Support setting build options via environment

Allow for overriding the build.config settings using environment
variables (of the same name, but upper-case).

.gitignore

@@ -13,6 +13,7 @@
 /doc/*.html
 /index.html
 /.make-binary.*/
+/.build.config
 #
 # ExDoc:
 #

CHANGELOG.md

@@ -5,6 +5,8 @@ project adheres to [Semantic Versioning][SemVer].
 
 ## [Unreleased]
 ### Added
+- Allow for overriding the `build.config` settings using environment variables
+  (of the same name, but upper-case).
 - Docker: Container images can now be pulled from Dockerhub as well. The name
   is `docker.io/eturnal/eturnal:latest`. When pulling with `Docker`, `docker.io`
   may be omitted.

INSTALL.md

@@ -27,8 +27,9 @@ and `openssl-devel`.
 This generates the archive file `_build/prod/rel/eturnal/eturnal-1.10.1.tar.gz`.
 The default installation prefix is set to `/opt/eturnal`, and it's assumed the
 server will be executed by a user named `eturnal`. To change these defaults,
-edit the [build.config][7] file, re-run `./rebar3 as prod tar`, and adapt the
-following installation instructions accordingly.
+edit the [build.config][7] file or override the settings using environment
+variables (of the same name, but upper-case), re-run `./rebar3 as prod tar`, and
+adapt the following installation instructions accordingly.
 
 ## Quick Test
 

PACKAGING.md

@@ -67,7 +67,9 @@ The `eturnal_prefix` option in the [build.config][7] file must be set to that
 location before building eturnal. The user for running eturnal can be adjusted
 in that file as well. Apart from that, you might want to set the `code_loading`
 option to `"dynamic"`, as that avoids hard-coding of Erlang dependency versions
-at build time.
+at build time. If patching that file is not desirable, the settings may be
+overridden using environment variables (of the same name, but upper-case)
+instead.
 
 The `bin` directory contains, among other things, the [eturnalctl][8] script,
 which can be moved (or symlinked) elsewhere (e.g., into `/usr/sbin`). If it's

build.config

@@ -1,4 +1,7 @@
 %%% eturnal build configuration.
+%%%
+%%% Note that these settings may be overridden using environment variables of
+%%% the same name, but upper-case.
 
 {eturnal_user, "eturnal"}.        % The user running eturnal.
 {eturnal_prefix, "/opt/eturnal"}. % The installation directory.

rebar.config.script

@@ -1,10 +1,20 @@
-% For each profile with a 'relx' configuration specified in the rebar.config
-% file, generate three additional profiles. E.g., for the 'prod' profile:
-%
-% - The 'prod_minimal' excludes non-essential dependencies.
-% - The 'prod_cross' profile includes ERTS and system libraries from the
-%   "lib/erlang" directory (for cross compilation).
-% - The 'prod_cross_minimal' does both.
+%%% eturnal STUN/TURN server.
+%%%
+%%% Copyright (c) 2020-2023 Holger Weiss <holger@zedat.fu-berlin.de>.
+%%% Copyright (c) 2020-2023 ProcessOne, SARL.
+%%% 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.
 
 Find =
     fun(K, L) ->
@@ -26,9 +36,61 @@ AddSuffix =
             list_to_atom(atom_to_list(N) ++ [$_ | atom_to_list(S)])
     end,
 
+%% If any of the build.conf settings are overridden via environment variables,
+%% create a .build.config file and point Relx to that one.
+
+{ok, OldSettings} = file:consult("build.config"),
+
+NewSettings =
+    lists:map(
+      fun({K, V}) ->
+              EnvKey = string:uppercase(atom_to_list(K)),
+              case os:getenv(EnvKey) of
+                  EnvVal when is_list(EnvVal) ->
+                      {K, EnvVal};
+                  false ->
+                      {K, V}
+              end
+      end, OldSettings),
+
+OldOverlayVars = "build.config",
+NewOverlayVars = ".build.config",
+
+OverlayVars =
+    case {lists:sort(OldSettings), lists:sort(NewSettings)} of
+        {Settings, Settings} ->
+            _ = file:delete(NewOverlayVars),
+            OldOverlayVars;
+        {_, _} ->
+            NewData = [io_lib:format("~p.~n", [S]) || S <- NewSettings],
+            ok = file:write_file(NewOverlayVars, NewData),
+            NewOverlayVars
+    end,
+
 {profiles, Profiles} = Find(profiles, CONFIG),
 
 Profiles1 =
+    lists:map(
+      fun({ProfName, ProfOpts} = Profile) ->
+              case Find(relx, ProfOpts) of
+                  {relx, RelxOpts} ->
+                      RelxOpts1 = Store(overlay_vars, OverlayVars, RelxOpts),
+                      ProfOpts1 = Store(relx, RelxOpts1, ProfOpts),
+                      {ProfName, ProfOpts1};
+                  false ->
+                      Profile
+              end
+      end, Profiles),
+
+%% For each profile with a 'relx' configuration specified in the rebar.config
+%% file, generate three additional profiles. E.g., for the 'prod' profile:
+%%
+%% - The 'prod_minimal' excludes non-essential dependencies.
+%% - The 'prod_cross' profile includes ERTS and system libraries from the
+%%   "lib/erlang" directory (for cross compilation).
+%% - The 'prod_cross_minimal' does both.
+
+Profiles2 =
     lists:flatmap(
       fun({ProfName, ProfOpts} = Profile) ->
               case Find(relx, ProfOpts) of
@@ -58,11 +120,12 @@ Profiles1 =
                   false ->
                       [Profile]
               end
-      end, Profiles),
+      end, Profiles1),
 
-Config1 = Store(profiles, Profiles1, CONFIG),
+Config1 = Store(profiles, Profiles2, CONFIG),
 
-% Remove the rebar.lock file and set SKIP_DEPS=true to skip dependency handling.
+%% Remove the rebar.lock file and set SKIP_DEPS=true to skip dependency
+%% handling.
 
 case os:getenv("SKIP_DEPS") of
     "true" ->