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

base/trace_event/memory_allocator_dump_guid.cc

Index: base/trace_event/memory_allocator_dump_guid.cc
diff --git a/base/trace_event/memory_allocator_dump_guid.cc b/base/trace_event/memory_allocator_dump_guid.cc
index a4ea50d416a0d5bd784e7c74501f77ad45d7b49f..2b5c1bba27d07deeb414714e71a997a29689537c 100644
--- a/base/trace_event/memory_allocator_dump_guid.cc
+++ b/base/trace_event/memory_allocator_dump_guid.cc
@@ -6,9 +6,25 @@
 
 #include "base/format_macros.h"
 #include "base/hash.h"
+#include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 
 namespace base {
+namespace {
+int unique_child_process_id = -1;
+
+uint64 InsertChildIdAndHash(std::string guid_str) {
+  const char kReplaceChildIdStr[] = "$$";

[picksi, 2015/06/16 16:18:37]

Random thought: Are we ever going to want to add other 'generated' strings here?
If so '$$' is not very extensible. If we used e.g. {P} for Process ID we could
easily extend it to {T} for timestamp (etc).

[ssid, 2015/06/19 10:18:14]

primiano@ WDYT?

+  size_t pos = guid_str.find(kReplaceChildIdStr);

[picksi, 2015/06/16 16:18:37]

Does this code need to work fast? If so we could add a magic first character
(E.g. a '*') that flags that this string contains a "replaceable" item. So if
guid_str[0] != '*' we can avoid the string search.

[ssid, 2015/06/19 10:18:14]

hm, The string length is going to be small, so i don't think that the search
will make a difference. Also this will add extra logic for each allocator to add
*.

+  if (pos != std::string::npos) {
+    DCHECK_NE(unique_child_process_id, -1);
+    guid_str.replace(pos, sizeof(kReplaceChildIdStr) - 1,
+                     IntToString(unique_child_process_id));
+  }
+  return Hash(guid_str);
+}
+}  // namespace
+
 namespace trace_event {
 
 MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid(uint64 guid) : guid_(guid) {
@@ -19,7 +35,12 @@ MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid()
 }
 
 MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid(const std::string& guid_str)
-    : MemoryAllocatorDumpGuid(Hash(guid_str)) {
+    : MemoryAllocatorDumpGuid(InsertChildIdAndHash(guid_str)) {
+}
+
+// static
+void MemoryAllocatorDumpGuid::SetUniqueChildProcessId(int child_process_id) {
+  unique_child_process_id = child_process_id;
 }
 
 std::string MemoryAllocatorDumpGuid::ToString() const {