jobs: avoid unnecessary system.job_progress updates

This avoid unnecessary job progress updates when a user issues job
control statements such as pause and unpause.

Epic: none
Release note: None

Author: stevendanna

pkg/bench/rttanalysis/testdata/benchmark_expectations

@@ -61,16 +61,16 @@ exp,benchmark
 8,Grant/grant_all_on_3_tables
 12,GrantRole/grant_1_role
 16,GrantRole/grant_2_roles
-12-15,Jobs/cancel_job
+9-12,Jobs/cancel_job
 3,Jobs/crdb_internal.system_jobs
 3-5,Jobs/jobs_page_default
 3,Jobs/jobs_page_latest_50
 3,Jobs/jobs_page_type_filtered
 1-3,Jobs/jobs_page_type_filtered_no_matches
 2,Jobs/non_admin_crdb_internal.system_jobs
 2,Jobs/non_admin_show_jobs
-12-15,Jobs/pause_job
-12-15,Jobs/resume_job
+9-12,Jobs/pause_job
+9-12,Jobs/resume_job
 3,Jobs/show_job
 3-5,Jobs/show_jobs
 3,ORMQueries/activerecord_type_introspection_query

pkg/jobs/update.go

@@ -293,7 +293,10 @@ WHERE id = $1
 		}
 	}
 
-	if progress != nil {
+	// NB: if ju.md.Progress was non-nil then progress has been set to the value
+	// from the updater. If it isn't set, progress has the value from the original
+	// scan.
+	if ju.md.Progress != nil {
 		var ts hlc.Timestamp
 		if hwm := progress.GetHighWater(); hwm != nil {
 			ts = *hwm