[MM-63314] Fix ClaimJob in HA environments (#30383)

* ClaimJob now returns newly claimed job

* internal code affected by change

* test changes required

* two branches: for mysql, use transaction; for postgres, use returning

* two branches: for mysql, use transaction; for postgres, use returning

* use same millis value for LastActivityAt and StartAt

* blank commit

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/jobs/s3_path_migration/s3_path_migration.go

@@ -12,7 +12,6 @@ import (
 
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
-	"github.com/mattermost/mattermost/server/public/shared/request"
 	"github.com/mattermost/mattermost/server/v8/channels/jobs"
 	"github.com/mattermost/mattermost/server/v8/channels/store"
 	"github.com/mattermost/mattermost/server/v8/platform/shared/filestore"
@@ -106,10 +105,12 @@ func (worker *S3PathMigrationWorker) DoJob(job *model.Job) {
 	logger.Debug("Worker: Received a new candidate job.")
 	defer worker.jobServer.HandleJobPanic(logger, job)
 
-	if claimed, err := worker.jobServer.ClaimJob(job); err != nil {
-		logger.Warn("S3PathMigrationWorker experienced an error while trying to claim job", mlog.Err(err))
+	var appErr *model.AppError
+	job, appErr = worker.jobServer.ClaimJob(job)
+	if appErr != nil {
+		logger.Warn("S3PathMigrationWorker experienced an error while trying to claim job", mlog.Err(appErr))
 		return
-	} else if !claimed {
+	} else if job == nil {
 		return
 	}
 
@@ -120,17 +121,6 @@ func (worker *S3PathMigrationWorker) DoJob(job *model.Job) {
 		return
 	}
 
-	c := request.EmptyContext(worker.logger)
-
-	var appErr *model.AppError
-	// We get the job again because ClaimJob changes the job status.
-	job, appErr = worker.jobServer.GetJob(c, job.Id)
-	if appErr != nil {
-		logger.Error("S3PathMigrationWorker: job execution error", mlog.Err(appErr))
-		worker.setJobError(logger, job, appErr)
-		return
-	}
-
 	// Check if there is metadata for that job.
 	// If there isn't, it will be empty by default, which is the right value.
 	startFileID := job.Data["start_file_id"]