chore: Speed up compilation in CI

.dockerignore

@@ -1,5 +1,3 @@
-_build
-deps
 cover
 .elixir_ls
 *.dump

.gitlab-ci.yml

@@ -18,6 +18,10 @@ variables:
 test:
   stage: test
   image: "elixir:1.10"
+  cache:
+    paths:
+      - _build/
+      - deps/
   services:
   - name: postgres:12
   script: script/cibuild
@@ -27,9 +31,14 @@ build_image_for_commit:
   image: "docker:20.10"
   only:
     - master
+  cache:
+    paths:
+      - _build/
+      - deps/
   services:
     - name: docker:20.10-dind
   script:
+    - script/ci/restore-timestamps
     - docker login "https://${CI_REGISTRY}" -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD
     - docker pull $CI_REGISTRY_IMAGE:latest || true
     # This enables fast parallel builds
@@ -38,6 +47,9 @@ build_image_for_commit:
     # Push the commit SHA tagged version to registry. We will later choose to tag that as stable
     #   if everything passes.
     - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
+    # Pull out the built _build/prod directory so we can cache it!
+    - docker cp `docker create $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA`:/root/app/_build/prod _build
+    - script/ci/restore-timestamps
 
 # If tests pass, tag the commit and update package versions
 deploy_to_tags:

Dockerfile

@@ -26,16 +26,18 @@ ADD ./apps/admin/mix.exs /root/app/apps/admin/
 ADD ./apps/app/mix.exs /root/app/apps/app/
 ADD ./apps/content/mix.exs /root/app/apps/content/
 ADD ./apps/core/mix.exs /root/app/apps/core/
+ADD ./_build/ /root/app/_build/
+ADD ./deps/ /root/app/deps/
 RUN mix deps.get
 RUN mix deps.compile
 
-ADD ./apps /root/app/apps
-
 # Leave off here so that we can built assets and compile the elixir app in parallel
 
 FROM node:15.0
 
 # Build assets in a node container
+ADD ./apps/app/assets/ /root/app/apps/app/assets/
+
 WORKDIR /root/app/apps/app/assets/
 COPY --from=0 /root/app/ /root/app/
 RUN npm install
@@ -43,6 +45,8 @@ RUN npm run deploy
 
 FROM elixir1
 
+ADD ./apps /root/app/apps
+
 # Resume compilation of the elixir app
 ADD ./script /root/app/script
 RUN MAKE=cmake mix compile

script/ci/restore-timestamps

@@ -0,0 +1,74 @@
+#!/usr/bin/env elixir
+
+defmodule TimestampRestorer do
+  @environment System.get_env("MIX_ENV", "dev")
+  @db_path "_build/#{@environment}/timestamp-database"
+
+  def sha_all do
+    timestamp_database = load_timestamp_database()
+
+    "**/*.{ex,exs,beam}"
+    |> Path.wildcard()
+    |> Enum.reduce(%{}, fn filename, acc ->
+      {sha, timestamp} = process(filename, timestamp_database)
+      Map.put(acc, sha, timestamp)
+    end)
+    |> write_timestamp_database()
+  end
+
+  defp load_timestamp_database() do
+    if File.exists?(@db_path) do
+      @db_path
+      |> File.read!()
+      |> String.split("\n")
+      |> Enum.reduce(%{}, fn line, acc ->
+        [sha, timestamp_string] = String.split(line, ":")
+        {timestamp, ""} = Integer.parse(timestamp_string)
+        Map.put(acc, sha, timestamp)
+      end)
+    else
+      %{}
+    end
+  end
+
+  defp write_timestamp_database(database) do
+    database
+    |> Enum.map(fn {key, value} -> "#{key}:#{value}" end)
+    |> Enum.join("\n")
+    |> (& File.write!(@db_path, &1)).()
+  end
+
+  defp process(filename, timestamp_database) do
+    sha = sha(filename)
+    {:ok, %{mtime: new_timestamp}} = File.lstat(filename, time: :posix)
+
+    case Map.get(timestamp_database, sha) do
+      nil ->
+        :logger.debug("[NEW SHA  ] #{filename}: #{new_timestamp}")
+      timestamp when timestamp < new_timestamp ->
+        :logger.debug("[RESTORED ] #{filename}: #{timestamp}")
+        File.touch(filename, timestamp)
+      timestamp when timestamp >= new_timestamp ->
+        :logger.debug("[UNCHANGED] #{filename}: #{timestamp}")
+    end
+
+    {sha, new_timestamp}
+  end
+
+  defp sha(filename) do
+    hash_ref = :crypto.hash_init(:sha)
+
+    File.stream!(filename)
+    |> Enum.reduce(hash_ref, fn chunk, prev_ref->
+      new_ref = :crypto.hash_update(prev_ref, chunk)
+      new_ref
+    end)
+    |> :crypto.hash_final()
+    |> Base.encode16()
+    |> String.downcase()
+  end
+end
+
+{time, _result} = :timer.tc(TimestampRestorer, :sha_all, [])
+
+:logger.info("Restored timestamps in #{time / 1_000_000}s")

script/cibuild

@@ -6,8 +6,12 @@ set -e
 mix local.hex --force
 mix local.rebar --force
 
+script/ci/restore-timestamps
+
 mix deps.get
 mix ecto.create
 mix ecto.migrate
 
 mix test
+
+script/ci/restore-timestamps