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

base/trace_event/memory_dump_manager.cc

 // 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.
 
 #include "base/trace_event/memory_dump_manager.h"
 
 #include <algorithm>
 
 #include "base/atomic_sequence_num.h"
 #include "base/compiler_specific.h"
+#include "base/hash.h"
+#include "base/strings/string_number_conversions.h"
 #include "base/thread_task_runner_handle.h"
 #include "base/trace_event/memory_dump_provider.h"
 #include "base/trace_event/memory_dump_session_state.h"
 #include "base/trace_event/process_memory_dump.h"
 #include "base/trace_event/trace_event_argument.h"
 #include "build/build_config.h"
 
 #if !defined(OS_NACL)
 #include "base/trace_event/process_memory_totals_dump_provider.h"
 #endif
@@ skipping 129 matching lines @@
 // static
 void MemoryDumpManager::SetInstanceForTesting(MemoryDumpManager* instance) {
   if (instance)
     instance->skip_core_dumpers_auto_registration_for_testing_ = true;
   g_instance_for_testing = instance;
 }
 
 MemoryDumpManager::MemoryDumpManager()
     : delegate_(nullptr),
       memory_tracing_enabled_(0),
+      tracing_process_id_(kInvalidTracingProcessId),
       skip_core_dumpers_auto_registration_for_testing_(false) {
   g_next_guid.GetNext();  // Make sure that first guid is not zero.
 }
 
 MemoryDumpManager::~MemoryDumpManager() {
   base::trace_event::TraceLog::GetInstance()->RemoveEnabledStateObserver(this);
 }
 
 void MemoryDumpManager::Initialize() {
   TRACE_EVENT0(kTraceCategory, "init");  // Add to trace-viewer category list.
@@ skipping 255 matching lines @@
   }
 }
 
 void MemoryDumpManager::OnTraceLogDisabled() {
   AutoLock lock(lock_);
   periodic_dump_timer_.Stop();
   subtle::NoBarrier_Store(&memory_tracing_enabled_, 0);
   session_state_ = nullptr;
 }
 
+// static
+int MemoryDumpManager::ChildProcessIdToTracingProcessId(int child_process_id) {
+  return Hash(IntToString(child_process_id));

[Primiano Tucci (use gerrit), 2015/06/24 17:47:35]

Ok for the Hash, but do:
Hash(reinterpret_cast<const char*>(&child_process_id),
sizeof(child_process_id));
as that will spread out the state space more.
I tested it and got the first collision after 260k elements, which should be
good enough.
If you convert to ascii, the state space is too small and collides more.

+}
+
 MemoryDumpManager::MemoryDumpProviderInfo::MemoryDumpProviderInfo(
     const scoped_refptr<SingleThreadTaskRunner>& task_runner)
     : task_runner(task_runner), consecutive_failures(0), disabled(false) {
 }
 MemoryDumpManager::MemoryDumpProviderInfo::~MemoryDumpProviderInfo() {
 }
 
 }  // namespace trace_event
 }  // namespace base