Increase usability of RefCountedBytes as a mutable buffer

This patch adds a new constructor for RefCountedBytes that initializes
it with a given length of zeros and non-const versions of the front()
and front_as<>() methods to accompany the non-const data() method.

This is split out from https://crrev.com/c/786680 which uses this class
as a replacement for net::IOBufferWithSize (but with an unsigned
buffer).

Change-Id: I36c16d02e0a524da3da8679b883321388febaf82
Reviewed-on: https://chromium-review.googlesource.com/822165Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524160}

base/memory/ref_counted_memory.cc

@@ -38,6 +38,8 @@ RefCountedBytes::RefCountedBytes(const std::vector<unsigned char>& initializer)
 RefCountedBytes::RefCountedBytes(const unsigned char* p, size_t size)
     : data_(p, p + size) {}
 
+RefCountedBytes::RefCountedBytes(size_t size) : data_(size, 0) {}
+
 scoped_refptr<RefCountedBytes> RefCountedBytes::TakeVector(
     std::vector<unsigned char>* to_destroy) {
   scoped_refptr<RefCountedBytes> bytes(new RefCountedBytes);

base/memory/ref_counted_memory.h

@@ -78,6 +78,10 @@ class BASE_EXPORT RefCountedBytes : public RefCountedMemory {
   // Constructs a RefCountedBytes object by copying |size| bytes from |p|.
   RefCountedBytes(const unsigned char* p, size_t size);
 
+  // Constructs a RefCountedBytes object by zero-initializing a new vector of
+  // |size| bytes.
+  explicit RefCountedBytes(size_t size);
+
   // Constructs a RefCountedBytes object by performing a swap. (To non
   // destructively build a RefCountedBytes, use the constructor that takes a
   // vector.)
@@ -91,6 +95,14 @@ class BASE_EXPORT RefCountedBytes : public RefCountedMemory {
   const std::vector<unsigned char>& data() const { return data_; }
   std::vector<unsigned char>& data() { return data_; }
 
+  // Non-const versions of front() and front_as() that are simply shorthand for
+  // data().data().
+  unsigned char* front() { return data_.data(); }
+  template <typename T>
+  T* front_as() {
+    return reinterpret_cast<T*>(front());
+  }
+
  private:
   ~RefCountedBytes() override;
 

base/memory/ref_counted_memory_unittest.cc

@@ -6,8 +6,12 @@
 
 #include <stdint.h>
 
+#include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
+using testing::Each;
+using testing::ElementsAre;
+
 namespace base {
 
 TEST(RefCountedMemoryUnitTest, RefCountedStaticMemory) {
@@ -41,6 +45,20 @@ TEST(RefCountedMemoryUnitTest, RefCountedBytes) {
   EXPECT_EQ(99U, mem2->front()[2]);
 }
 
+TEST(RefCountedMemoryUnitTest, RefCountedBytesMutable) {
+  auto mem = base::MakeRefCounted<RefCountedBytes>(10);
+
+  ASSERT_EQ(10U, mem->size());
+  EXPECT_THAT(mem->data(), Each(0U));
+
+  // Test non-const versions of data(), front() and front_as<>().
+  mem->data()[0] = 1;
+  mem->front()[1] = 2;
+  mem->front_as<char>()[2] = 3;
+
+  EXPECT_THAT(mem->data(), ElementsAre(1, 2, 3, 0, 0, 0, 0, 0, 0, 0));
+}
+
 TEST(RefCountedMemoryUnitTest, RefCountedString) {
   std::string s("destroy me");
   scoped_refptr<RefCountedMemory> mem = RefCountedString::TakeString(&s);