Move most of shell scripts to Erlang code

A large part of the rabbitmq-server(8) and CLI scripts, both
Bourne-shell and Windows Batch versions, was moved to Erlang code and
the RabbitMQ startup procedure was reorganized to be closer to a regular
Erlang application.

A new application called `rabbitmq_prelaunch` is responsible for:

  1. Querying the environment variables to initialize important
     variables (using the new `rabbit_env` module in rabbitmq-common).
  2. Checking the compatibility with the Erlang/OTP runtime.
  3. Configuring Erlang distribution.
  5. Writing the PID file.

The application is started early (i.e. it is started before `rabbit`).

The `rabbit` application runs the second half of the prelaunch sequence
at the beginning of the application `start()` function. This second
phase is responsible for the following steps:

  1. Preparing the feature flags registry.
  2. Reading and validating the configuration.
  3. Configuring logging.
  4. Running the various cluster checks.

In addition to this prelaunch sequence, the `rabbit` application start
procedure ends with a "postlaunch" sequence which takes care of
starting enabled plugins.

Thanks to this, RabbitMQ can be started with `application:start(rabbit)`
as any other Erlang application. The only caveats are:

  * Mnesia must be stopped at the time `rabbit_prelaunch` is started,
    and must remain stopped when `rabbit` is started, to allow the
    Erlang distribution setup and cluster checks. `rabbit` takes care of
    starting Mnesia.

  * Likewise for Ra, because it relies on the `ra` application
    environment to be configured.

Transitioning from scripts to Erlang code has the following benefits:

  * RabbitMQ start behavior should be identical between Unix and
    Windows. Also, features should be on par now. For instance, RabbitMQ
    now writes a PID file on Windows, like it always did on Unix-based
    systems.

  * The difference between published packages and a development
    environment are greatly reduced. In fact, we removed all the "if
    this is a dev working copy, then ..." blocks.

    As part of that, the `rabbit` application is now treated like its
    plugins: it is packaged as an `.ez` archive and written to the
    `plugins` directory (even though it is not technically a plugin).

    Also in a development copy, the CLI is copied to the top-level
    project. So when testing a plugin for instance, the CLI to use is
    `sbin/rabbitmqctl` in the current directory, not the master copy in
    `rabbit/scripts`.

  * As a consequence of the previous two points, maintaining and testing
    on Windows is now made easy. It should even be possible to setup CI
    on Windows.

  * There are less issues with paths containing non-US-ASCII characters,
    which can happen on Windows because RabbitMQ stores its data in user
    directories by default.

This process brings at least one more benefit: we now have early logging
during this prelaunch phase, which eases diagnostics and debugging.

There are also behavior changes:

  * The new format configuration files used to be converted to an
    Erlang-term-based file by the Cuttlefish CLI. To do that,
    configuration schemas were copied to a temporary directory and the
    generated configuration file was written to RabbitMQ data directory.

    Now, Cuttlefish is used as a library: everything happens in memory.
    No schemas are copied, no generated configuration is written to
    disk.

  * The PID file is removed when the Erlang VM exits.

  * The `rabbit_config` module was trimmed significantly because most of
    the configuration handling is done in `rabbit_prelaunch_conf` now.

  * The RabbitMQ nodename does not appear on the command line, therefore
    it is missing from ps(1) and top(1) output.

  * The `rabbit:start()` function will probably behave differently in
    some ways because it defers everything to the Erlang application
    controller (instead of reimplementing it).

1 files changed, 58 insertions, 0 deletions

diff --git a/scripts/rabbitmq-rel b/scripts/rabbitmq-rel
new file mode 100755
index 0000000000..a96ec78764
--- /dev/null
+++ b/scripts/rabbitmq-rel
@@ -0,0 +1,58 @@
+#!/usr/bin/env escript
+%% vim:ft=erlang:sw=2:et:
+
+main(["show-rel"]) ->
+  Rel = get_rel(),
+  io:format("~p.~n", [Rel]);
+main(["gen-boot"]) ->
+  generate_rel(),
+  generate_boot().
+
+get_rel() ->
+  ok = application:load(rabbit),
+  Apps0 = get_apps(rabbit),
+  Apps1 = lists:sort(
+            fun
+              (_, rabbitmq_prelaunch) -> false;
+              (rabbitmq_prelaunch, _) -> true;
+              (_, mnesia)             -> true;
+              (mnesia, _)             -> false;
+              (A, B)                  -> A =< B
+            end, Apps0),
+  Apps = [{App, get_vsn(App)} || App <- Apps1],
+
+  ERTSVersion = erlang:system_info(version),
+  RabbitVersion = get_vsn(rabbit),
+
+  {release,
+   {"RabbitMQ", RabbitVersion},
+   {erts, ERTSVersion},
+   Apps}.
+
+get_apps(App) ->
+  ok = load_app(App),
+  {ok, DirectDeps} = application:get_key(App, applications),
+  lists:umerge(
+    [lists:usort(get_apps(Dep)) || Dep <- DirectDeps] ++
+    [lists:usort([kernel, stdlib, sasl, App, mnesia])]).
+
+load_app(App) ->
+  case application:load(App) of
+    ok                             -> ok;
+    {error, {already_loaded, App}} -> ok
+  end.
+
+generate_rel() ->
+  Rel = get_rel(),
+  io:format("~p.~n", [Rel]),
+  Output = io_lib:format("~p.~n", [Rel]),
+  ok = file:write_file("rabbit.rel", Output).
+
+generate_boot() ->
+  Options = [local, {path, code:get_path()}],
+  ok = systools:make_script("rabbit", Options).
+
+get_vsn(App) ->
+  load_app(App),
+  {ok, Vsn} = application:get_key(App, vsn),
+  Vsn.