Revert of Mojo: Armed Watchers

mojo/public/c/system/functions.h

Index: mojo/public/c/system/functions.h
diff --git a/mojo/public/c/system/functions.h b/mojo/public/c/system/functions.h
index 750a29ee0728c368c1ac8761f1129979ef0783b1..f0a23d931dee03583dc464afb4b2b7dd4ef9d2a1 100644
--- a/mojo/public/c/system/functions.h
+++ b/mojo/public/c/system/functions.h
@@ -18,6 +18,14 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
+
+// A callback used to notify watchers registered via |MojoWatch()|. Called when
+// some watched signals are satisfied or become unsatisfiable. See the
+// documentation for |MojoWatch()| for more details.
+typedef void (*MojoWatchCallback)(uintptr_t context,
+                                  MojoResult result,
+                                  struct MojoHandleSignalsState signals_state,
+                                  MojoWatchNotificationFlags flags);
 
 // Note: Pointer parameters that are labelled "optional" may be null (at least
 // under some circumstances). Non-const pointer parameters are also labeled
@@ -130,6 +138,74 @@
              uint32_t* result_index,                          // Optional out
              struct MojoHandleSignalsState* signals_states);  // Optional out
 
+// Watches the given handle for one of the following events to happen:
+//   - A signal indicated by |signals| is satisfied.
+//   - It becomes known that no signal indicated by |signals| will ever be
+//     satisfied. (See the description of the |MOJO_RESULT_CANCELLED| and
+//     |MOJO_RESULT_FAILED_PRECONDITION| return values below.)
+//   - The handle is closed.
+//
+// |handle|: The handle to watch. Must be an open message pipe or data pipe
+//     handle.
+// |signals|: The signals to watch for.
+// |callback|: A function to be called any time one of the above events happens.
+//     The function must be safe to call from any thread at any time.
+// |context|: User-provided context passed to |callback| when called. |context|
+//     is used to uniquely identify a registered watch and can be used to cancel
+//     the watch later using |MojoCancelWatch()|.
+//
+// Returns:
+//   |MOJO_RESULT_OK| if the watch has been successfully registered. Note that
+//       if the signals are already satisfied this may synchronously invoke
+//       |callback| before returning.
+//   |MOJO_RESULT_CANCELLED| if the watch was cancelled. In this case it is not
+//       necessary to explicitly call |MojoCancelWatch()|, and in fact it may be
+//       an error to do so as the handle may have been closed.
+//   |MOJO_RESULT_INVALID_ARGUMENT| if |handle| is not an open message pipe
+//       handle.
+//   |MOJO_RESULT_FAILED_PRECONDITION| if it is already known that |signals| can
+//       never be satisfied.
+//   |MOJO_RESULT_ALREADY_EXISTS| if there is already a watch registered for
+//       the same combination of |handle| and |context|.
+//
+// Callback result codes:
+//   The callback may be called at any time on any thread with one of the
+//   following result codes to indicate various events:
+//
+//   |MOJO_RESULT_OK| indicates that some signal in |signals| has been
+//       satisfied.
+//   |MOJO_RESULT_FAILED_PRECONDITION| indicates that no signals in |signals|
+//       can ever be satisfied again.
+//   |MOJO_RESULT_CANCELLED| indicates that the handle has been closed. In this
+//       case the watch is implicitly cancelled and there is no need to call
+//       |MojoCancelWatch()|.
+MOJO_SYSTEM_EXPORT MojoResult
+MojoWatch(MojoHandle handle,
+          MojoHandleSignals signals,
+          MojoWatchCallback callback,
+          uintptr_t context);
+
+// Cancels a handle watch corresponding to some prior call to |MojoWatch()|.
+//
+// NOTE: If the watch callback corresponding to |context| is currently running
+// this will block until the callback completes execution. It is therefore
+// illegal to call |MojoCancelWatch()| on a given |handle| and |context| from
+// within the associated callback itself, as this will always deadlock.
+//
+// After |MojoCancelWatch()| function returns, the watch's associated callback
+// will NEVER be called again by Mojo.
+//
+// |context|: The same user-provided context given to some prior call to
+//     |MojoWatch()|. Only the watch corresponding to this context will be
+//     cancelled.
+//
+// Returns:
+//     |MOJO_RESULT_OK| if the watch corresponding to |context| was cancelled.
+//     |MOJO_RESULT_INVALID_ARGUMENT| if no watch was registered with |context|
+//         for the given |handle|, or if |handle| is invalid.
+MOJO_SYSTEM_EXPORT MojoResult
+MojoCancelWatch(MojoHandle handle, uintptr_t context);
+
 // Retrieves system properties. See the documentation for |MojoPropertyType| for
 // supported property types and their corresponding output value type.
 //