HttpCache::Transaction layer allowing parallel validation

net/http/http_cache.h

 // Copyright (c) 2012 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.
 
 // This file declares a HttpTransactionFactory implementation that can be
 // layered on top of another HttpTransactionFactory to add HTTP caching.  The
 // caching logic follows RFC 7234 (any exceptions are called out in the code).
 //
 // The HttpCache takes a disk_cache::Backend as a parameter, and uses that for
 // the cache storage.
@@ skipping 226 matching lines @@
   };
 
   class MetadataWriter;
   class QuicServerInfoFactoryAdaptor;
   class Transaction;
   class WorkItem;
   friend class Transaction;
   friend class ViewCacheHelper;
   struct PendingOp;  // Info for an entry under construction.
 
+  // To help with testing.
+  friend class MockHttpCache;
+
   using TransactionList = std::list<Transaction*>;
   using TransactionSet = std::unordered_set<Transaction*>;
   typedef std::list<std::unique_ptr<WorkItem>> WorkItemList;
 
   struct ActiveEntry {
     explicit ActiveEntry(disk_cache::Entry* entry);
     ~ActiveEntry();
     size_t EstimateMemoryUsage() const;
 
     // Returns true if no transactions are associated with this entry.
     bool HasNoTransactions();
 
-    disk_cache::Entry* disk_entry;
-    Transaction*       writer;
+    // Returns true if no active readers/writer transactions are associated
+    // with this entry.
+    bool HasNoActiveTransactions();
+
+    disk_cache::Entry* disk_entry = nullptr;
+
+    // We implement a basic reader/writer lock for the disk cache entry. If
+    // there is a writer, then all read-only transactions must wait. Non
+    // read-only transactions can proceed to their validation phase. Validation
+    // is allowed for one transaction at a time so that we do not end up with
+    // wasted network requests.
+
+    // The following variables represent the various transactions associated
+    // with this entry in different stages. A transaction goes through these
+    // transitions.
+    //
+    // Write mode transactions:
+    // add_to_entry_queue-> headers_transaction -> done_headers_queue ->
+    // writer
+    // add_to_entry_queue-> headers_transaction -> done_headers_queue ->
+    // readers (once the data is written to the cache by another writer)
+    //
+    // Read only transactions:
+    // add_to_entry_queue-> readers (once the data is written to the cache by
+    // the writer)
+
+    // Transactions waiting to be added to entry.
+    TransactionList add_to_entry_queue;
+
+    // Transaction currently in the headers phase, either validating the
+    // response or getting new headers. This can exist simultaneously with
+    // writer or readers while validating existing headers.
+    Transaction* headers_transaction = nullptr;
+
+    // Transactions that have completed their headers phase and are waiting
+    // to read the response body or write the response body.
+    TransactionList done_headers_queue;
+
+    // Transaction currently reading from the network and writing to the cache.
+    Transaction* writer = nullptr;
+
+    // Transactions that can only read from the cache. Only one of writer or
+    // readers can exist at a time.
     TransactionSet readers;
-    TransactionList    pending_queue;
-    bool               will_process_pending_queue;
-    bool               doomed;
+
+    // The following variables are true if OnProcessQueuedTransactions is posted
+    bool will_process_queued_transactions = false;
+
+    // True if entry is doomed.
+    bool doomed = false;
   };
 
   using ActiveEntriesMap =
       std::unordered_map<std::string, std::unique_ptr<ActiveEntry>>;
   using PendingOpsMap = std::unordered_map<std::string, PendingOp*>;
   using ActiveEntriesSet = std::map<ActiveEntry*, std::unique_ptr<ActiveEntry>>;
   using PlaybackCacheMap = std::unordered_map<std::string, int>;
 
   // Methods ------------------------------------------------------------------
 
@@ skipping 60 matching lines @@
 
   // Creates the disk cache entry associated with |key|, returning an
   // ActiveEntry in |*entry|. |trans| will be notified via its IO callback if
   // this method returns ERR_IO_PENDING.
   int CreateEntry(const std::string& key, ActiveEntry** entry,
                   Transaction* trans);
 
   // Destroys an ActiveEntry (active or doomed).
   void DestroyEntry(ActiveEntry* entry);
 
-  // Adds a transaction to an ActiveEntry. If this method returns ERR_IO_PENDING
-  // the transaction will be notified about completion via its IO callback. This
-  // method returns ERR_CACHE_RACE to signal the transaction that it cannot be
-  // added to the provided entry, and it should retry the process with another
-  // one (in this case, the entry is no longer valid).
-  int AddTransactionToEntry(ActiveEntry* entry, Transaction* trans);
+  // Adds a transaction to an ActiveEntry. This method returns ERR_IO_PENDING
+  // and the transaction will be notified about completion via its IO callback.
+  int AddTransactionToEntry(ActiveEntry* entry, Transaction* transaction);
+
+  // Transaction invokes this when its response headers phase is complete
+  // either on validating existing headers/skipping validation or getting new
+  // headers. The transaction is added to a queue and a task is posted
+  // to process queued transactions of the entry. Returns ERR_IO_PENDING.
+  int DoneWithResponseHeaders(ActiveEntry* entry, Transaction* transaction);
 
