fix: check if server_metadata file is empty before unmarshall

agent/agent.go

@@ -4708,6 +4708,11 @@ func (a *Agent) checkServerLastSeen(readFn consul.ServerMetadataReadFunc) error
 		return fmt.Errorf("error reading server metadata: %w", err)
 	}
 
+	// Return early if the file exists, but content is empty.
+	if md == nil {
+		return nil
+	}
+
 	maxAge := a.config.ServerRejoinAgeMax
 	if md.IsLastSeenStale(maxAge) {
 		return fmt.Errorf("refusing to rejoin cluster because server has been offline for more than the configured server_rejoin_age_max (%s) - consider wiping your data dir", maxAge)

agent/agent_test.go

@@ -6412,6 +6412,15 @@ func TestAgent_checkServerLastSeen(t *testing.T) {
 		require.ErrorContains(t, err, "refusing to rejoin cluster because server has been offline for more than the configured server_rejoin_age_max")
 	})
 
+	t.Run("TestIsLastSeenEmptyFile", func(t *testing.T) {
+		readFn := func(filename string) (*consul.ServerMetadata, error) {
+			return nil, nil
+		}
+
+		err := agent.checkServerLastSeen(readFn)
+		require.NoError(t, err)
+	})
+
 	// Test that a server with a 6h old last seen timestamp is not treated as an error.
 	t.Run("TestNoErr", func(t *testing.T) {
 		agent.config.ServerRejoinAgeMax = 24 * 7 * time.Hour

agent/consul/server_metadata.go

@@ -46,6 +46,10 @@ func ReadServerMetadata(filename string) (*ServerMetadata, error) {
 		return nil, err
 	}
 
+	if len(b) == 0 {
+		return nil, nil
+	}
+
 	var md ServerMetadata
 	if err := json.Unmarshal(b, &md); err != nil {
 		return nil, err