add support for checking if an ERL_NIF_TERM is an exception

Add the enif_is_exception function to allow callers to determine
whether an ERL_NIF_TERM represents an exception. (Currently the only
supported exception is badarg since only enif_make_badarg exists, but
this will likely be expanded in future releases.) This allows NIF code
to call other NIF functions that return ERL_NIF_TERM and properly
check to see if the returned terms are exceptions. Without the
enif_is_exception function, developers have to create their own means
of checking whether a function creates an exception, such as returning
boolean success/failure indicators or some other special value
indicating that an exception is in effect.

The declaration of enif_is_exception in erl_nif_api_funcs.h respects
the order of declarations required to keep compatibility on Windows.

Add a new test to verify the operation of enif_is_exception.

Modify the erl_nif man page to add a description of enif_is_exception
and also to clarify the requirements of calling the enif_make_badarg
function. If code calls enif_make_badarg, the env passed in gets set
with exception information and so the return value of the calling
function MUST be the badarg term returned from enif_make_badarg. Also
clarify that the result of enif_make_badarg may be passed only to
enif_is_exception and not to any other NIF API functions.

6 files changed, 65 insertions, 5 deletions

diff --git a/erts/doc/src/erl_nif.xml b/erts/doc/src/erl_nif.xml
index 4bbd4e2a54..cdce4ec0b8 100644
--- a/erts/doc/src/erl_nif.xml
+++ b/erts/doc/src/erl_nif.xml
@@ -688,6 +688,10 @@ typedef enum {
       <fsummary>Determine if a term is an empty list</fsummary>
       <desc><p>Return true if <c>term</c> is an empty list.</p></desc>
     </func>
+    <marker id="enif_is_exception"/><func><name><ret>int</ret><nametext>enif_is_exception(ErlNifEnv* env, ERL_NIF_TERM term)</nametext></name>
+      <fsummary>Determine if a term is an exception</fsummary>
+      <desc><p>Return true if <c>term</c> is an exception.</p></desc>
+    </func>
     <func><name><ret>int</ret><nametext>enif_is_fun(ErlNifEnv* env, ERL_NIF_TERM term)</nametext></name>
       <fsummary>Determine if a term is a fun</fsummary>
       <desc><p>Return true if <c>term</c> is a fun.</p></desc>
@@ -738,7 +742,14 @@ typedef enum {
     </func>
     <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_badarg(ErlNifEnv* env)</nametext></name>
       <fsummary>Make a badarg exception.</fsummary>
-      <desc><p>Make a badarg exception to be returned from a NIF.</p></desc>
+      <desc><p>Make a badarg exception to be returned from a NIF, and set
+      an associated exception reason in <c>env</c>. If
+      <c>enif_make_badarg</c> is called, the term it returns <em>must</em>
+      be returned from the function that called it. No other return value
+      is allowed. Also, the term returned from <c>enif_make_badarg</c> may
+      be passed only to
+      <seealso marker="#enif_is_exception">enif_is_exception</seealso> and
+      not to any other NIF API function.</p></desc>
     </func>
     <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_binary(ErlNifEnv* env, ErlNifBinary* bin)</nametext></name>
       <fsummary>Make a binary term.</fsummary>
diff --git a/erts/emulator/beam/erl_nif.c b/erts/emulator/beam/erl_nif.c
index 135c6b0ccc..8b48444904 100644
--- a/erts/emulator/beam/erl_nif.c
+++ b/erts/emulator/beam/erl_nif.c
@@ -430,6 +430,11 @@ int enif_is_list(ErlNifEnv* env, ERL_NIF_TERM term)
     return is_list(term) || is_nil(term);
 }
 
+int enif_is_exception(ErlNifEnv* env, ERL_NIF_TERM term)
+{
+    return term == THE_NON_VALUE;
+}
+
 static void aligned_binary_dtor(struct enif_tmp_obj_t* obj)
 {
     erts_free_aligned_binary_bytes_extra((byte*)obj,ERTS_ALC_T_TMP);
diff --git a/erts/emulator/beam/erl_nif.h b/erts/emulator/beam/erl_nif.h
index 8050b3640a..d028567faf 100644
--- a/erts/emulator/beam/erl_nif.h
+++ b/erts/emulator/beam/erl_nif.h
@@ -31,9 +31,10 @@
 ** 1.0: R13B04
 ** 2.0: R14A
 ** 2.1: R14B02 "vm_variant"
+** 2.2: R14B03 enif_is_exception
 */
 #define ERL_NIF_MAJOR_VERSION 2
-#define ERL_NIF_MINOR_VERSION 1
+#define ERL_NIF_MINOR_VERSION 2
 
 #include <stdlib.h>
 
diff --git a/erts/emulator/beam/erl_nif_api_funcs.h b/erts/emulator/beam/erl_nif_api_funcs.h
index eca506593d..c991b61abe 100644
--- a/erts/emulator/beam/erl_nif_api_funcs.h
+++ b/erts/emulator/beam/erl_nif_api_funcs.h
@@ -1,7 +1,7 @@
 /*
  * %CopyrightBegin%
  *
- * Copyright Ericsson AB 2009-2010. All Rights Reserved.
+ * Copyright Ericsson AB 2009-2011. All Rights Reserved.
  *
  * The contents of this file are subject to the Erlang Public License,
  * Version 1.1, (the "License"); you may not use this file except in
@@ -21,6 +21,13 @@
 #  error This file should not be included directly
 #endif
 
