Make SNTP connection failure (more) reliable.

There are two reliability issues in the SNTP code.

Right now if we fail in the last step of `ntp_time_update` (when we do
`Smtp_ReceiveTimeResponse`) because of a network error, we run into an
infinite loop of retries. Even worse, the loop does not print anything
(the system looks frozen).

Also, although `sntp_update` is run in a loop in the examples, we do not
reset the timeout before calling `sntp_update`, which leads subsequent
calls to always fail.

Address this by not calling again `Smtp_ReceiveTimeResponse` if we reach
the timeout and reset-ing the timeout before calling again `sntp_update`
in examples.

Signed-off-by: Hugo Lefeuvre <[email protected]>

Author: hlef

examples/01.SNTP/sntp.cc

@@ -19,6 +19,7 @@ void __cheri_compartment("sntp_example") example()
 	{
 		Debug::log("Failed to update NTP time");
 		Timeout oneSecond{MS_TO_TICKS(1000)};
+		t = Timeout{MS_TO_TICKS(5000)};
 	}
 	Debug::log("Updating NTP took {} ticks", t.elapsed);
 	t = UnlimitedTimeout;

examples/03.HTTPS/https.cc

@@ -32,6 +32,7 @@ void __cheri_compartment("https_example") example()
 	{
 		Debug::log("Failed to update NTP time");
 		Timeout oneSecond{MS_TO_TICKS(1000)};
+		t = Timeout{MS_TO_TICKS(5000)};
 	}
 	Debug::log("Updating NTP took {} ticks", t.elapsed);
 	t = UnlimitedTimeout;

examples/04.MQTT/mqtt.cc

@@ -96,6 +96,7 @@ void __cheri_compartment("mqtt_example") example()
 	{
 		Debug::log("Failed to update NTP time");
 		Timeout oneSecond{MS_TO_TICKS(1000)};
+		t = Timeout{MS_TO_TICKS(5000)};
 	}
 	Debug::log("Updating NTP took {} ticks", t.elapsed);
 	t = UnlimitedTimeout;

lib/sntp/sntp.cc

@@ -410,6 +410,7 @@ namespace
 				SntpStatus_t lastStatus = SntpSuccess;
 				do
 				{
+					// `timeout` is updated through `context`.
 					status = Sntp_ReceiveTimeResponse(&context, 0);
 					if (status != lastStatus)
 					{
@@ -423,8 +424,21 @@ namespace
 						thread_sleep(&t);
 						timeout->elapse(t.elapsed);
 					}
-				} while (status == SntpNoResponseReceived);
-				Debug::log("Received new time fron NTP!");
+				} while (status == SntpNoResponseReceived &&
+				         timeout->may_block());
+
+				if (status != SntpSuccess)
+				{
+					Debug::log("Failed to receive SNTP time response: {}",
+					           status);
+					Timeout t{UnlimitedTimeout};
+					network_socket_close(
+					  &t, MALLOC_CAPABILITY, udpContext.socket);
+					timeout->elapse(t.elapsed);
+					return ntp_error_to_errno(status);
+				}
+
+				Debug::log("Received new time from NTP!");
 			}
 			else
 			{