Add safe artifact previews

coordinator/internal/domain/job.go

@@ -119,6 +119,10 @@ func (p JobProgress) DeriveStatus() JobStatus {
 	switch {
 	case p.Job.Status == JobCancelled:
 		return JobCancelled
+	case p.Job.Status == JobFailed:
+		// A reducer may fail after every shard has completed. That terminal
+		// failure must not be overwritten by an otherwise-complete task count.
+		return JobFailed
 	case p.Job.Status == JobReducing:
 		return JobReducing
 	case p.Total == 0:

coordinator/internal/domain/job_test.go

@@ -82,6 +82,7 @@ func TestDeriveStatus(t *testing.T) {
 		{"done and failed", JobProgress{Total: 3, Done: 2, Failed: 1}, JobFailed},
 		{"failed but work remains", JobProgress{Total: 3, Pending: 1, Failed: 2}, JobRunning},
 		{"cancelled job wins over task histogram", JobProgress{Job: Job{Status: JobCancelled}, Total: 3, Done: 1, Cancelled: 2}, JobCancelled},
+		{"persisted reducer failure wins over completed tasks", JobProgress{Job: Job{Status: JobFailed}, Total: 3, Done: 3}, JobFailed},
 	}
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {