Modify SpdyHeaderBlock's internals to consolidate header values only on first access.

net/spdy/spdy_header_block.h

Index: net/spdy/spdy_header_block.h
diff --git a/net/spdy/spdy_header_block.h b/net/spdy/spdy_header_block.h
index fef27e8f51a6577b7d725635f1ae38751c62093e..fe8827f5c8d06aba8c0932439c06aaea956dab3e 100644
--- a/net/spdy/spdy_header_block.h
+++ b/net/spdy/spdy_header_block.h
@@ -11,6 +11,7 @@
 #include <map>
 #include <memory>
 #include <string>
+#include <vector>
 
 #include "base/macros.h"
 #include "base/strings/string_piece.h"
@@ -27,6 +28,7 @@ namespace net {
 class NetLogCaptureMode;
 
 namespace test {
+class SpdyHeaderBlockPeer;
 class ValueProxyPeer;
 }
 
@@ -43,16 +45,93 @@ class ValueProxyPeer;
 // It's expected that keys are rarely deleted from a SpdyHeaderBlock.
 class NET_EXPORT SpdyHeaderBlock {
  private:
-  using MapType = linked_hash_map<base::StringPiece,
-                                  base::StringPiece,
-                                  base::StringPieceHash>;
   class Storage;
 
+  // Stores a list of value fragments that can be joined later with a
+  // key-dependent separator.
+  class NET_EXPORT HeaderValue {
+   public:
+    HeaderValue(Storage* storage,
+                base::StringPiece key,
+                base::StringPiece initial_value);
+
+    // Moves are allowed.
+    HeaderValue(HeaderValue&& other);
+    HeaderValue& operator=(HeaderValue&& other);
+
+    // Copies are not.
+    HeaderValue(const HeaderValue& other) = delete;
+    HeaderValue& operator=(const HeaderValue& other) = delete;
+
+    ~HeaderValue();
+
+    // Consumes at most |fragment.size()| bytes of memory.
+    void Append(base::StringPiece fragment);
+
+    base::StringPiece value() const { return as_pair().second; }
+    const std::pair<base::StringPiece, base::StringPiece>& as_pair() const;
+
+   private:
+    // May allocate a large contiguous region of memory to hold the concatenated
+    // fragments and separators.
+    base::StringPiece ConsolidatedValue() const;
+
+    mutable Storage* storage_;
+    mutable std::vector<base::StringPiece> fragments_;
+    // The first element is the key; the second is the consolidated value.
+    mutable std::pair<base::StringPiece, base::StringPiece> pair_;
+  };
+
+  typedef linked_hash_map<base::StringPiece, HeaderValue, base::StringPieceHash>
+      MapType;
+
  public:
-  using iterator = MapType::iterator;
-  using const_iterator = MapType::const_iterator;
-  using value_type = MapType::value_type;
-  using reverse_iterator = MapType::reverse_iterator;
+  typedef std::pair<base::StringPiece, base::StringPiece> value_type;
+
+  // Provides iteration over a sequence of std::pair<StringPiece, StringPiece>,
+  // even though the underlying MapType::value_type is different. Dereferencing
+  // the iterator will result in memory allocation for multi-value headers.
+  class NET_EXPORT iterator {
+   public:
+    // The following type definitions fulfill the requirements for iterator
+    // implementations.
+    typedef std::pair<base::StringPiece, base::StringPiece> value_type;
+    typedef value_type& reference;
+    typedef value_type* pointer;
+    typedef std::forward_iterator_tag iterator_category;
+    typedef MapType::iterator::difference_type difference_type;
+
+    // In practice, this iterator only offers access to const value_type.
+    typedef const value_type& const_reference;
+    typedef const value_type* const_pointer;
+
+    explicit iterator(MapType::const_iterator it);
+    iterator(const iterator& other);
+    ~iterator();
+
+    // This will result in memory allocation if the value consists of multiple
+    // fragments.
+    const_reference operator*() const { return it_->second.as_pair(); }
+
+    const_pointer operator->() const { return &(this->operator*()); }
+    bool operator==(const iterator& it) const { return it_ == it.it_; }
+    bool operator!=(const iterator& it) const { return !(*this == it); }
+
+    iterator& operator++() {
+      it_++;
+      return *this;
+    }
+
+    iterator operator++(int) {
+      auto ret = *this;
+      this->operator++();
+      return ret;
+    }
+
+   private:
+    MapType::const_iterator it_;
+  };
+  typedef iterator const_iterator;
 
   class ValueProxy;
 
@@ -72,16 +151,16 @@ class NET_EXPORT SpdyHeaderBlock {
   // keys and values.
   std::string DebugString() const;
 
-  // These methods delegate to our MapType member.
-  iterator begin() { return block_.begin(); }
-  iterator end() { return block_.end(); }
-  const_iterator begin() const { return block_.begin(); }
-  const_iterator end() const { return block_.end(); }
+  iterator begin() { return iterator(block_.begin()); }
+  iterator end() { return iterator(block_.end()); }
+  const_iterator begin() const { return const_iterator(block_.begin()); }
+  const_iterator end() const { return const_iterator(block_.end()); }
   bool empty() const { return block_.empty(); }
   size_t size() const { return block_.size(); }
-  iterator find(base::StringPiece key) { return block_.find(key); }
-  const_iterator find(base::StringPiece key) const { return block_.find(key); }
-  reverse_iterator rbegin() { return block_.rbegin(); }
+  iterator find(base::StringPiece key) { return iterator(block_.find(key)); }
+  const_iterator find(base::StringPiece key) const {
+    return const_iterator(block_.find(key));
+  }
   void erase(base::StringPiece key) { block_.erase(key); }
 
   // Clears both our MapType member and the memory used to hold headers.
@@ -91,7 +170,7 @@ class NET_EXPORT SpdyHeaderBlock {
 
   // If key already exists in the block, replaces the value of that key. Else
   // adds a new header to the end of the block.
-  void insert(const MapType::value_type& value);
+  void insert(const value_type& value);
 
   // If a header with the key is already present, then append the value to the
   // existing header value, NUL ("\0") separated unless the key is cookie, in
@@ -140,8 +219,11 @@ class NET_EXPORT SpdyHeaderBlock {
   };
 
  private:
+  friend class test::SpdyHeaderBlockPeer;
+
   void AppendHeader(const base::StringPiece key, const base::StringPiece value);
   Storage* GetStorage();
+  size_t bytes_allocated() const;
 
   // StringPieces held by |block_| point to memory owned by |*storage_|.
   // |storage_| might be nullptr as long as |block_| is empty.
@@ -149,6 +231,12 @@ class NET_EXPORT SpdyHeaderBlock {
   std::unique_ptr<Storage> storage_;
 };
 
+// Writes |fragments| to |dst|, joined by |separator|. |dst| must be large
+// enough to hold the result. Returns the number of bytes written.
+NET_EXPORT size_t Join(char* dst,
+                       const std::vector<base::StringPiece>& fragments,
+                       base::StringPiece separator);
+
 // Converts a SpdyHeaderBlock into NetLog event parameters.
 NET_EXPORT std::unique_ptr<base::Value> SpdyHeaderBlockNetLogCallback(
     const SpdyHeaderBlock* headers,