metamorphic: add a missing simulated excise

We weren't simulating the excise when the batch wasn't empty but it
yielded an empty sst (e.g. it only contained a `LogData`). Fix that
and refactor the code a bit to avoid three copies of this code.

Informs #3448

metamorphic/ops.go

@@ -898,19 +898,14 @@ func (o *ingestAndExciseOp) run(t *Test, h historyRecorder) {
 	}
 	db := t.getDB(o.dbID)
 	if b.Empty() {
-		// Ingestion will be a no-op. But still do a DeleteRange and RangeKeyDelete
-		// as the generator/key manager expect this to succeed regardless.
-		//
-		// TODO(bilal): Take out this case when we support standalone Excises with
-		// no ingestions.
-		err = firstError(err, db.DeleteRange(o.exciseStart, o.exciseEnd, t.writeOpts))
-		err = firstError(err, db.RangeKeyDelete(o.exciseStart, o.exciseEnd, t.writeOpts))
-		h.Recordf("%s // %v", o, err)
+		h.Recordf("%s // %v", o, o.simulateExcise(db, t))
 		return
 	}
+
 	if o.sstContainsExciseTombstone {
 		// Add a rangedel and rangekeydel to the batch. This ensures it'll end up
-		// inside the sstable.
+		// inside the sstable. Note that all entries in the sstable will have the
+		// same sequence number, so the ordering within the batch doesn't matter.
 		err = firstError(err, b.DeleteRange(o.exciseStart, o.exciseEnd, t.writeOpts))
 		err = firstError(err, b.RangeKeyDelete(o.exciseStart, o.exciseEnd, t.writeOpts))
 	}
@@ -919,38 +914,39 @@ func (o *ingestAndExciseOp) run(t *Test, h historyRecorder) {
 		h.Recordf("Build(%s) // %v", o.batchID, err2)
 		return
 	}
-	err = firstError(err, err2)
 	err = firstError(err, b.Close())
 
 	if writerMeta.Properties.NumEntries == 0 && writerMeta.Properties.NumRangeKeys() == 0 {
-		// No-op.
-		h.Recordf("%s // %v", o, err)
+		h.Recordf("%s // %v", o, o.simulateExcise(db, t))
 		return
 	}
-	if !t.testOpts.useExcise {
-		// Do a rangedel and rangekeydel before the ingestion. This mimics the
-		// behaviour of an excise.
-		err = firstError(err, db.DeleteRange(o.exciseStart, o.exciseEnd, t.writeOpts))
-		err = firstError(err, db.RangeKeyDelete(o.exciseStart, o.exciseEnd, t.writeOpts))
-	}
 
 	if t.testOpts.useExcise {
 		err = firstError(err, t.withRetries(func() error {
-			_, err := t.getDB(o.dbID).IngestAndExcise([]string{path}, nil /* shared */, nil /* external */, pebble.KeyRange{
+			_, err := db.IngestAndExcise([]string{path}, nil /* shared */, nil /* external */, pebble.KeyRange{
 				Start: o.exciseStart,
 				End:   o.exciseEnd,
 			}, o.sstContainsExciseTombstone)
 			return err
 		}))
 	} else {
+		err = firstError(err, o.simulateExcise(db, t))
 		err = firstError(err, t.withRetries(func() error {
-			return t.getDB(o.dbID).Ingest([]string{path})
+			return db.Ingest([]string{path})
 		}))
 	}
 
 	h.Recordf("%s // %v", o, err)
 }
 
+func (o *ingestAndExciseOp) simulateExcise(db *pebble.DB, t *Test) error {
+	// Simulate the excise using a DeleteRange and RangeKeyDelete.
+	return errors.CombineErrors(
+		db.DeleteRange(o.exciseStart, o.exciseEnd, t.writeOpts),
+		db.RangeKeyDelete(o.exciseStart, o.exciseEnd, t.writeOpts),
+	)
+}
+
 func (o *ingestAndExciseOp) receiver() objID { return o.dbID }
 func (o *ingestAndExciseOp) syncObjs() objIDSlice {
 	// Ingest should not be concurrent with mutating the batches that will be