnDataReceived(int result);
+
+  // Helper function for writing to cache.
+  int WriteToEntry(int index,
+                   int offset,
+                   IOBuffer* data,
+                   int data_len,
+                   const CompletionCallback& callback);
+
+  // Notifies the transactions waiting on Read of the result, by posting a task
+  // for each of them.
+  void ProcessWaitingForReadTransactions(int result);
+
+  // Sets the state of idle writers so that they can fail any subsequent
+  // Read.
+  void SetIdleWritersFailState(int result);
+
+  RequestPriority GetCurrentHighestPriority();
+
+  bool IsResponseCompleted();
+
+  int WriteResponseInfo(bool truncated);
+
+  // IO Completion callback function.
+  void OnIOComplete(int result);
+
+  State next_state_ = State::NONE;
+
+  // True if only reading from network and not writing to cache.
+  bool network_read_only_ = false;
+
+  // Http Cache.
+  HttpCache* cache_;
+
+  // Owner of this object.
+  ActiveEntry* entry_ = nullptr;
+
+  std::unique_ptr<HttpTransaction> network_transaction_;
+
+  scoped_refptr<IOBuffer> read_buf_;
+
+  int io_buf_len_ = 0;
+  int write_len_ = 0;
+
+  // The cache transaction that is the current consumer of network_transaction_
+  // ::Read or writing to the entry and is waiting for the operation to be
+  // completed. This is used to ensure there is at most one consumer of
+  // network_transaction_ at a time.
+  Transaction* active_transaction_ = nullptr;
+
+  // Transactions whose consumers have invoked Read, but another transaction is
+  // currently the |active_transaction_|. After the network read and cache write
+  // is complete, the waiting transactions will be notified.
+  WaitingForReadList waiting_for_read_;
+
+  // Includes all transactions.
+  TransactionSet all_writers_;
+
+  // True if multiple transactions are not allowed e.g. for partial requests.
+  bool is_exclusive_ = false;
+
+  // Current priority of the request. If a higher priority transaction is
+  // added, the priority of network transaction will be increased.
+  RequestPriority priority_ = MINIMUM_PRIORITY;
+
+  bool truncated_ = false;  // used for testing.
+
+  CompletionCallback callback_;  // Consumer's callback.
+  CompletionCallback io_callback_;
+  base::WeakPtrFactory<Writers> weak_factory_;
+  DISALLOW_COPY_AND_ASSIGN(Writers);
+};
+
+}  // namespace net
+#endif  // NET_HTTP_HTTP_CACHE_WRITERS_H_