Use unsigned_mut in age and update comment.

rust/src/events/serialize.rs

@@ -468,15 +468,21 @@ fn serialize_event_value(
         Value::String(event.event_id().to_owned()),
     );
 
-    // age_ts -> age
-    if let Some(Value::Object(unsigned)) = d.get_mut(UNSIGNED) {
-        if let Some(age_ts) = unsigned.get(unsigned_field::AGE_TS).and_then(Value::as_i64) {
-            unsigned.insert(
-                unsigned_field::AGE.to_owned(),
-                Value::Number(Number::from(time_now_ms - age_ts)),
-            );
-            unsigned.remove(unsigned_field::AGE_TS);
-        }
+    // Replace `age_ts` with `age`, with `age` calculated as the difference
+    // between the current time and `age_ts`. This is an optional field in the
+    // spec.
+    //
+    // We might not have an `age_ts`, e.g. if a remote server did not include
+    // the `age` field in the event it sent us. Since `age_ts` is generated by
+    // us it *should* be an integer, but it is possible for it to be out of i64
+    // range. In that case, just omit `age` rather than erroring.
+    let unsigned = unsigned_mut(&mut d);
+    if let Some(age_ts) = unsigned.get(unsigned_field::AGE_TS).and_then(Value::as_i64) {
+        unsigned.insert(
+            unsigned_field::AGE.to_owned(),
+            Value::Number(Number::from(time_now_ms - age_ts)),
+        );
+        unsigned.remove(unsigned_field::AGE_TS);
     }
 
     // Include the transaction_id / delay_id in the unsigned section if the event