Add a log handler helper (#188)

sharding: add a log-handler helper for tests

Former-commit-id: fb083848d8615f36786f5c6b96a2182a8be21b9d [formerly 1c62e207943d9265a59f0ff6577479c6a2eaf76b]
Former-commit-id: 0677762c40db862185c7b951e419473949b761c9

sharding/utils/log_helper_test.go

@@ -0,0 +1,44 @@
+package utils
+
+import (
+	"testing"
+
+	"github.com/ethereum/go-ethereum/log"
+)
+
+var _ = log.Handler(&LogHandler{})
+
+// LogHandler provides methods for testing ethereum logs.
+type LogHandler struct {
+	records []*log.Record
+	t       *testing.T
+}
+
+// Log adds records to the record slice.
+func (t *LogHandler) Log(r *log.Record) error {
+	t.records = append(t.records, r)
+	return nil
+}
+
+// Pop the record at index 0.
+func (t *LogHandler) Pop() *log.Record {
+	var r *log.Record
+	r, t.records = t.records[0], t.records[1:]
+	return r
+}
+
+// Length of the records slice.
+func (t *LogHandler) Len() int {
+	return len(t.records)
+}
+
+// VerifyLogMsg verfies that the log at index 0 matches the string exactly.
+// This method removes the verified message from the slice.
+func (h *LogHandler) VerifyLogMsg(str string) {
+	if h.Len() == 0 {
+		h.t.Error("Expected a log, but there were none!")
+	}
+	if l := h.Pop(); l.Msg != str {
+		h.t.Errorf("Unexpected log: %v. Wanted: %s", l, str)
+	}
+}