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

base/trace_event/memory_dump_manager.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TRACE_EVENT_MEMORY_DUMP_MANAGER_H_
 #define BASE_TRACE_EVENT_MEMORY_DUMP_MANAGER_H_
 
 #include <vector>
 
 #include "base/atomicops.h"
@@ skipping 18 matching lines @@
 class MemoryDumpManagerDelegate;
 class MemoryDumpProvider;
 class ProcessMemoryDump;
 class MemoryDumpSessionState;
 
 // This is the interface exposed to the rest of the codebase to deal with
 // memory tracing. The main entry point for clients is represented by
 // 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();
 
   // Invoked once per process to register the TraceLog observer.
   void Initialize();
 
   // See the lifetime and thread-safety requirements on the delegate below in
   // the |MemoryDumpManagerDelegate| docstring.
   void SetDelegate(MemoryDumpManagerDelegate* delegate);
@@ skipping 25 matching lines @@
   void OnTraceLogEnabled() override;
   void OnTraceLogDisabled() override;
 
   // Returns the MemoryDumpSessionState object, which is shared by all the
   // ProcessMemoryDump and MemoryAllocatorDump instances through all the tracing
   // session lifetime.
   const scoped_refptr<MemoryDumpSessionState>& session_state() const {
     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.
   struct MemoryDumpProviderInfo {
     MemoryDumpProviderInfo(
         const scoped_refptr<SingleThreadTaskRunner>& task_runner);
     ~MemoryDumpProviderInfo();
 
     scoped_refptr<SingleThreadTaskRunner> task_runner;  // Optional.
     int consecutive_failures;  // Number of times the provider failed (to
@@ skipping 38 matching lines @@
   // to guard against disabling logging while dumping on another thread.
   Lock lock_;
 
   // Optimization to avoid attempting any memory dump (i.e. to not walk an empty
   // dump_providers_enabled_ list) when tracing is not enabled.
   subtle::AtomicWord memory_tracing_enabled_;
 
   // 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_;
 
   DISALLOW_COPY_AND_ASSIGN(MemoryDumpManager);
 };
 
 // The delegate is supposed to be long lived (read: a Singleton) and thread
 // safe (i.e. should expect calls from any thread and handle thread hopping).
 class BASE_EXPORT MemoryDumpManagerDelegate {
  public:
@@ skipping 14 matching lines @@
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MemoryDumpManagerDelegate);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_MEMORY_DUMP_MANAGER_H_