[#292] pruning: grid: add all snapshots that do not match the regex to the rule's destroy list

Before this patch, multiple grids with disjoint regexes would result in
no snapshots being destroyed at all.

fixes #292

pruning/pruning_test.go

@@ -75,6 +75,10 @@ func TestPruneSnapshots(t *testing.T) {
 		},
 	}
 
+	reltime := func(secs int64) time.Time {
+		return time.Unix(secs, 0)
+	}
+
 	tcs := map[string]testCase{
 		"simple": {
 			inputs: inputs["s1"],
@@ -121,6 +125,24 @@ func TestPruneSnapshots(t *testing.T) {
 			},
 			expDestroy: map[string]bool{},
 		},
+		"multiple_grids_with_disjoint_regexes": {
+			inputs: []Snapshot{
+				stubSnap{"p1_a", false, reltime(4)},
+				stubSnap{"p2_a", false, reltime(5)},
+				stubSnap{"p1_b", false, reltime(14)},
+				stubSnap{"p2_b", false, reltime(15)},
+				stubSnap{"p1_c", false, reltime(29)},
+				stubSnap{"p2_c", false, reltime(30)},
+			},
+			rules: []KeepRule{
+				MustNewKeepGrid("^p1_", `1x10s | 1x10s`),
+				MustNewKeepGrid("^p2_", `1x10s | 1x10s`),
+			},
+			expDestroy: map[string]bool{
+				"p1_a": true,
+				"p2_a": true,
+			},
+		},
 	}
 
 	testTable(tcs, t)