Idempotent snapshot handle setting

If a transaction handle is already a snapshot, make setting it a snapshot again
a no-op.

src/erlfdb.erl

@@ -172,7 +172,10 @@ transactional(?IS_SS = SS, UserFun) when is_function(UserFun, 1) ->
 
 
 snapshot(?IS_TX = Tx) ->
-    {erlfdb_snapshot, Tx}.
+    {erlfdb_snapshot, Tx};
+
+snapshot(?IS_SS = SS) ->
+    SS.
 
 
 set_option(DbOrTx, Option) ->
@@ -318,7 +321,10 @@ get(?IS_SS = SS, Key) ->
 
 
 get_ss(?IS_TX = Tx, Key) ->
-    erlfdb_nif:transaction_get(Tx, Key, true).
+    erlfdb_nif:transaction_get(Tx, Key, true);
+
+get_ss(?IS_SS = SS, Key) ->
+    get_ss(?GET_TX(SS), Key).
 
 
 get_key(?IS_DB = Db, Key) ->

test/erlfdb_04_snapshot_test.erl

@@ -0,0 +1,44 @@
+% Licensed under the Apache License, Version 2.0 (the "License"); you may not
+% use this file except in compliance with the License. You may obtain a copy of
+% the License at
+%
+%   http://www.apache.org/licenses/LICENSE-2.0
+%
+% Unless required by applicable law or agreed to in writing, software
+% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+% License for the specific language governing permissions and limitations under
+% the License.
+
+-module(erlfdb_04_snapshot_test).
+
+-include_lib("eunit/include/eunit.hrl").
+
+
+snapshot_from_tx_test() ->
+    Db = erlfdb_util:get_test_db(),
+    Key = gen(10),
+    Val = gen(10),
+    erlfdb:set(Db, Key, Val),
+    erlfdb:transactional(Db, fun(Tx) ->
+         ?assertEqual(Val, erlfdb:wait(erlfdb:get_ss(Tx, Key))),
+         Ss = erlfdb:snapshot(Tx),
+         ?assertEqual(Val, erlfdb:wait(erlfdb:get(Ss, Key)))
+    end).
+
+
+snapshot_from_a_snapshot_test() ->
+    Db = erlfdb_util:get_test_db(),
+    Key = gen(10),
+    Val = gen(10),
+    erlfdb:set(Db, Key, Val),
+    erlfdb:transactional(Db, fun(Tx) ->
+         Ss = erlfdb:snapshot(Tx),
+         ?assertEqual(Val, erlfdb:wait(erlfdb:get_ss(Ss, Key))),
+         Ss = erlfdb:snapshot(Ss)
+    end).
+
+
+gen(Size) when is_integer(Size), Size > 1 ->
+    RandBin = crypto:strong_rand_bytes(Size - 1),
+    <<0, RandBin/binary>>.