From f013960d87e05d28cccbb38a7b22f62cbcd4176b Mon Sep 17 00:00:00 2001
From: Josh Bleecher Snyder <josh@tailscale.com>
Date: Wed, 4 Aug 2021 10:36:51 -0700
Subject: [PATCH] tstime/mono: make json.Unmarshal of a zero time.Time yield a
 zero Time

This was the proximate cause of #2579.
#2582 is a deeper fix, but this will remain
as a footgun, so may as well fix it too.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
---
 tstime/mono/mono.go      |  4 ++++
 tstime/mono/mono_test.go | 17 +++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/tstime/mono/mono.go b/tstime/mono/mono.go
index 3e582f644..c639c47a7 100644
--- a/tstime/mono/mono.go
+++ b/tstime/mono/mono.go
@@ -121,6 +121,10 @@ func (t *Time) UnmarshalJSON(data []byte) error {
 	if err != nil {
 		return err
 	}
+	if tt.IsZero() {
+		*t = 0
+		return nil
+	}
 	*t = Now().Add(-time.Since(tt))
 	return nil
 }
diff --git a/tstime/mono/mono_test.go b/tstime/mono/mono_test.go
index afa05799f..4d678c0bc 100644
--- a/tstime/mono/mono_test.go
+++ b/tstime/mono/mono_test.go
@@ -5,6 +5,7 @@
 package mono
 
 import (
+	"encoding/json"
 	"testing"
 	"time"
 )
@@ -17,6 +18,22 @@ func TestNow(t *testing.T) {
 	}
 }
 
+func TestUnmarshalZero(t *testing.T) {
+	var tt time.Time
+	buf, err := json.Marshal(tt)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var m Time
+	err = json.Unmarshal(buf, &m)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !m.IsZero() {
+		t.Errorf("expected unmarshal of zero time to be 0, got %d (~=%v)", m, m)
+	}
+}
+
 func BenchmarkMonoNow(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Now()