Add Get/SetUniqueIdForProcess.

base/process/process_handle.h

Index: base/process/process_handle.h
diff --git a/base/process/process_handle.h b/base/process/process_handle.h
index 77f2c585cfcc71087596855fa6f83ac643ce9a6c..6255abb4b250df48694620fcbca286ac12259944 100644
--- a/base/process/process_handle.h
+++ b/base/process/process_handle.h
@@ -34,16 +34,37 @@ const ProcessHandle kNullProcessHandle = 0;
 const ProcessId kNullProcessId = 0;
 #endif  // defined(OS_WIN)
 
+// This is not a valid process ID on Linux, Mac, or Windows.

[danakj, 2015/06/17 17:17:16]

Is it on android?

[rickyz (no longer on Chrome), 2015/06/17 19:53:41]

Yeah, I considered that to be part of Linux since this is a kernel detail -
added a mention to the comment.

+const uint32 kInvalidUniqueId = 0;
+
 // Returns the id of the current process.
+// Note that on some platforms, this is not guaranteed to be unique across
+// processes (use GetUniqueIdForProcess if uniqueness is required).
 BASE_EXPORT ProcessId GetCurrentProcId();
 
+// Returns a unique ID for the current process. The ID will be unique across all
+// currently running processes within the chrome session, but note that IDs of
+// terminated processes may be reused. This defaults to the process's process
+// ID, but may return a different value if SetUniqueIdForProcess has been
+// called.
+BASE_EXPORT uint32 GetUniqueIdForProcess();
+
+// Sets the unique ID for the current process. |unique_id| may not be
+// kInvalidUniqueId. Not thread safe.
+// WARNING: To avoid inconsistent results from GetUniqueIdForProcess, this

[danakj, 2015/06/17 17:17:16]

Can you DCHECK that when SetUniqueId is called, GetUniqueId hasn't been called
yet?

[rickyz (no longer on Chrome), 2015/06/17 19:53:41]

It's not required to call SetUniqueId before GetUniqueId because it defaults to
the process id - we only call SetUniqueId if separate PID namespaces are being
used on Linux, which is what causes the getpid() = 1 situation. Would folks
prefer not to have a default behavior and to always require that SetUniqueId be
called even if it's to set it to the pid? I avoided doing that for this change
because it'd involve adding calls to every place (on all platforms) where we
fork.

[danakj, 2015/06/17 19:56:23]

What I'd like is for GetUniqueId to always return a single thing for every call,
and situations that would deviate would trigger a DCHECK. Ever getting 2
different return values sounds like a bug.

+// should only be called very early after process startup - ideally as soon
+// after process creation as possible.
+BASE_EXPORT void SetUniqueIdForProcess(uint32 unique_id);
+
 // Returns the ProcessHandle of the current process.
 BASE_EXPORT ProcessHandle GetCurrentProcessHandle();
 
-// Returns the unique ID for the specified process. This is functionally the
-// same as Windows' GetProcessId(), but works on versions of Windows before
-// Win XP SP1 as well.
+// Returns the process ID for the specified process. This is functionally the
+// same as Windows' GetProcessId(), but works on versions of Windows before Win
+// XP SP1 as well.
 // DEPRECATED. New code should be using Process::Pid() instead.
+// Note that on some platforms, this is not guaranteed to be unique across
+// processes.
 BASE_EXPORT ProcessId GetProcId(ProcessHandle process);
 
 #if defined(OS_POSIX)