feat(variform): add a builtin function to get env vars

apps/emqx_rule_engine/src/emqx_rule_funcs.erl

@@ -252,6 +252,9 @@
     timezone_to_offset_seconds/1
 ]).
 
+%% System functions
+-export([getenv/1]).
+
 %% See extra_functions_module/0 and set_extra_functions_module/1 in the
 %% emqx_rule_engine module
 -callback handle_rule_function(atom(), list()) -> any() | {error, no_match_for_function}.
@@ -1262,3 +1265,9 @@ convert_timestamp(MillisecondsTimestamp) ->
 
 uuid_str(UUID, DisplayOpt) ->
     uuid:uuid_to_string(UUID, DisplayOpt).
+
+%%------------------------------------------------------------------------------
+%% System Funcs
+%%------------------------------------------------------------------------------
+getenv(Env) ->
+    emqx_variform_bif:getenv(Env).

apps/emqx_utils/src/emqx_variform_bif.erl

@@ -79,6 +79,12 @@
 %% Number compare functions
 -export([num_comp/2, num_eq/2, num_lt/2, num_lte/2, num_gt/2, num_gte/2]).
 
+%% System
+-export([getenv/1]).
+
+-define(CACHE(Key), {?MODULE, Key}).
+-define(ENV_CACHE(Env), ?CACHE({env, Env})).
+
 %%------------------------------------------------------------------------------
 %% String Funcs
 %%------------------------------------------------------------------------------
@@ -569,3 +575,24 @@ num_lte(A, B) ->
 num_gte(A, B) ->
     R = num_comp(A, B),
     R =:= gt orelse R =:= eq.
+
+%%------------------------------------------------------------------------------
+%% System
+%%------------------------------------------------------------------------------
+getenv(Bin) when is_binary(Bin) ->
+    EnvKey = ?ENV_CACHE(Bin),
+    case persistent_term:get(EnvKey, undefined) of
+        undefined ->
+            Name = erlang:binary_to_list(Bin),
+            Result =
+                case os:getenv(Name) of
+                    false ->
+                        <<>>;
+                    Value ->
+                        erlang:list_to_binary(Value)
+                end,
+            persistent_term:put(EnvKey, Result),
+            Result;
+        Result ->
+            Result
+    end.

apps/emqx_utils/test/emqx_variform_bif_tests.erl

@@ -72,3 +72,10 @@ base64_encode_decode_test() ->
     RandBytes = crypto:strong_rand_bytes(100),
     Encoded = emqx_variform_bif:base64_encode(RandBytes),
     ?assertEqual(RandBytes, emqx_variform_bif:base64_decode(Encoded)).
+
+system_test() ->
+    EnvName = erlang:atom_to_list(?MODULE),
+    EnvVal = erlang:atom_to_list(?FUNCTION_NAME),
+    EnvNameBin = erlang:list_to_binary(EnvName),
+    os:putenv(EnvName, EnvVal),
+    ?assertEqual(erlang:list_to_binary(EnvVal), emqx_variform_bif:getenv(EnvNameBin)).