Use buildpack to set git commit SHA during Heroku build

Heroku does not provide access to Git via the command line, so the methods that work for the dev and test environments won't work here. Instead, we can utilize an additional buildpack in .buildpacks to write the SOURCE_VERSION environment variable to a file named .source_version in the application root. We cannot reference this environment variable directly from the app because Heroku only makes it available during the compilation phase of the build, hence the need to use a buildpack to write it to a file for later use by the app.

Author: robwise

Diff for: .buildpacks

@@ -1,2 +1,3 @@
 https://github.com/heroku/heroku-buildpack-nodejs.git
 https://github.com/heroku/heroku-buildpack-ruby.git
+https://github.com/sreid/heroku-buildpack-sourceversion.git

Diff for: app/helpers/pages_helper.rb

@@ -5,6 +5,6 @@ def git_commit_sha
 
   def git_commit_sha_short
     full_sha = git_commit_sha
-    full_sha[-7..-1]
+    full_sha[-8..-1]
   end
 end

Diff for: app/models/git_commit_sha.rb

@@ -1,6 +1,7 @@
+# Retrieves the current git commit SHA of the project
 class GitCommitSha
   def self.current_sha
-    @commit_sha ||= retrieve_sha_from_env_var.presence || retrieve_sha_from_git
+    @commit_sha ||= retrieve_sha_from_file.presence || retrieve_sha_from_git
   end
 
   def self.current_sha=(sha)
@@ -15,8 +16,8 @@ def self.retrieve_sha_from_git
     `git rev-parse HEAD 2>/dev/null`.to_s.strip
   end
 
-  def self.retrieve_sha_from_env_var
-    env_var = ENV["DEPLOYMENT_SHA"]
-    env_var.blank? ? nil : env_var
+  def self.retrieve_sha_from_file
+    expected_filepath = Rails.root.join(".source_version")
+    File.exist?(expected_filepath) ? File.read(expected_filepath) : nil
   end
 end

Diff for: scripts/deploy

@@ -11,19 +11,20 @@
 end
 
 feature "Git Commit SHA" do
-  context "when env var is not set" do
+  context "when .source_version file does not exist" do
     let(:sha) { "94d92356828a56db25fccff9d50f41c525eead5x" }
     let(:expected_text) { "5eead5x" }
     before { GitCommitSha.current_sha = sha }
     it_behaves_like "Git Commit SHA"
   end
-  context "when env var is set" do
+  context "when .source_version file exists" do
     let(:sha) { "94d92356828a56db25fccff9d50f41c525eead5y" }
     let(:expected_text) { "5eead5y" }
     before do
-      ENV["DEPLOYMENT_SHA"] = sha
+      `cd #{Rails.root} && echo #{sha} > .source_version`
       GitCommitSha.reset_current_sha
     end
+    after { `cd #{Rails.root} && rm .source_version` }
     it_behaves_like "Git Commit SHA"
   end
 end