[Tracing] Introduce HeapDumpWriter helper class

base/trace_event/memory_profiler_allocation_context.h

Index: base/trace_event/memory_profiler_allocation_context.h
diff --git a/base/trace_event/memory_profiler_allocation_context.h b/base/trace_event/memory_profiler_allocation_context.h
index c9b8f737f095f61c33a5928aa45cbd1ce4e5d313..44b1367a7154df3b28cb449845d9de5cb5a1fc62 100644
--- a/base/trace_event/memory_profiler_allocation_context.h
+++ b/base/trace_event/memory_profiler_allocation_context.h
@@ -69,23 +69,22 @@ class BASE_EXPORT AllocationStack {
 //
 // See the design doc (https://goo.gl/4s7v7b) for more details.
 
-// The allocation context is context metadata that is kept for every allocation
-// when heap profiling is enabled. To simplify memory management for
-// bookkeeping, this struct has a fixed size. All |const char*|s here
-// must have static lifetime.
-struct BASE_EXPORT AllocationContext {
-  struct Backtrace {
-    // Unused backtrace frames are filled with nullptr frames. If the stack is
-    // higher than what can be stored here, the bottom frames are stored. Based
-    // on the data above, a depth of 12 captures the full stack in the vast
-    // majority of the cases.
-    StackFrame frames[12];
-  } backtrace;
+struct BASE_EXPORT Backtrace {

[Primiano Tucci (use gerrit), 2015/10/26 11:52:06]

not really sure this is more readable than before. I'd probably keep that inside
the AllocationContext

[Ruud van Asseldonk, 2015/10/26 14:51:26]

Going to leave it outside for now to avoid rightward drift.

+  // Unused backtrace frames are filled with nullptr frames. If the stack is
+  // higher than what can be stored here, the bottom frames are stored. Based
+  // on the data above, a depth of 12 captures the full stack in the vast
+  // majority of the cases.
+  StackFrame frames[12];
+};
 
-  // There is room for two arbitrary context fields, which can be set by the
-  // |TRACE_ALLOCATION_CONTEXT| macro. A nullptr key indicates that the field is
-  // unused.
-  std::pair<const char*, const char*> fields[2];
+bool BASE_EXPORT operator==(const Backtrace& lhs, const Backtrace& rhs);
+
+// A hasher for |Backtrace| that allows using it as a key for |base::hash_map|.
+class BASE_EXPORT BacktraceHash {

[Primiano Tucci (use gerrit), 2015/10/26 11:52:06]

A search in the codebase suggests that the way we usually deal with this is by
defining at the end of the file a specialization of hash in the std namespace.
See
https://code.google.com/p/chromium/codesearch#chromium/src/cc/surfaces/surfac...
as an example

[Primiano Tucci (use gerrit), 2015/10/26 11:52:06]

+er (Hasher)?

[Ruud van Asseldonk, 2015/10/26 14:51:26]

Done.

[Ruud van Asseldonk, 2015/10/26 14:51:26]

Would be called |hash<Backtrace>| then.

+ public:
+  using result_type = uint32_t;
+  using argument_type = Backtrace;
+  uint32_t operator()(const Backtrace& backtrace) const;
 };
 
 // A data structure that allows grouping a set of backtraces in a space-
@@ -117,10 +116,9 @@ class BASE_EXPORT StackFrameDeduplicator {
   StackFrameDeduplicator();
   ~StackFrameDeduplicator();
 
-  // Inserts a backtrace and returns the index of its leaf node in the range
-  // defined by |begin| and |end|. I.e. if this returns |n|, the node is
-  // |begin() + n|. Returns -1 if the backtrace is empty.
-  int Insert(const AllocationContext::Backtrace& bt);
+  // Inserts a backtrace and returns the index of its leaf node in |frames_|.
+  // Returns -1 if the backtrace is empty.
+  int Insert(const Backtrace& bt);
 
   // Iterators over the frame nodes in the call tree.
   ConstIterator begin() const { return frames_.begin(); }
@@ -133,6 +131,19 @@ class BASE_EXPORT StackFrameDeduplicator {
   DISALLOW_COPY_AND_ASSIGN(StackFrameDeduplicator);
 };
 
+// The allocation context is context metadata that is kept for every allocation
+// when heap profiling is enabled. To simplify memory management for
+// bookkeeping, this struct has a fixed size. All |const char*|s here
+// must have static lifetime.
+struct BASE_EXPORT AllocationContext {
+  Backtrace backtrace;
+
+  // There is room for two arbitrary context fields, which can be set by the
+  // |TRACE_ALLOCATION_CONTEXT| macro. A nullptr key indicates that the field is
+  // unused.
+  std::pair<const char*, const char*> fields[2];
+};
+
 // The allocation context tracker keeps track of thread-local context for heap
 // profiling. It includes a pseudo stack of trace events, and it might contain
 // arbitrary (key, value) context. On every allocation the tracker provides a