wal: add TestValidSnapshotEntriesAfterPurgeWal testcase

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

wal/wal_test.go

@@ -1003,3 +1003,59 @@ func TestValidSnapshotEntries(t *testing.T) {
 		t.Errorf("expected walSnaps %+v, got %+v", expected, walSnaps)
 	}
 }
+
+// TestValidSnapshotEntriesAfterPurgeWal ensure that there are many wal files, and after cleaning the first wal file,
+// it can work well.
+func TestValidSnapshotEntriesAfterPurgeWal(t *testing.T) {
+	oldSegmentSizeBytes := SegmentSizeBytes
+	SegmentSizeBytes = 64
+	defer func() {
+		SegmentSizeBytes = oldSegmentSizeBytes
+	}()
+	p, err := ioutil.TempDir(os.TempDir(), "waltest")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(p)
+	snap0 := walpb.Snapshot{Index: 0, Term: 0}
+	snap1 := walpb.Snapshot{Index: 1, Term: 1}
+	state1 := raftpb.HardState{Commit: 1, Term: 1}
+	snap2 := walpb.Snapshot{Index: 2, Term: 1}
+	snap3 := walpb.Snapshot{Index: 3, Term: 2}
+	state2 := raftpb.HardState{Commit: 3, Term: 2}
+	func() {
+		w, werr := Create(zap.NewExample(), p, nil)
+		if werr != nil {
+			t.Fatal(werr)
+		}
+		defer w.Close()
+
+		// snap0 is implicitly created at index 0, term 0
+		if err = w.SaveSnapshot(snap1); err != nil {
+			t.Fatal(err)
+		}
+		if err = w.Save(state1, nil); err != nil {
+			t.Fatal(err)
+		}
+		if err = w.SaveSnapshot(snap2); err != nil {
+			t.Fatal(err)
+		}
+		if err = w.SaveSnapshot(snap3); err != nil {
+			t.Fatal(err)
+		}
+		for i := 0; i < 128; i++ {
+			if err = w.Save(state2, nil); err != nil {
+				t.Fatal(err)
+			}
+		}
+	}()
+	files, _, ferr := selectWALFiles(nil, p, snap0)
+	if ferr != nil {
+		t.Fatal(ferr)
+	}
+	os.Remove(p + "/" + files[0])
+	_, err = ValidSnapshotEntries(zap.NewExample(), p)
+	if err != nil {
+		t.Fatal(err)
+	}
+}