[mojo-edk] Expose notification source to MojoWatch callbacks

mojo/public/c/system/types.h

 // Copyright 2014 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.
 
 // This file contains types and constants/macros common to different Mojo system
 // APIs.
 //
 // Note: This header should be compilable as C.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_TYPES_H_
@@ skipping 164 matching lines @@
 // Note: This struct is not extensible (and only has 32-bit quantities), so it's
 // 32-bit-aligned.
 MOJO_STATIC_ASSERT(MOJO_ALIGNOF(int32_t) == 4, "int32_t has weird alignment");
 struct MOJO_ALIGNAS(4) MojoHandleSignalsState {
   MojoHandleSignals satisfied_signals;
   MojoHandleSignals satisfiable_signals;
 };
 MOJO_STATIC_ASSERT(sizeof(MojoHandleSignalsState) == 8,
                    "MojoHandleSignalsState has wrong size");
 
+// |MojoWatchNotificationFlags|: Passed to a callback invoked as a result of
+// signals being raised on a handle watched by |MojoWatch()|. May take the
+// following values:
+//   |MOJO_WATCH_NOTIFICATION_EXTERNAL_PROCESS| - The callback is being invoked

[Anand Mistry (off Chromium), 2016/03/16 23:04:14]

Not a fan of the "EXTERNAL_PROCESS" name, since it:
1. Leaks implementation details
2. Doesn't really reflect the meaning

As discussed, other possibilities:
- Re-entrant safe
- System
- Something, something, empty stack something

[Ken Rockot(use gerrit already), 2016/03/17 00:15:34]

Went with FROM_SYSTEM for the public API flag. The internal Source name is just
SYSTEM now.

+//       as a result of an incoming event from out-of-process. This may be used
+//       as an indication that user code is safe to call without fear of
+//       reentry.
+
+typedef uint32_t MojoWatchNotificationFlags;
+
+#ifdef __cplusplus
+const MojoWatchNotificationFlags MOJO_WATCH_NOTIFICATION_FLAG_NONE = 0;
+const MojoWatchNotificationFlags MOJO_WATCH_NOTIFICATION_FLAG_EXTERNAL_PROCESS =
+    1 << 0;
+#else
+#define MOJO_WATCH_NOTIFICATION_FLAG_NONE ((MojoWatchNotificationFlags)0)
+#define MOJO_WATCH_NOTIFICATION_FLAG_EXTERNAL_PROCESS \
+    ((MojoWatchNotificationFlags)1 << 0);
+#endif
+
 #endif  // MOJO_PUBLIC_C_SYSTEM_TYPES_H_