Implement discard_all for executions to be used to clear queues

Clearing a queue applies only to ready executions, but it's easy to
implement this for all types of executions.

Author: rosa

app/models/solid_queue/claimed_execution.rb

@@ -24,7 +24,11 @@ def release_all
       includes(:job).each(&:release)
     end
 
-    def discard_all(*)
+    def discard_all_in_batches(*)
+      raise UndiscardableError, "Can't discard jobs in progress"
+    end
+
+    def discard_all_from_jobs(*)
       raise UndiscardableError, "Can't discard jobs in progress"
     end
   end

app/models/solid_queue/execution.rb

@@ -23,6 +23,23 @@ def execution_data_from_jobs(jobs)
         end
       end
 
+      def discard_all_in_batches(batch_size: 500)
+        pending = count
+        discarded = 0
+
+        loop do
+          transaction do
+            job_ids = limit(batch_size).order(:job_id).lock.pluck(:job_id)
+
+            discard_jobs job_ids
+            discarded = where(job_id: job_ids).delete_all
+            pending -= discarded
+          end
+
+          break if pending <= 0 || discarded == 0
+        end
+      end
+
       def discard_all_from_jobs(jobs)
         transaction do
           job_ids = lock_all_from_jobs(jobs)

app/models/solid_queue/queue.rb

@@ -33,7 +33,7 @@ def resume
     end
 
     def clear
-      Job.where(queue_name: name).each(&:discard)
+      ReadyExecution.queued_as(name).discard_all_in_batches
     end
 
     def size

test/models/solid_queue/ready_execution_test.rb

@@ -127,6 +127,26 @@ class SolidQueue::ReadyExecutionTest < ActiveSupport::TestCase
     end
   end
 
+  test "discard all" do
+    3.times { |i| AddToBufferJob.perform_later(i) }
+
+    assert_difference [ -> { SolidQueue::ReadyExecution.count }, -> { SolidQueue::Job.count } ], -8 do
+      SolidQueue::ReadyExecution.discard_all_in_batches
+    end
+  end
+
+  test "discard all by queue" do
+    3.times { |i| AddToBufferJob.perform_later(i) }
+
+    assert_difference [ -> { SolidQueue::ReadyExecution.count }, -> { SolidQueue::Job.count } ], -5 do
+      SolidQueue::ReadyExecution.queued_as(:backend).discard_all_in_batches
+    end
+
+    assert_no_difference [ -> { SolidQueue::ReadyExecution.count }, -> { SolidQueue::Job.count } ] do
+      SolidQueue::ReadyExecution.queued_as(:backend).discard_all_in_batches
+    end
+  end
+
   private
     def assert_claimed_jobs(count, &block)
       assert_difference -> { SolidQueue::ClaimedExecution.count } => +count, -> { SolidQueue::ReadyExecution.count } => -count do