Avoids a nil pointer dereference error in Geo() (#227)

* Avoids a nil pointer dereference error in Geo()

There was a case in staging where Locate was returning this for a host:

  "neubot-mlab4-syd04.mlab-staging.measurement-lab.org": {
    "Health": null,
    "Registration": null,
    "Prometheus": {
      "Health": false
    }
  },

... this was causing a panic in Locate. This commit avoids this, and adds a new
unit test to be sure it working as intended.

* Fixes a few typos in a comment in Geo()

Author: nkinkade

siteinfo/siteinfo.go

@@ -35,22 +35,31 @@ func Geo(msgs map[string]v2.HeartbeatMessage, v url.Values) (*geojson.FeatureCol
 		return nil, err
 	}
 
-	for k, v := range hosts {
+	for k, h := range hosts {
 		parts, err := host.Parse(k)
 		if err != nil {
 			returnError := fmt.Errorf("failed to parse hostname: %s", k)
 			return fc, returnError
 		}
 
-		f = geojson.NewFeature(orb.Point{v.Registration.Longitude, v.Registration.Latitude})
+		// We have witnessed a case in staging where a v2.HeartbeatMessage's
+		// Registration field was nil for some reason. It's not clear under
+		// what circumstances this error condition can happen, but skip this
+		// host if Registration is nil to avoid panics when trying to
+		// dereference a nil pointer.
+		if h.Registration == nil {
+			continue
+		}
+
+		f = geojson.NewFeature(orb.Point{h.Registration.Longitude, h.Registration.Latitude})
 		f.Properties = map[string]interface{}{
-			"health":      v.Health.Score,
-			"hostname":    v.Registration.Hostname,
+			"health":      h.Health.Score,
+			"hostname":    h.Registration.Hostname,
 			"machine":     fmt.Sprintf("%s-%s", parts.Site, parts.Machine),
 			"org":         parts.Org,
-			"probability": v.Registration.Probability,
-			"uplink":      v.Registration.Uplink,
-			"type":        v.Registration.Type,
+			"probability": h.Registration.Probability,
+			"uplink":      h.Registration.Uplink,
+			"type":        h.Registration.Type,
 		}
 
 		fc.Append(f)

siteinfo/siteinfo_test.go

@@ -299,6 +299,19 @@ func TestGeo(t *testing.T) {
 				"ndt-oma7777-217f832a.mlab.sandbox.measurement-lab.org",
 			},
 		},
+		{
+			name: "success-missing-data",
+			instances: map[string]v2.HeartbeatMessage{
+				"ndt-mlab3-mia0t.mlab-sandbox.measurement-lab.org": {
+					Health:       nil,
+					Registration: nil,
+					Prometheus: &v2.Prometheus{
+						Health: false,
+					},
+				},
+			},
+			wantErr: false,
+		},
 		{
 			name: "error",
 			instances: map[string]v2.HeartbeatMessage{