Use latest stable tag in the repo as fallback for stable branches

Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

lib/gitlab/build/info.rb

@@ -9,6 +9,8 @@ require_relative 'image'
 module Build
   class Info
+    TagsNotFoundError = Class.new(StandardError)
+
     DEPLOYER_OS_MAPPING = {
       'AUTO_DEPLOY_ENVIRONMENT' => 'ubuntu-xenial',
       'PATCH_DEPLOY_ENVIRONMENT' => 'ubuntu-bionic',
@@ -91,7 +93,17 @@ module Build
         # Level decides tag at which position you want. Level one gives you
         # latest stable tag, two gives you the one just before it and so on.
-        `git -c versionsort.prereleaseSuffix=rc tag -l '#{version}#{Info.tag_match_pattern}' --sort=-v:refname | awk '!/rc/' | head -#{level}`.split("\n").last
+        output = `git -c versionsort.prereleaseSuffix=rc tag -l '#{version}#{Info.tag_match_pattern}' --sort=-v:refname | awk '!/rc/' | head -#{level}`&.split("\n")&.last
+
+        # If no tags exist that start with the specified version, we need to
+        # fall back to the available latest stable tag. For that, we run the
+        # same command without the version in the filter argument.
+        raise TagsNotFoundError if output.nil?
+
+        output
+      rescue TagsNotFoundError
+        puts "No tags found in #{version}.x series. Falling back to latest available tag."
+        `git -c versionsort.prereleaseSuffix=rc tag -l '#{Info.tag_match_pattern}' --sort=-v:refname | awk '!/rc/' | head -#{level}`.split("\n").last
       end
       def docker_tag

spec/lib/gitlab/build/info_spec.rb

@@ -162,6 +162,42 @@ RSpec.describe Build::Info do
         expect(described_class.latest_stable_tag).to eq('12.121.12+ee.0')
       end
     end
+
+    describe 'on stable branches' do
+      before do
+        stub_env_var('CI_COMMIT_BRANCH', '15-11-stable')
+        stub_is_ee(false)
+      end
+
+      context 'when no tags exist in the stable branch' do
+        before do
+          allow(described_class).to receive(:`).with("git -c versionsort.prereleaseSuffix=rc tag -l '15.11*[+.]ce.*' --sort=-v:refname | awk '!/rc/' | head -1").and_return(nil)
+        end
+
+        it 'returns the latest available stable tag' do
+          # Twice because we are calling the method two times - once to check
+          # the return value and another time to check if the message is
+          # printed
+          expect(described_class).to receive(:`).with("git -c versionsort.prereleaseSuffix=rc tag -l '15.11*[+.]ce.*' --sort=-v:refname | awk '!/rc/' | head -1").twice
+          expect(described_class).to receive(:`).with("git -c versionsort.prereleaseSuffix=rc tag -l '*[+.]ce.*' --sort=-v:refname | awk '!/rc/' | head -1").twice.and_return('15.10.3+ce.0')
+
+          expect(described_class.latest_stable_tag).to eq('15.10.3+ce.0')
+          expect { described_class.latest_stable_tag }.to output(/No tags found in 15.11.x series/).to_stdout
+        end
+      end
+
+      context 'when tags exist in the stable branch' do
+        before do
+          allow(described_class).to receive(:`).with("git -c versionsort.prereleaseSuffix=rc tag -l '15.11*[+.]ce.*' --sort=-v:refname | awk '!/rc/' | head -1").and_return('15.11.0+ce.0')
+        end
+
+        it 'returns the latest available stable tag' do
+          expect(described_class).to receive(:`).with("git -c versionsort.prereleaseSuffix=rc tag -l '15.11*[+.]ce.*' --sort=-v:refname | awk '!/rc/' | head -1")
+          expect(described_class).not_to receive(:`).with("git -c versionsort.prereleaseSuffix=rc tag -l '*[+.]ce.*' --sort=-v:refname | awk '!/rc/' | head -1")
+          expect(described_class.latest_stable_tag).to eq('15.11.0+ce.0')
+        end
+      end
+    end
   end
   describe '.gitlab_version' do