[tracing] Send unique tracing process id for cross-process memory dumps identification.

base/trace_event/memory_dump_manager.h

Index: base/trace_event/memory_dump_manager.h
diff --git a/base/trace_event/memory_dump_manager.h b/base/trace_event/memory_dump_manager.h
index e34bdb0853cc6200d3a494969fe8582eb2040f79..60aeab98ee6a1111875a110b43e319d9c2eeabbb 100644
--- a/base/trace_event/memory_dump_manager.h
+++ b/base/trace_event/memory_dump_manager.h
@@ -36,6 +36,7 @@ class MemoryDumpSessionState;
 // RequestDumpPoint(). The extension by Un(RegisterDumpProvider).
 class BASE_EXPORT MemoryDumpManager : public TraceLog::EnabledStateObserver {
  public:
+  static const int kInvalidTracingProcessId = -1;
   static const char* const kTraceCategoryForTesting;
 
   static MemoryDumpManager* GetInstance();
@@ -81,6 +82,20 @@ class BASE_EXPORT MemoryDumpManager : public TraceLog::EnabledStateObserver {
     return session_state_;
   }
 
+  // Sets the unique hash value for identifying child process to create cross

[Primiano Tucci (use gerrit), 2015/06/23 13:35:20]

Don't expose the "hash" thing here. Just clarify that is neither the PID nor the
child process ID
See below

[ssid, 2015/06/24 08:37:57]

hm, base/ shouldn't really know about child process id. It is content's
implementation. That is why I din't mention that in the comment. I changed the
comments now, WDYT?

+  // process unique guids. Pass kInvalidTracingProcessId for invalidating the
+  // id.
+  void set_tracing_process_id(int id) {

[Primiano Tucci (use gerrit), 2015/06/23 13:35:20]

Can you make the setter private and use the "friend backdoor"? so it is explicit
that only the MDMDelegate should set it?
The way you use the "backdoor" is  as follows (see CreateProcessDump as a
pattern):

- make this setter a private: member of MDManger
- Create a set_tracing_process_id method in the protected section
MemoryDumpManagerDelegate, which calls MDM::set_tracing_process_id
- Call the proxy MDMDelegate::set_tracing_process_id method from
child_trace_message_filter

[ssid, 2015/06/24 08:37:57]

Done.

+    DCHECK(tracing_process_id_ == kInvalidTracingProcessId ||
+           id == kInvalidTracingProcessId);
+    tracing_process_id_ = id;
+  }
+
+  int tracing_process_id() {

[Primiano Tucci (use gerrit), 2015/06/23 13:35:20]

+const

[ssid, 2015/06/24 08:37:57]

Done.

+    DCHECK_NE(tracing_process_id_, kInvalidTracingProcessId);

[Primiano Tucci (use gerrit), 2015/06/23 13:35:20]

add a comment saying that if you are requesting this you might be out of the
scope of a trace

[ssid, 2015/06/24 08:37:57]

Done.

+    return tracing_process_id_;
+  }
+
  private:
   // Descriptor struct used to hold information about registered MDPs. It is
   // deliberately copyable, in order to allow to be used as hash_map value.
@@ -139,6 +154,10 @@ class BASE_EXPORT MemoryDumpManager : public TraceLog::EnabledStateObserver {
   // For time-triggered periodic dumps.
   RepeatingTimer<MemoryDumpManager> periodic_dump_timer_;
 
+  // The unique id of the child process for tracing. The value is expected to

[Primiano Tucci (use gerrit), 2015/06/23 13:35:20]

Can you clarify that this is neither the PID nor the child process ID but a hash
that can be obtained via ...

+  // be valid only when tracing is enabled.
+  int tracing_process_id_;
+
   // Skips the auto-registration of the core dumpers during Initialize().
   bool skip_core_dumpers_auto_registration_for_testing_;