Implement off-the-wire validation scheme for enum types.

ipc/ipc_message_macros.h

Index: ipc/ipc_message_macros.h
diff --git a/ipc/ipc_message_macros.h b/ipc/ipc_message_macros.h
index 2993014f0468539f89afaf4c0089d6c8d2689e7f..cd036802528211db7f582bf9e1a1740d07eeb19e 100644
--- a/ipc/ipc_message_macros.h
+++ b/ipc/ipc_message_macros.h
@@ -130,7 +130,9 @@
 // to proclaim equivalent struct declarations for use by callers, as well
 // as later registering the type with the message generation.  Note that
 // IPC_STRUCT_MEMBER() is only permitted inside matching calls to
-// IPC_STRUCT_BEGIN() / IPC_STRUCT_END().
+// IPC_STRUCT_BEGIN() / IPC_STRUCT_END(). There is also an
+// IPC_STRUCT_BEGIN_WITH_PARENT(), which behaves like IPC_STRUCT_BEGIN(),
+// but also accomodates structs that inherit from other structs.
 //
 // Externally-defined structs are registered with IPC_STRUCT_TRAITS_BEGIN(),
 // IPC_STRUCT_TRAITS_MEMBER(), and IPC_STRUCT_TRAITS_END() macros. These
@@ -141,8 +143,16 @@
 // inside matching calls to IPC_STRUCT_TRAITS_BEGIN() /
 // IPC_STRUCT_TRAITS_END().
 //
-// Enum types are registered with a single IPC_ENUM_TRAITS() macro.  There
-// is no need to enumerate each value to the IPC mechanism.
+// Enum types are registered with a single IPC_ENUM_TRAITS_VALIDATE() macro.
+// There is no need to enumerate each value to the IPC mechanism. Instead,
+// pass an expression in terms of the parameter |value| to provide
+// range-checking.  For convenience, the IPC_ENUM_TRAITS() is provided which
+// performs no checking, passing everything including out-of-range values.
+// Its use is discouraged. The IPC_ENUM_TRAITS_MAX_VALUE() macro can be used
+// for the typical case where the enum must be in the range 0..maxvalue
+// inclusive. The IPC_ENUM_TRAITS_MIN_MAX_VALUE() macro can be used for the
+// less typical case where the enum must be in the range minvalue..maxvalue
+// inclusive.
 //
 // Do not place semicolons following these IPC_ macro invocations.  There
 // is no reason to expect that their expansion corresponds one-to-one with
@@ -192,9 +202,12 @@
 #include "ipc/ipc_message_utils_impl.h"
 #endif
 
-// Macros for defining structs.  May be subsequently redefined.
+// Convenience macro for defining structs without inheritence. Should not need
+// to be subsequently redefined.
 #define IPC_STRUCT_BEGIN(struct_name) \
   IPC_STRUCT_BEGIN_WITH_PARENT(struct_name, IPC::NoParams)
+
+// Macros for defining structs. Will be subsequently redefined.
 #define IPC_STRUCT_BEGIN_WITH_PARENT(struct_name, parent) \
   struct struct_name; \
   IPC_STRUCT_TRAITS_BEGIN(struct_name) \