Add service_opt() strict_mbit

There are differing opinions on whether or not reception of an arbitrary
AVP setting the M-bit is an error. 1.3.4 of RFC 6733 says this about
how an existing Diameter application may be modified:

   o  The M-bit allows the sender to indicate to the receiver whether or
      not understanding the semantics of an AVP and its content is
      mandatory.  If the M-bit is set by the sender and the receiver
      does not understand the AVP or the values carried within that AVP,
      then a failure is generated (see Section 7).

   It is the decision of the protocol designer when to develop a new
   Diameter application rather than extending Diameter in other ways.
   However, a new Diameter application MUST be created when one or more
   of the following criteria are met:

   M-bit Setting

      An AVP with the M-bit in the MUST column of the AVP flag table is
      added to an existing Command/Application.  An AVP with the M-bit
      in the MAY column of the AVP flag table is added to an existing
      Command/Application.

The point here is presumably interoperability: that the command grammar
should specify explicitly what mandatory AVPs much be understood, and
that anything more is an error.

On the other hand, 3.2 says thus about command grammars:

   avp-name         = avp-spec / "AVP"
                      ; The string "AVP" stands for *any* arbitrary AVP
                      ; Name, not otherwise listed in that Command Code
                      ; definition.  The inclusion of this string
                      ; is recommended for all CCFs to allow for
                      ; extensibility.

This renders 1.3.4 pointless unless "*any* AVP" is qualified by "not
setting the M-bit", since the sender can effectively violate 1.3.4
without this necessitating an error at the receiver. If clients add
arbitrary AVPs setting the M-bit then request handling becomes more
implementation-dependent.

The current interpretation in diameter is strict: if a command grammar
doesn't explicitly allow an AVP setting the M-bit then reception of such
an AVP is regarded as an error. The strict_mbit option now allows this
behaviour to be changed, false turning all responsibility for the M-bit
over to the user.

5 files changed, 16 insertions, 6 deletions

diff --git a/lib/diameter/src/base/diameter.erl b/lib/diameter/src/base/diameter.erl
index 010f977b97..9d71bcbbf8 100644
--- a/lib/diameter/src/base/diameter.erl
+++ b/lib/diameter/src/base/diameter.erl
@@ -311,6 +311,7 @@ call(SvcName, App, Message) ->
     | {sequence, sequence() | evaluable()}
     | {share_peers, remotes()}
     | {string_decode, boolean()}
+    | {strict_mbit, boolean()}
     | {incoming_maxlen, message_length()}
     | {use_shared_peers, remotes()}
     | {spawn_opt, list()}.
diff --git a/lib/diameter/src/base/diameter_codec.erl b/lib/diameter/src/base/diameter_codec.erl
index f900bb0c5e..aab8b5887e 100644
--- a/lib/diameter/src/base/diameter_codec.erl
+++ b/lib/diameter/src/base/diameter_codec.erl
@@ -76,9 +76,10 @@ setopts(Opts)
   when is_list(Opts) ->
     lists:foreach(fun setopt/1, Opts).
 
-%% Decode stringish types to string()? The default true is for
-%% backwards compatibility.
-setopt({string_decode = K, false = B}) ->
+%% The default string_decode true is for backwards compatibility.
+setopt({K, false = B})
+  when K == string_decode;
+       K == strict_mbit ->
     setopt(K, B);
 
 %% Regard anything but the generated RFC 3588 dictionary as modern.
@@ -96,7 +97,8 @@ setopt(Key, Value) ->
 
 getopt(Key) ->
     case get({diameter, Key}) of
-        undefined when Key == string_decode ->
+        undefined when Key == string_decode;
+                       Key == strict_mbit ->
             true;
         undefined when Key == rfc ->
             6733;
diff --git a/lib/diameter/src/base/diameter_config.erl b/lib/diameter/src/base/diameter_config.erl
index 89b26707ad..242b6b4d08 100644
--- a/lib/diameter/src/base/diameter_config.erl
+++ b/lib/diameter/src/base/diameter_config.erl
@@ -646,6 +646,7 @@ make_config(SvcName, Opts) ->
                          {?NOMASK, sequence},
                          {nodes, restrict_connections},
                          {16#FFFFFF, incoming_maxlen},
+                         {true, strict_mbit},
                          {true, string_decode},
                          {[], spawn_opt}]),
 
@@ -684,12 +685,14 @@ opt(K, false = B)
        K == use_shared_peers;
        K == monitor;
        K == restrict_connections;
+       K == strict_mbit;
        K == string_decode ->
     B;
 
 opt(K, true = B)
   when K == share_peers;
        K == use_shared_peers;
+       K == strict_mbit;
        K == string_decode ->
     B;
 
diff --git a/lib/diameter/src/base/diameter_service.erl b/lib/diameter/src/base/diameter_service.erl
index a31cef2c8c..7d4a24f7a0 100644
--- a/lib/diameter/src/base/diameter_service.erl
+++ b/lib/diameter/src/base/diameter_service.erl
@@ -128,6 +128,7 @@
              | {share_peers, diameter:remotes()}       %% broadcast to
              | {use_shared_peers, diameter:remotes()}  %% use from
              | {restrict_connections, diameter:restriction()}
+             | {strict_mbit, boolean()}
              | {string_decode, boolean()}
              | {incoming_maxlen, diameter:message_length()}]}).
 %% shared_peers reflects the peers broadcast from remote nodes.
@@ -697,7 +698,8 @@ service_options(Opts) ->
                                                 ?RESTRICT)},
      {spawn_opt, proplists:get_value(spawn_opt, Opts, [])},
      {string_decode, proplists:get_value(string_decode, Opts, true)},
-     {incoming_maxlen, proplists:get_value(incoming_maxlen, Opts, 16#FFFFFF)}].
+     {incoming_maxlen, proplists:get_value(incoming_maxlen, Opts, 16#FFFFFF)},
+     {strict_mbit, proplists:get_value(strict_mbit, Opts, true)}].
 %% The order of options is significant since we match against the list.
 
 mref(false = No) ->
diff --git a/lib/diameter/src/base/diameter_traffic.erl b/lib/diameter/src/base/diameter_traffic.erl
index 230a05fa11..6be2c35d48 100644
--- a/lib/diameter/src/base/diameter_traffic.erl
+++ b/lib/diameter/src/base/diameter_traffic.erl
@@ -79,6 +79,7 @@
          apps         :: [#diameter_app{}],
          sequence     :: diameter:sequence(),
          codec        :: [{string_decode, boolean()}
+                          | {strict_mbit, boolean()}
                           | {incoming_maxlen, diameter:message_length()}]}).
 %% Note that incoming_maxlen is currently handled in diameter_peer_fsm,
 %% so that any message exceeding the maximum is discarded. Retain the
@@ -108,7 +109,8 @@ make_recvdata([SvcName, PeerT, Apps, SvcOpts | _]) ->
               sequence = Mask,
               codec = [T || {K,_} = T <- SvcOpts,
                             lists:member(K, [string_decode,
-                                             incoming_maxlen])]}.
+                                             incoming_maxlen,
+                                             strict_mbit])]}.
 
 %% ---------------------------------------------------------------------------
 %% peer_up/1