Add OpenSSL FIPS mode detection and env var

Detects FIPS mode enablement via OpenSSL or
via the FIPS_MODE environment variable

Changelog: added

config/feature_flags/development/fips_mode.yml

----
-name: fips_mode
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81418/diffs?view=inline
-rollout_issue_url:
-milestone: '14.9'
-type: development
-group: group::source code
-default_enabled: false

lib/gitlab/fips.rb

@@ -10,7 +10,13 @@ module Gitlab
       #
       # @return [Boolean]
       def enabled?
-        Feature.enabled?(:fips_mode, default_enabled: :yaml)
+        # Attempt to auto-detect FIPS mode from OpenSSL
+        return true if OpenSSL.fips_mode
+
+        # Otherwise allow it to be set manually via the env vars
+        return true if ENV["FIPS_MODE"] == "true"
+
+        false
       end
     end
   end

spec/lib/gitlab/fips_spec.rb

@@ -6,16 +6,46 @@ RSpec.describe Gitlab::FIPS do
   describe ".enabled?" do
     subject { described_class.enabled? }
 
-    context "feature flag is enabled" do
-      it { is_expected.to be_truthy }
+    let(:openssl_fips_mode) { false }
+    let(:fips_mode_env_var) { nil }
+
+    before do
+      expect(OpenSSL).to receive(:fips_mode).and_return(openssl_fips_mode)
+      stub_env("FIPS_MODE", fips_mode_env_var)
+    end
+
+    describe "OpenSSL auto-detection" do
+      context "OpenSSL is in FIPS mode" do
+        let(:openssl_fips_mode) { true }
+
+        it { is_expected.to be_truthy }
+      end
+
+      context "OpenSSL is not in FIPS mode" do
+        let(:openssl_fips_mode) { false }
+
+        it { is_expected.to be_falsey }
+      end
     end
 
-    context "feature flag is disabled" do
-      before do
-        stub_feature_flags(fips_mode: false)
+    describe "manual configuration via env var" do
+      context "env var is not set" do
+        let(:fips_mode_env_var) { nil }
+
+        it { is_expected.to be_falsey }
       end
 
-      it { is_expected.to be_falsey }
+      context "env var is set to true" do
+        let(:fips_mode_env_var) { "true" }
+
+        it { is_expected.to be_truthy }
+      end
+
+      context "env var is set to false" do
+        let(:fips_mode_env_var) { "false" }
+
+        it { is_expected.to be_falsey }
+      end
     end
   end
 end