Don't validate gitaly timeouts that didn't change

This avoids validating Gitaly timeouts, that could have become invalid
based on configuration in omnibus.

The invalid values would not be used, instead the mininum value from
omnibus would be used. But this would prevent the application settings
from being updated.

Skipping these validations makes sure the settings can still be saved
if unrelated values are changed.

app/models/application_setting.rb

@@ -172,6 +172,7 @@ class ApplicationSetting < ApplicationRecord
 
   validates :gitaly_timeout_default,
             presence: true,
+            if: :gitaly_timeout_default_changed?,
             numericality: {
               only_integer: true,
               greater_than_or_equal_to: 0,
@@ -180,6 +181,7 @@ class ApplicationSetting < ApplicationRecord
 
   validates :gitaly_timeout_medium,
             presence: true,
+            if: :gitaly_timeout_medium_changed?,
             numericality: { only_integer: true, greater_than_or_equal_to: 0 }
   validates :gitaly_timeout_medium,
             numericality: { less_than_or_equal_to: :gitaly_timeout_default },
@@ -190,6 +192,7 @@ class ApplicationSetting < ApplicationRecord
 
   validates :gitaly_timeout_fast,
             presence: true,
+            if: :gitaly_timeout_fast_changed?,
             numericality: { only_integer: true, greater_than_or_equal_to: 0 }
   validates :gitaly_timeout_fast,
             numericality: { less_than_or_equal_to: :gitaly_timeout_default },

changelogs/unreleased/bvl-validate-changed-values.yml

+---
+title: Fix saving preferences with unrelated changes when gitaly timeouts became invalid.
+merge_request: 26292
+author:
+type: fixed

spec/models/application_setting_spec.rb

@@ -380,6 +380,12 @@ describe ApplicationSetting do
 
         expect(subject).to be_invalid
       end
+
+      it 'does not prevent from saving when gitaly timeouts were previously invalid' do
+        subject.update_column(:gitaly_timeout_default, Settings.gitlab.max_request_duration_seconds + 1)
+
+        expect(subject.reload).to be_valid
+      end
     end
 
     describe 'enforcing terms' do