1 files changed, 45 insertions, 17 deletions

diff --git a/lib/gitlab/redis/multi_store.rb b/lib/gitlab/redis/multi_store.rb
index d36ef6b99ee..7f4d611a490 100644
--- a/lib/gitlab/redis/multi_store.rb
+++ b/lib/gitlab/redis/multi_store.rb
@@ -6,15 +6,15 @@ module Gitlab
       include Gitlab::Utils::StrongMemoize
 
       class PipelinedDiffError < StandardError
-        def initialize(result_primary, result_secondary)
-          @result_primary = result_primary
-          @result_secondary = result_secondary
+        def initialize(non_default_store_result, default_store_result)
+          @non_default_store_result = non_default_store_result
+          @default_store_result = default_store_result
         end
 
         def message
           "Pipelined command executed on both stores successfully but results differ between them. " \
-            "Result from the primary: #{@result_primary.inspect}. " \
-            "Result from the secondary: #{@result_secondary.inspect}."
+            "Result from the non-default store: #{@non_default_store_result.inspect}. " \
+            "Result from the default store: #{@default_store_result.inspect}."
         end
       end
 
@@ -24,11 +24,17 @@ module Gitlab
         end
       end
 
+      class NestedReadonlyPipelineError < StandardError
+        def message
+          'Nested use of with_readonly_pipeline is detected.'
+        end
+      end
+
       attr_reader :primary_store, :secondary_store, :instance_name
 
       FAILED_TO_READ_ERROR_MESSAGE = 'Failed to read from the redis default_store.'
-      FAILED_TO_WRITE_ERROR_MESSAGE = 'Failed to write to the redis primary_store.'
-      FAILED_TO_RUN_PIPELINE = 'Failed to execute pipeline on the redis primary_store.'
+      FAILED_TO_WRITE_ERROR_MESSAGE = 'Failed to write to the redis non_default_store.'
+      FAILED_TO_RUN_PIPELINE = 'Failed to execute pipeline on the redis non_default_store.'
 
       SKIP_LOG_METHOD_MISSING_FOR_COMMANDS = %i[info].freeze
 
@@ -100,6 +106,25 @@ module Gitlab
         validate_stores!
       end
 
+      # Pipelines are sent to both instances by default since
+      # they could execute both read and write commands.
+      #
+      # But for pipelines that only consists of read commands, this method
+      # can be used to scope the pipeline and send it only to the default store.
+      def with_readonly_pipeline
+        raise NestedReadonlyPipelineError if readonly_pipeline?
+
+        Thread.current[:readonly_pipeline] = true
+
+        yield
+      ensure
+        Thread.current[:readonly_pipeline] = false
+      end
+
+      def readonly_pipeline?
+        Thread.current[:readonly_pipeline].present?
+      end
+
       # rubocop:disable GitlabSecurity/PublicSend
       READ_COMMANDS.each do |name|
         define_method(name) do |*args, **kwargs, &block|
@@ -123,7 +148,7 @@ module Gitlab
 
       PIPELINED_COMMANDS.each do |name|
         define_method(name) do |*args, **kwargs, &block|
-          if use_primary_and_secondary_stores?
+          if use_primary_and_secondary_stores? && !readonly_pipeline?
             pipelined_both(name, *args, **kwargs, &block)
           else
             send_command(default_store, name, *args, **kwargs, &block)
@@ -192,7 +217,7 @@ module Gitlab
         use_primary_store_as_default? ? primary_store : secondary_store
       end
 
-      def fallback_store
+      def non_default_store
         use_primary_store_as_default? ? secondary_store : primary_store
       end
 
@@ -252,36 +277,39 @@ module Gitlab
       end
 
       def write_both(command_name, *args, **kwargs, &block)
+        result = send_command(default_store, command_name, *args, **kwargs, &block)
+
+        # write to the non-default store only if write on default store is successful
         begin
-          send_command(primary_store, command_name, *args, **kwargs, &block)
+          send_command(non_default_store, command_name, *args, **kwargs, &block)
         rescue StandardError => e
           log_error(e, command_name,
             multi_store_error_message: FAILED_TO_WRITE_ERROR_MESSAGE)
         end
 
-        send_command(secondary_store, command_name, *args, **kwargs, &block)
+        result
       end
 
       # Run the entire pipeline on both stores. We assume that `&block` is idempotent.
       def pipelined_both(command_name, *args, **kwargs, &block)
+        result_default = send_command(default_store, command_name, *args, **kwargs, &block)
+
         begin
-          result_primary = send_command(primary_store, command_name, *args, **kwargs, &block)
+          result_non_default = send_command(non_default_store, command_name, *args, **kwargs, &block)
         rescue StandardError => e
           log_error(e, command_name, multi_store_error_message: FAILED_TO_RUN_PIPELINE)
         end
 
-        result_secondary = send_command(secondary_store, command_name, *args, **kwargs, &block)
-
         # Pipelined commands return an array with all results. If they differ, log an error
-        if result_primary && result_primary != result_secondary
-          error = PipelinedDiffError.new(result_primary, result_secondary)
+        if result_non_default && result_non_default != result_default
+          error = PipelinedDiffError.new(result_non_default, result_default)
           error.set_backtrace(Thread.current.backtrace[1..]) # Manually set backtrace, since the error is not `raise`d
 
           log_error(error, command_name)
           increment_pipelined_command_error_count(command_name)
         end
 
-        result_secondary
+        result_default
       end
 
       def same_redis_store?