Protect history usage vs grpc protocol changes

Change all uses of history data protobuf messages so that they will not crash the calling script in the case where the grpc protocol removes the message or field being accessed. Instead, they will return None for the affected field (which most calling scripts interpret as "no data") or raise the same error as when the dish is not reachable.

Same caveats about code readability and adherence to the advertised type data as the change for protecting status usage.

This is for issue #66.

starlink_grpc.py

@@ -955,6 +955,9 @@ def get_history(context: Optional[ChannelContext] = None):
 
     Raises:
         grpc.RpcError: Communication or service error.
+        AttributeError, ValueError: Protocol error. Either the target is not a
+            Starlink user terminal or the grpc protocol has changed in a way
+            this module cannot handle.
     """
     def grpc_call(channel: grpc.Channel):
         if imports_pending:
@@ -970,8 +973,12 @@ def _compute_sample_range(history,
                           parse_samples: int,
                           start: Optional[int] = None,
                           verbose: bool = False):
+    try:
         current = int(history.current)
         samples = len(history.pop_ping_drop_rate)
+    except (AttributeError, TypeError):
+        # Without current and pop_ping_drop_rate, history is unusable.
+        return range(0), 0, None
 
     if verbose:
         print("current counter:       " + str(current))
@@ -1046,8 +1053,14 @@ def concatenate_history(history1,
         An object with the unwrapped history data and the same attribute
         fields as a grpc history object.
     """
+    try:
         size2 = len(history2.pop_ping_drop_rate)
         new_samples = history2.current - history1.current
+    except (AttributeError, TypeError):
+        # Something is wrong. Probably both history objects are bad, so no
+        # point in trying to combine them.
+        return history1
+
     if new_samples < 0:
         if verbose:
             print("Dish reboot detected. Appending anyway.")
@@ -1062,6 +1075,7 @@ def concatenate_history(history1,
 
     unwrapped = UnwrappedHistory()
     for field in HISTORY_FIELDS:
+        if hasattr(history1, field) and hasattr(history2, field):
             setattr(unwrapped, field, [])
     unwrapped.unwrapped = True
 
@@ -1069,12 +1083,20 @@ def concatenate_history(history1,
         history1, samples1, start=start1)
     for i in sample_range:
         for field in HISTORY_FIELDS:
+            if hasattr(unwrapped, field):
+                try:
                     getattr(unwrapped, field).append(getattr(history1, field)[i])
+                except (IndexError, TypeError):
+                    pass
 
     sample_range, ignore1, ignore2 = _compute_sample_range(history2, new_samples)  # pylint: disable=unused-variable
     for i in sample_range:
         for field in HISTORY_FIELDS:
+            if hasattr(unwrapped, field):
+                try:
                     getattr(unwrapped, field).append(getattr(history2, field)[i])
+                except (IndexError, TypeError):
+                    pass
 
     unwrapped.current = history2.current
     return unwrapped
@@ -1124,7 +1146,7 @@ def history_bulk_data(parse_samples: int,
     if history is None:
         try:
             history = get_history(context)
-        except grpc.RpcError as e:
+        except (AttributeError, ValueError, grpc.RpcError) as e:
             raise GrpcError(e) from e
 
     sample_range, parsed_samples, current = _compute_sample_range(history,
@@ -1138,11 +1160,30 @@ def history_bulk_data(parse_samples: int,
     uplink_throughput_bps = []
 
     for i in sample_range:
+        # pop_ping_drop_rate is checked in _compute_sample_range
         pop_ping_drop_rate.append(history.pop_ping_drop_rate[i])
-        pop_ping_latency_ms.append(
-            history.pop_ping_latency_ms[i] if history.pop_ping_drop_rate[i] < 1 else None)
-        downlink_throughput_bps.append(history.downlink_throughput_bps[i])
-        uplink_throughput_bps.append(history.uplink_throughput_bps[i])
+
+        latency = None
+        try:
+            if history.pop_ping_drop_rate[i] < 1:
+                latency = history.pop_ping_latency_ms[i]
+        except (AttributeError, IndexError, TypeError):
+            pass
+        pop_ping_latency_ms.append(latency)
+
+        downlink = None
+        try:
+            downlink = history.downlink_throughput_bps[i]
+        except (AttributeError, IndexError, TypeError):
+            pass
+        downlink_throughput_bps.append(downlink)
+
+        uplink = None
+        try:
+            uplink = history.uplink_throughput_bps[i]
+        except (AttributeError, IndexError, TypeError):
+            pass
+        uplink_throughput_bps.append(uplink)
 
     return {
         "samples": parsed_samples,
@@ -1209,7 +1250,7 @@ def history_stats(
     if history is None:
         try:
             history = get_history(context)
-        except grpc.RpcError as e:
+        except (AttributeError, ValueError, grpc.RpcError) as e:
             raise GrpcError(e) from e
 
     sample_range, parsed_samples, current = _compute_sample_range(history,
@@ -1258,12 +1299,25 @@ def history_stats(
             init_run_length = 0
         tot += d
 
+        down = 0.0
+        try:
             down = history.downlink_throughput_bps[i]
+        except (AttributeError, IndexError, TypeError):
+            pass
         usage_down += down
+
+        up = 0.0
+        try:
             up = history.uplink_throughput_bps[i]
+        except (AttributeError, IndexError, TypeError):
+            pass
         usage_up += up
 
+        rtt = 0.0
+        try:
             rtt = history.pop_ping_latency_ms[i]
+        except (AttributeError, IndexError, TypeError):
+            pass
         # note that "full" here means the opposite of ping drop full
         if d == 0.0:
             rtt_full.append(rtt)