Add a persistent term storage

Persistent terms are useful for storing Erlang terms that are never
or infrequently updated. They have the following advantages:

* Constant time access. A persistent term is not copied when it is
  looked up. The constant factor is lower than for ETS, and no locks
  are taken when looking up a term.

* Persistent terms are not copied in garbage collections.

* There is only ever one copy of a persistent term (until it is
  deleted). That makes them useful for storing configuration data
  that needs to be easily accessible by all processes.

Persistent terms have the following drawbacks:

* Updates are expensive. The hash table holding the keys for the
  persistent terms are updated whenever a persistent term is added,
  updated or deleted.

* Updating or deleting a persistent term triggers a "global GC", which
  will schedule a heap scan of all processes to search the heap of all
  processes for the deleted term. If a process still holds a reference
  to the deleted term, the process will be garbage collected and the
  term copied to the heap of the process. This global GC can make the
  system less responsive for some time.

Three BIFs (implemented in C in the emulator) is the entire
interface to the persistent term functionality:

* put(Key, Value) to store a persistent term.

* get(Key) to look up a persistent term.

* erase(Key) to delete a persistent term.

There are also two additional BIFs to obtain information about
persistent terms:

* info() to return a map with information about persistent terms.

* get() to return a list of a {Key,Value} tuples for all persistent
  terms. (The values are not copied.)

1 files changed, 55 insertions, 0 deletions

diff --git a/erts/preloaded/src/persistent_term.erl b/erts/preloaded/src/persistent_term.erl
new file mode 100644
index 0000000000..5d0c266127
--- /dev/null
+++ b/erts/preloaded/src/persistent_term.erl
@@ -0,0 +1,55 @@
+%%
+%% %CopyrightBegin%
+%%
+%% Copyright Ericsson AB 2018. All Rights Reserved.
+%%
+%% 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.
+%%
+%% %CopyrightEnd%
+%%
+-module(persistent_term).
+
+-export([erase/1,get/0,get/1,info/0,put/2]).
+
+-type key() :: term().
+-type value() :: term().
+
+-spec erase(Key) -> Result when
+      Key :: key(),
+      Result :: boolean().
+erase(_Key) ->
+    erlang:nif_error(undef).
+
+-spec get() -> List when
+      List :: [{key(),value()}].
+get() ->
+    erlang:nif_error(undef).
+
+-spec get(Key) -> Value when
+      Key :: key(),
+      Value :: value().
+get(_Key) ->
+    erlang:nif_error(undef).
+
+-spec info() -> Info when
+      Info :: #{'count':=Count,'memory':=Memory},
+      Count :: non_neg_integer(),
+      Memory :: non_neg_integer().
+info() ->
+    erlang:nif_error(undef).
+
+-spec put(Key, Value) -> 'ok' when
+      Key :: key(),
+      Value :: value().
+put(_Key, _Value) ->
+    erlang:nif_error(undef).