+/*
+** WARNING: add new ERL_NIF_API_FUNC_DECL entries at the bottom of the list
+** to keep compatibility on Windows!!!
+**
+** And don't forget to increase ERL_NIF_MINOR_VERSION in erl_nif.h
+** when adding functions to the API.
+*/
 #ifdef ERL_NIF_API_FUNC_DECL
 ERL_NIF_API_FUNC_DECL(void*,enif_priv_data,(ErlNifEnv*));
 ERL_NIF_API_FUNC_DECL(void*,enif_alloc,(size_t size));
@@ -128,12 +135,17 @@ ERL_NIF_API_FUNC_DECL(int,enif_get_uint64,(ErlNifEnv*, ERL_NIF_TERM term, ErlNif
 ERL_NIF_API_FUNC_DECL(ERL_NIF_TERM,enif_make_int64,(ErlNifEnv*, ErlNifSInt64));
 ERL_NIF_API_FUNC_DECL(ERL_NIF_TERM,enif_make_uint64,(ErlNifEnv*, ErlNifUInt64));
 #endif
+ERL_NIF_API_FUNC_DECL(int,enif_is_exception,(ErlNifEnv*, ERL_NIF_TERM term));
 
 /*
-** Add last to keep compatibility on Windows!!!
+** Add new entries here to keep compatibility on Windows!!!
 */
 #endif
 
+/*
+** Please keep the ERL_NIF_API_FUNC_MACRO list below in the same order
+** as the ERL_NIF_API_FUNC_DECL list above
+*/
 #ifdef ERL_NIF_API_FUNC_MACRO
 #  define enif_priv_data ERL_NIF_API_FUNC_MACRO(enif_priv_data)
 #  define enif_alloc ERL_NIF_API_FUNC_MACRO(enif_alloc)
@@ -243,6 +255,11 @@ ERL_NIF_API_FUNC_DECL(ERL_NIF_TERM,enif_make_uint64,(ErlNifEnv*, ErlNifUInt64));
 #  define enif_make_uint64 ERL_NIF_API_FUNC_MACRO(enif_make_uint64)
 #endif
 
+#  define enif_is_exception ERL_NIF_API_FUNC_MACRO(enif_is_exception)
+
+/*
+** Add new entries here
+*/
 #endif
 
 #ifndef enif_make_list1
diff --git a/erts/emulator/test/nif_SUITE.erl b/erts/emulator/test/nif_SUITE.erl
index b79c30d8d9..91d695d979 100644
--- a/erts/emulator/test/nif_SUITE.erl
+++ b/erts/emulator/test/nif_SUITE.erl
@@ -1121,7 +1121,14 @@ is_checks(Config) when is_list(Config) ->
     ?line ensure_lib_loaded(Config, 1),
     ?line ok = check_is(hejsan, <<19,98>>, make_ref(), ok, fun() -> ok end,
 			self(), hd(erlang:ports()), [], [1,9,9,8],
-			{hejsan, "hejsan", [$h,"ejs",<<"an">>]}).
+			{hejsan, "hejsan", [$h,"ejs",<<"an">>]}),
+    try
+        ?line error = check_is_exception(),
+        ?line throw(expected_badarg)
+    catch
+        error:badarg ->
+            ?line ok
+    end.
 
 get_length(doc) -> ["Test all enif_get_length functions"];
 get_length(Config) when is_list(Config) ->
@@ -1245,6 +1252,7 @@ release_resource(_) -> ?nif_stub.
 last_resource_dtor_call() -> ?nif_stub.
 make_new_resource(_,_) -> ?nif_stub.
 check_is(_,_,_,_,_,_,_,_,_,_) -> ?nif_stub.
+check_is_exception() -> ?nif_stub.
 length_test(_,_,_,_,_) -> ?nif_stub.
 make_atoms() -> ?nif_stub.
 make_strings() -> ?nif_stub.
diff --git a/erts/emulator/test/nif_SUITE_data/nif_SUITE.c b/erts/emulator/test/nif_SUITE_data/nif_SUITE.c
index 8489124966..dc047394b5 100644
--- a/erts/emulator/test/nif_SUITE_data/nif_SUITE.c
+++ b/erts/emulator/test/nif_SUITE_data/nif_SUITE.c
@@ -802,6 +802,23 @@ static ERL_NIF_TERM check_is(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[]
 }
 
 /*
+ * no arguments
+ *
+ * This function is separate from check_is because it calls enif_make_badarg
+ * and so it must return the badarg exception as its return value. Thus, the
+ * badarg exception indicates success. Failure is indicated by returning an
+ * error atom.
+ */
+static ERL_NIF_TERM check_is_exception(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
+{
+    ERL_NIF_TERM error_atom = enif_make_atom(env, "error");
+    ERL_NIF_TERM badarg = enif_make_badarg(env);
+    if (enif_is_exception(env, error_atom)) return error_atom;
+    if (!enif_is_exception(env, badarg)) return error_atom;
+    return badarg;
+}
+
+/*
  * argv[0] atom with length of 6
  * argv[1] list with length of 6
  * argv[2] empty list
@@ -1383,6 +1400,7 @@ static ErlNifFunc nif_funcs[] =
     {"last_resource_dtor_call", 0, last_resource_dtor_call},
     {"make_new_resource", 2, make_new_resource},
     {"check_is", 10, check_is},
+    {"check_is_exception", 0, check_is_exception},
     {"length_test", 5, length_test},
     {"make_atoms", 0, make_atoms},
     {"make_strings", 0, make_strings},