tuning the edge for merge-task execution loop

Adjusting the merge task creation loop to accommodate
the newly merged segments so that the eventual merge
results/ number of segments stay within the calculated budget.

index/scorch/mergeplan/merge_plan.go

@@ -186,13 +186,13 @@ func plan(segmentsIn []Segment, o *MergePlanOptions) (*MergePlan, error) {
 
 	// While we’re over budget, keep looping, which might produce
 	// another MergeTask.
-	for len(eligibles) > budgetNumSegments {
+	for len(eligibles) > 0 && (len(eligibles)+len(rv.Tasks)) > budgetNumSegments {
 		// Track a current best roster as we examine and score
 		// potential rosters of merges.
 		var bestRoster []Segment
 		var bestRosterScore float64 // Lower score is better.
 
-		for startIdx := 0; startIdx < len(eligibles)-o.SegmentsPerMergeTask; startIdx++ {
+		for startIdx := 0; startIdx < len(eligibles); startIdx++ {
 			var roster []Segment
 			var rosterLiveSize int64
 

index/scorch/mergeplan/merge_plan_test.go

@@ -73,7 +73,16 @@ func TestSimplePlan(t *testing.T) {
 				segs[2],
 			},
 			nil,
-			&MergePlan{},
+			&MergePlan{
+				Tasks: []*MergeTask{
+					&MergeTask{
+						Segments: []Segment{
+							segs[2],
+							segs[1],
+						},
+					},
+				},
+			},
 			nil,
 		},
 		{"3 segments",
@@ -83,7 +92,17 @@ func TestSimplePlan(t *testing.T) {
 				segs[9],
 			},
 			nil,
-			&MergePlan{},
+			&MergePlan{
+				Tasks: []*MergeTask{
+					&MergeTask{
+						Segments: []Segment{
+							segs[9],
+							segs[2],
+							segs[1],
+						},
+					},
+				},
+			},
 			nil,
 		},
 		{"many segments",