[Branch] Add build failure concept

Use the Branch record to keep track of build failures that we find when
polling the Travis API.  The main purpose is to avoid spamming gitter
with extra notifications, but also can be used to identify that a branch
has previously been broken and now has been fixed.

Makes sense to also notify on the passing cases so others aren't taking
the time to investigate a failure when this has already been
investigated/fixed by another.

app/models/branch.rb

@@ -99,4 +99,37 @@ def fq_branch_name
   def git_service
     GitService::Branch.new(self)
   end
+
+  # Branch Failure
+
+  def notify_of_failure
+    if passing?
+      BuildFailureNotifier.new(self).report_passing
+      update(:last_build_failure_notified_at => nil, :travis_build_failure_id => nil)
+    elsif should_notify_of_failure?
+      update(:last_build_failure_notified_at => Time.zone.now)
+
+      BuildFailureNotifier.new(self).post_failure
+    end
+  end
+
+  def previously_failing?
+    !!travis_build_failure_id
+  end
+
+  def should_notify_of_failure?
+    last_build_failure_notified_at.nil? || last_build_failure_notified_at < 1.day.ago
+  end
+
+  # If we have reported a failure before and the branch is now green.
+  #
+  # The other advantage of checking `last_build_failure_notified_at.nil?` here
+  # is that we save a Travis API call, since we shouldn't be creating
+  # BuildFailure records without having found a build failure elsewhere (e.g.
+  # TravisBranchMonitor).
+  #
+  # New records will short circut before hitting `Travis::Repository.find`.
+  def passing?
+    !last_build_failure_notified_at.nil? && Travis::Repository.find(repo.name).branch(name).green?
+  end
 end

db/migrate/20200109223351_add_branch_build_failures.rb

@@ -0,0 +1,6 @@
+class AddBranchBuildFailures < ActiveRecord::Migration[5.2]
+  def change
+    add_column :branches, :travis_build_failure_id, :integer
+    add_column :branches, :last_build_failure_notified_at, :datetime
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2017_10_06_050814) do
+ActiveRecord::Schema.define(version: 2020_01_09_223351) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -44,6 +44,8 @@
     t.string "merge_target"
     t.string "pr_title"
     t.integer "linter_offense_count"
+    t.integer "travis_build_failure_id"
+    t.datetime "last_build_failure_notified_at"
     t.index ["repo_id"], name: "index_branches_on_repo_id"
   end