Add Get/SetUniqueIdForProcess.

base/process/process_handle.cc

Index: base/process/process_handle.cc
diff --git a/base/process/process_handle.cc b/base/process/process_handle.cc
new file mode 100644
index 0000000000000000000000000000000000000000..59751b1ae5ca6585b76046d7f27bbc9bb987e50d
--- /dev/null
+++ b/base/process/process_handle.cc
@@ -0,0 +1,40 @@
+// 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 <stdint.h>
+
+#include "base/logging.h"
+#include "base/process/process_handle.h"
+
+namespace base {
+
+namespace {
+uint32_t g_unique_id = kInvalidUniqueId;
+
+#if DCHECK_IS_ON()
+// The process which set |g_unique_id|.
+uint32_t g_procid;
+#endif
+}  // namespace
+
+uint32_t GetUniqueIdForProcess() {
+  if (g_unique_id == kInvalidUniqueId) {
+    return static_cast<uint32_t>(GetCurrentProcId());

[danakj, 2015/08/05 19:52:26]

Is this branch basically for the browser process/other OS? Can we just call
SetUniqueIdForProcess always, and this always returns the set id?

[rickyz (no longer on Chrome), 2015/08/13 23:16:55]

Yeah, I think it ends up being just the browser process and other OSes. I was
afraid to require SetUniqueIdForProcess to be called because there are still
some situations where we might forget to call it - for example, there are tests
which fork, and there is also code which may call GetUniqueIdForProcess, but
goes through a different main() function than chrome (tests are also an example
of this). 

[danakj, 2015/09/15 18:36:32]

This still really bothers me :/ It feels very unclear as a caller what I can
expect here. Sometimes I can take this and use it as a PID sometimes I can't.

Why can't you just DCHECK that SetUniqueIdForProcess was called before calling
this. Then if any tests that you're worried about use this incorrectly, they'll
DCHECK and be easily fixable.

I really hate having warts in production code that is just for tests.

+  }
+
+  // Make sure we are the same process that set |g_procid|. This check may have
+  // false negatives (if a process ID was reused) but should have no false
+  // positives.
+  DCHECK_EQ(static_cast<uint32_t>(GetCurrentProcId()), g_procid);
+  return g_unique_id;
+}
+
+void SetUniqueIdForProcess(uint32_t unique_id) {
+  g_unique_id = unique_id;

[danakj, 2015/08/05 19:52:26]

can you DCHECK_GT(unique_id, 0) ?

[rickyz (no longer on Chrome), 2015/08/13 23:16:55]

Done.

+#if DCHECK_IS_ON()
+  g_procid = static_cast<uint32_t>(GetCurrentProcId());
+#endif
+}
+
+}  // namespace base