[tracing] Add method to create "weak" global dumps

base/trace_event/memory_allocator_dump.h

Index: base/trace_event/memory_allocator_dump.h
diff --git a/base/trace_event/memory_allocator_dump.h b/base/trace_event/memory_allocator_dump.h
index 6c514fa6a62380b9ec6180792451a40b3a9dbbf9..62964aea933e651ee0f96f18473bdc7676857b67 100644
--- a/base/trace_event/memory_allocator_dump.h
+++ b/base/trace_event/memory_allocator_dump.h
@@ -26,9 +26,19 @@ class TracedValue;
 // Data model for user-land memory allocator dumps.
 class BASE_EXPORT MemoryAllocatorDump {
  public:
+  enum Flag {
+    // A dump marked weak will be removed from tracing. It is used when dump

[petrcermak, 2016/01/18 11:23:46]

nit: "... removed from tracing, unless it was/will be dumped by the same or a
different process without the weak flag."

[ssid, 2016/01/18 16:28:35]

Hm, I think as far as mad is concerned, it means that a dump marked weak is
removed. It can be marked strong later so that it is not removed. These cases
are handled by pmd.
Also "will be dumped without weak" would mean same dump can be dumped twice and
is confusing to the user. The existing comment would prevent the user from using
this flag directly from here instead of usign pmd to modify.

[petrcermak, 2016/01/18 17:18:12]

Good point. My concern is that this suggests that the trace will NOT contain
weak memory allocator dumps, which it WILL (they will be removed in Trace
Viewer).

What about "A dump marked as weak will be removed together with its transitive
owners and descendants in trace post-processing"?

+    // provider is unsure of the existance of memory.

[petrcermak, 2016/01/18 11:23:46]

s/existance/existence/

[petrcermak, 2016/01/18 11:23:46]

nit: s/memory/a block of memory/ (memory always exists)

[ssid, 2016/01/18 16:28:35]

Done.

[ssid, 2016/01/18 16:28:35]

Done.

+    WEAK = 1 << 0,
+  };
+
   // MemoryAllocatorDump is owned by ProcessMemoryDump.
   MemoryAllocatorDump(const std::string& absolute_name,
                       ProcessMemoryDump* process_memory_dump,
+                      const MemoryAllocatorDumpGuid& guid,
+                      uint8_t flags);

[petrcermak, 2016/01/18 11:23:46]

As explained in my comment in ProcessMemoryDump, adding this constructor might
not be necessary.

[ssid, 2016/01/18 16:28:35]

true.

+  MemoryAllocatorDump(const std::string& absolute_name,
+                      ProcessMemoryDump* process_memory_dump,
                       const MemoryAllocatorDumpGuid& guid);
   MemoryAllocatorDump(const std::string& absolute_name,
                       ProcessMemoryDump* process_memory_dump);
@@ -68,6 +78,11 @@ class BASE_EXPORT MemoryAllocatorDump {
     return process_memory_dump_;
   }
 
+  // Use enum Flag to set values.
+  void set_flags(uint8_t flags) { flags_ |= flags; }
+  void unset_flags(uint8_t flags) { flags_ &= ~flags; }
+  uint8_t flags() { return flags_; }
+
   // |guid| is an optional global dump identifier, unique across all processes
   // within the scope of a global dump. It is only required when using the
   // graph APIs (see TODO_method_name) to express retention / suballocation or
@@ -83,6 +98,7 @@ class BASE_EXPORT MemoryAllocatorDump {
   ProcessMemoryDump* const process_memory_dump_;  // Not owned (PMD owns this).
   scoped_refptr<TracedValue> attributes_;
   MemoryAllocatorDumpGuid guid_;
+  uint8_t flags_;  // See enum Flag.
 
   // A local buffer for Sprintf conversion on fastpath. Avoids allocating
   // temporary strings on each AddScalar() call.