   // Called when the transaction has finished working with this entry. |cancel|
   // is true if the operation was cancelled by the caller instead of running
   // to completion.
-  void DoneWithEntry(ActiveEntry* entry, Transaction* trans, bool cancel);
+  void DoneWithEntry(ActiveEntry* entry, Transaction* transaction, bool cancel);
+
+  // Returns true if this transaction can write headers to the entry.
+  bool CanTransactionWriteResponseHeaders(ActiveEntry* entry,
+                                          Transaction* transaction,
+                                          int response_code) const;
+
+  // Returns true if this transaction is currently writing or about to start
+  // writing the response body.

[Randy Smith (Not in Mondays), 2017/04/04 21:21:16]

I feel as if the comments and the name don't quite match.  Is this about whether
the transaction can write the response body at some point in the future, or is
it about whether it's committed to writing the response body?

[shivanisha, 2017/04/05 15:00:37]

Since there can be only one writer, "can" implies the transaction is committed
to write. Let me know if you think the name or comments should change.

[Randy Smith (Not in Mondays), 2017/04/05 18:39:17]

I have a preference for a name that indicates that the transaction will be
writing the response body, but up to you.

+  bool CanTransactionWriteResponseBody(ActiveEntry* entry,
+                                       Transaction* transaction,
+                                       const std::string& method) const;
 
   // Called when the transaction has finished writing to this entry. |success|
   // is false if the cache entry should be deleted.
-  void DoneWritingToEntry(ActiveEntry* entry, bool success);
+  void DoneWritingToEntry(ActiveEntry* entry,
+                          bool success,
+                          Transaction* transaction);
 
   // Called when the transaction has finished reading from this entry.
-  void DoneReadingFromEntry(ActiveEntry* entry, Transaction* trans);
-
-  // Converts the active writer transaction to a reader so that other
-  // transactions can start reading from this entry.
-  void ConvertWriterToReader(ActiveEntry* entry);
+  void DoneReadingFromEntry(ActiveEntry* entry, Transaction* transaction);
 
   // Returns the LoadState of the provided pending transaction.
   LoadState GetLoadStateForPendingTransaction(const Transaction* trans);
 
   // Removes the transaction |trans|, from the pending list of an entry
   // (PendingOp, active or doomed entry).
   void RemovePendingTransaction(Transaction* trans);
 
   // Removes the transaction |trans|, from the pending list of |entry|.
   bool RemovePendingTransactionFromEntry(ActiveEntry* entry,
                                          Transaction* trans);
 
   // Removes the transaction |trans|, from the pending list of |pending_op|.
   bool RemovePendingTransactionFromPendingOp(PendingOp* pending_op,
                                              Transaction* trans);
-  // Resumes processing the pending list of |entry|.
-  void ProcessPendingQueue(ActiveEntry* entry);
+
+  // Removes and returns all queued transactions in |entry| in FIFO order. This
+  // includes transactions that have completed the headers phase and those that
+  // have not been added to the entry yet in that order. |list| is the output
+  // argument.
+  void RemoveAllQueuedTransactions(ActiveEntry* entry, TransactionList* list);
+
+  // Resumes processing the queued transactions of |entry|.
+  void ProcessQueuedTransactions(ActiveEntry* entry);
+
+  // Checks if a transaction can be added to the entry. If yes, it will
+  // invoke the IO callback of the transaction. This is a helper function for
+  // OnProcessQueuedTransactions.
+  void ProcessAddToEntryQueue(ActiveEntry* entry);
+
+  // Checks if a transaction that has already completed the response headers
+  // phase can resume reading/writing the response body. If yes, it will
+  // invoke the IO callback of the transaction. This is a helper function for
+  // OnProcessQueuedTransactions.
+  void ProcessDoneHeadersQueue(ActiveEntry* entry);
+
+  // Processes either writer's failure to write response body or
+  // headers_transactions's failure to write headers.
+  void ProcessEntryFailure(ActiveEntry* entry);
 
   // Events (called via PostTask) ---------------------------------------------
 
-  void OnProcessPendingQueue(ActiveEntry* entry);
+  void OnProcessQueuedTransactions(ActiveEntry* entry);
 
   // Callbacks ----------------------------------------------------------------
 
   // Processes BackendCallback notifications.
   void OnIOComplete(int result, PendingOp* entry);
 
   // Helper to conditionally delete |pending_op| if the HttpCache object it
   // is meant for has been deleted.
   //
   // TODO(ajwong): The PendingOp lifetime management is very tricky.  It might
@@ skipping 37 matching lines @@
   std::unique_ptr<base::Clock> clock_;
 
   base::WeakPtrFactory<HttpCache> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(HttpCache);
 };
 
 }  // namespace net
 
 #endif  // NET_HTTP_HTTP_CACHE_H_