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..ef57953007b11aee21d23c6ac1dad516286d68e0
--- /dev/null
+++ b/base/process/process_handle.cc
@@ -0,0 +1,53 @@
+// 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"
+#include "build/build_config.h"
+
+namespace base {
+
+namespace {
+// This is never a valid mangled process ID.

[danakj, 2015/09/23 21:56:32]

This is getting kinda magical and hard to really believe. Maybe just use a bool
and set it true when you set the unique_id? That'd be much safer.

[rickyz (no longer on Chrome), 2015/09/24 01:22:03]

Good idea, done

+const uint32_t kInvalidUniqueId = 0xffffffffU;
+
+uint32_t g_unique_id = kInvalidUniqueId;
+
+// The process which set |g_unique_id|.
+ProcessId g_procid;
+
+// Mangle IDs so that they are not accidentally used as PIDs, e.g. as an
+// argument to kill or waitpid.
+uint32_t MangleProcessId(ProcessId process_id) {
+  // Add a large power of 10 so that the pid is still the pid is still readable

[danakj, 2015/09/23 21:56:31]

"the pid is still the pid is still readable"

"the pid is still readable inside the mangled id"?

[rickyz (no longer on Chrome), 2015/09/24 01:22:03]

Done

+  // for debugging
+  return static_cast<uint32_t>(process_id) + 1000000000U;
+}
+
+}  // namespace
+
+uint32_t GetUniqueIdForProcess() {
+  if (g_unique_id == kInvalidUniqueId) {
+    return MangleProcessId(GetCurrentProcId());
+  }
+
+  // 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(GetCurrentProcId(), g_procid);
+  return g_unique_id;
+}
+
+#if defined(OS_LINUX)
+
+void InitUniqueIdForProcessInPidNamespace(ProcessId pid_outside_of_namespace) {
+  g_unique_id = MangleProcessId(pid_outside_of_namespace);
+  g_procid = GetCurrentProcId();
+}
+
+#endif
+
+}  // namespace base