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

content/common/discardable_shared_memory_heap.cc

Index: content/common/discardable_shared_memory_heap.cc
diff --git a/content/common/discardable_shared_memory_heap.cc b/content/common/discardable_shared_memory_heap.cc
index 263f83b24ade3e4b2e910681d8446baa7546e4fc..0ef38c9c7454ea7d53897b89ac89da59fca948b3 100644
--- a/content/common/discardable_shared_memory_heap.cc
+++ b/content/common/discardable_shared_memory_heap.cc
@@ -423,14 +423,18 @@ void DiscardableSharedMemoryHeap::OnMemoryDump(
           ->GetTracingProcessId();
   base::trace_event::MemoryAllocatorDumpGuid shared_segment_guid =
       GetSegmentGUIDForTracing(tracing_process_id, segment_id);
-  pmd->CreateSharedGlobalAllocatorDump(shared_segment_guid);
+  base::trace_event::MemoryAllocatorDump* global_dump =
+      pmd->CreateSharedGlobalAllocatorDump(shared_segment_guid);
+
+  // Set the dump as weak so that it is invalidated if the browser had purged
+  // the segment.
+  global_dump->set_flags(base::trace_event::MemoryAllocatorDump::Flags::WEAK);

[petrcermak, 2016/01/13 17:46:14]

This will work as long as there is no other user of the global dump within the
process. However, even if there's another user of the global dump who created it
first *without* the weak flag, the global dump will now be weak. We want the
inverse semantics. To address this, I think that it would be better to add the
following method:

  pmd->GetOrCreateWeakSharedGlobalAllocatorDump(shared_segment_guid);

which would simply return an existing global dump (possibly strong) or create a
new one and mark is as weak. On the other hand, the existing method:

  pmd->GetOrCreateSharedGlobalAllocatorDump(shared_segment_guid);

would mark an existing global dump as strong and return it, or create a new one
(and keep it marked as strong).

The only issue here is combinatorial explosion of the method signatures.

ssid, primiano: WDYT?

[ssid, 2016/01/15 17:14:56]

Done.

 
   // The size is added to the global dump so that it gets propagated to both the
   // dumps associated.
-  pmd->GetSharedGlobalAllocatorDump(shared_segment_guid)
-      ->AddScalar(base::trace_event::MemoryAllocatorDump::kNameSize,
-                  base::trace_event::MemoryAllocatorDump::kUnitsBytes,
-                  allocated_objects_size_in_bytes);
+  global_dump->AddScalar(base::trace_event::MemoryAllocatorDump::kNameSize,
+                         base::trace_event::MemoryAllocatorDump::kUnitsBytes,
+                         allocated_objects_size_in_bytes);
 
   // By creating an edge with a higher |importance| (w.r.t. browser-side dumps)
   // the tracing UI will account the effective size of the segment to the child.