Safe render ERB files

Avoid overwriting generated files blindly, and instead show the diff
with the changes and ask the user what to do. Just like `bin/safe-sed`
was introduced in:
https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/628

6 files changed, 63 insertions, 12 deletions

diff --git a/Makefile b/Makefile
index b0e0b5689de..2f033bc585e 100644
--- a/Makefile
+++ b/Makefile
@@ -325,7 +325,7 @@ support-setup: .ruby-version foreman Procfile redis gitaly-setup jaeger-setup po
 gdk.yml:
 	touch $@
 
-Procfile: gdk.yml gdk-defaults.yml.erb auto_devops_enabled auto_devops_gitlab_port auto_devops_registry_port
+Procfile: Procfile.erb gdk.yml gdk-defaults.yml.erb auto_devops_enabled auto_devops_gitlab_port auto_devops_registry_port
 	rake $@
 
 redis: redis/redis.conf
diff --git a/Rakefile b/Rakefile
index da58ca53201..06d0979bba6 100644
--- a/Rakefile
+++ b/Rakefile
@@ -2,23 +2,16 @@
 
 $LOAD_PATH.unshift('.')
 
-require 'lib/gdk/config'
+require 'lib/gdk'
 
 def config
   @config ||= GDK::Config.new
 end
 
-def render_erb(source, target)
-  str = File.read(source)
-  result = ERB.new(str).result
-
-  IO.write(target, result)
-end
-
 file 'Procfile' => ['Procfile.erb', GDK::Config::FILE, GDK::Defaults::FILE] do |t|
-  render_erb(t.source, t.name)
+  GDK::ErbRenderer.new(t.source, t.name).safe_render!
 end
 
 file 'nginx/conf/nginx.conf' => ['nginx/conf/nginx.conf.erb', GDK::Config::FILE, GDK::Defaults::FILE] do |t|
-  render_erb(t.source, t.name)
+  GDK::ErbRenderer.new(t.source, t.name).safe_render!
 end
diff --git a/gdk-defaults.yml.erb b/gdk-defaults.yml.erb
index de6f0c95f0b..dc881bc4b3e 100644
--- a/gdk-defaults.yml.erb
+++ b/gdk-defaults.yml.erb
@@ -39,7 +39,7 @@ auto_devops:
   gitlab:
     port: <%= read_or_write!('auto_devops_gitlab_port', rand(20000..24999)) %>
   registry:
-    port: <%= config.auto_devops.gitlab.port + 5000 %>
+    port: <%= config.auto_devops.gitlab.port.to_i + 5000 %>
 
 geo:
   enabled: false
diff --git a/lib/gdk.rb b/lib/gdk.rb
index 2d71e24bbcb..7275c0c40c2 100644
--- a/lib/gdk.rb
+++ b/lib/gdk.rb
@@ -4,6 +4,8 @@
 # part of the gitlab-development-kit gem so that we can iterate faster.
 
 require_relative 'gdk/env'
+require_relative 'gdk/config'
+require_relative 'gdk/erb_renderer'
 
 module GDK
   PROGNAME = 'gdk'.freeze
diff --git a/lib/gdk/env.rb b/lib/gdk/env.rb
index 8a14d52b3d8..0e2a44eaa32 100644
--- a/lib/gdk/env.rb
+++ b/lib/gdk/env.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'pathname'
 require 'shellwords'
 
diff --git a/lib/gdk/erb_renderer.rb b/lib/gdk/erb_renderer.rb
new file mode 100644
index 00000000000..d9ebdadfc82
--- /dev/null
+++ b/lib/gdk/erb_renderer.rb
@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+require 'erb'
+require 'tempfile'
+
+module GDK
+  class ErbRenderer
+    attr_reader :source, :target
+
+    def initialize(source, target)
+      @source = source
+      @target = target
+    end
+
+    def render!(target = @target)
+      str = File.read(source)
+      result = ERB.new(str).result
+
+      File.write(target, result)
+    end
+
+    def safe_render!
+      temp_file = Tempfile.new(target)
+
+      render!(temp_file.path)
+
+      return File.rename(temp_file.path, target) unless File.exist?(target)
+
+      warn!(temp_file) unless FileUtils.identical?(target, temp_file.path)
+    ensure
+      temp_file.close!
+    end
+
+    private
+
+    def warn!(temp_file)
+      diff = `git --no-pager diff --no-index --color -u "#{target}" "#{temp_file.path}"`
+
+      puts <<~EOF
+        -------------------------------------------------------------------------------------------------------------
+        Warning: Your `#{target}` is outdated. Below are the changes GDK wanted to apply.
+         - To automatically update: `rm #{target}`
+           and re-run `gdk update`.
+         - To silence this warning (at your own peril): `touch #{target}`
+        -------------------------------------------------------------------------------------------------------------
+        #{diff}
+        -------------------------------------------------------------------------------------------------------------
+        Waiting 5 seconds for previous warning to be noticed...."
+        -------------------------------------------------------------------------------------------------------------
+      EOF
+      sleep 5
+    end
+  end
+end