MB-66112: fix an edge case related to index convergence (#2200)

- As an edge case, the segment layout can be such that the merge
planning generates tasks having only a single non-empty segment.
- When this task is processed and introduced into the scorch system, the
root doesn't change and you'd be stuck in an infinite loop because the
same segment is still present in the system and the plan keeps
generating the task with the same single segment, which doesn't converge
the index to a steady state.

Author: Thejas-bhat

index/scorch/mergeplan/merge_plan.go

@@ -295,8 +295,10 @@ func plan(segmentsIn []Segment, o *MergePlanOptions) (*MergePlan, error) {
 		if len(bestRoster) == 0 {
 			return rv, nil
 		}
-
-		rv.Tasks = append(rv.Tasks, &MergeTask{Segments: bestRoster})
+		// create tasks with valid merges - i.e. there should be atleast 2 non-empty segments
+		if len(bestRoster) > 1 {
+			rv.Tasks = append(rv.Tasks, &MergeTask{Segments: bestRoster})
+		}
 
 		eligibles = removeSegments(eligibles, bestRoster)
 	}

index/scorch/mergeplan/merge_plan_test.go

@@ -719,3 +719,46 @@ func TestPlanMaxSegmentFileSize(t *testing.T) {
 		})
 	}
 }
+
+func TestSingleTaskMergePlan(t *testing.T) {
+	o := &DefaultMergePlanOptions
+	o.FloorSegmentFileSize = 209715200
+
+	// borrowing the spec values from MB-66112
+	//
+	// both segments are eligible, but the roster with a single segment is scored
+	// higher than the roster with two segments
+	// in this case the merge plan returns task with a single segment non-empty
+	// segment which when introduced into the scorch system doesn't cause any change
+	// and you'd be stuck in an infinite loop where the plan keeps generating the
+	// same task with the same single segment, which doesn't converge the index to
+	// a steady state
+	spec := testCyclesSpec{
+		descrip: "mssswdbm",
+		verbose: os.Getenv("VERBOSE") == "mssswdbm" || os.Getenv("VERBOSE") == "y",
+		o:       o,
+		segments: []Segment{
+			&segment{
+				MyId:       2,
+				MyFullSize: 78059,
+				MyLiveSize: 78059,
+				MyFileSize: 129475914,
+			},
+			&segment{
+				MyId:       1,
+				MyFullSize: 3959,
+				MyLiveSize: 3959,
+				MyFileSize: 24805725,
+			},
+		},
+	}
+
+	plan, err := Plan(spec.segments, spec.o)
+	if err != nil {
+		t.Fatalf("Plan failed, err: %v", err)
+	}
+
+	if len(plan.Tasks) > 0 {
+		t.Fatalf("expected 0 tasks, got: %d", len(plan.Tasks))
+	}
+}