fix: don't try to calculate sMax if there are no viable servers

When the primary is unknown the `maxStalenessReducer` attempts
to determine the server with the greatest `lastWriteDate` in
order to calculate each server's staleness. There is a bug where
the reducer currently assumes it will always be able to find such
a server, which is not possible when working with an empty set
of servers.

NODE-2641

lib/sdam/server_selection.js

@@ -59,7 +59,13 @@ function maxStalenessReducer(readPreference, topologyDescription, servers) {
       if (staleness <= readPreference.maxStalenessSeconds) result.push(server);
       return result;
     }, []);
-  } else if (topologyDescription.type === TopologyType.ReplicaSetNoPrimary) {
+  }
+
+  if (topologyDescription.type === TopologyType.ReplicaSetNoPrimary) {
+    if (servers.length === 0) {
+      return servers;
+    }
+
     const sMax = servers.reduce((max, s) => (s.lastWriteDate > max.lastWriteDate ? s : max));
     return servers.reduce((result, server) => {
       const stalenessMS =

test/spec/max-staleness/ReplicaSetNoPrimary/MaxStalenessTooSmall.json

@@ -0,0 +1,20 @@
+{
+  "topology_description": {
+    "type": "ReplicaSetNoPrimary",
+    "servers": [
+      {
+        "address": "a:27017",
+        "type": "Unknown"
+      },
+      {
+        "address": "b:27017",
+        "type": "Unknown"
+      }
+    ]
+  },
+  "read_preference": {
+    "mode": "Nearest",
+    "maxStalenessSeconds": 1
+  },
+  "error": true
+}

test/spec/max-staleness/ReplicaSetNoPrimary/MaxStalenessTooSmall.yml

@@ -0,0 +1,15 @@
+# maxStalenessSeconds must be at least 90 seconds, even with no known servers.
+---
+topology_description:
+  type: ReplicaSetNoPrimary
+  servers:
+  - &1
+    address: a:27017
+    type: Unknown
+  - &2
+    address: b:27017
+    type: Unknown
+read_preference:
+  mode: Nearest
+  maxStalenessSeconds: 1  # Too small.
+error: true

test/spec/max-staleness/ReplicaSetNoPrimary/NoKnownServers.json

@@ -14,7 +14,8 @@
   },
   "read_preference": {
     "mode": "Nearest",
-    "maxStalenessSeconds": 1
+    "maxStalenessSeconds": 90
   },
-  "error": true
+  "suitable_servers": [],
+  "in_latency_window": []
 }

test/spec/max-staleness/ReplicaSetNoPrimary/NoKnownServers.yml

@@ -1,4 +1,4 @@
-# maxStalenessSeconds must be at least 90 seconds, even with no known servers.
+# valid maxStalenessSeconds and no known servers results in an empty set of suitable servers
 ---
 topology_description:
   type: ReplicaSetNoPrimary
@@ -11,5 +11,6 @@ topology_description:
     type: Unknown
 read_preference:
   mode: Nearest
-  maxStalenessSeconds: 1  # Too small.
-error: true
+  maxStalenessSeconds: 90
+suitable_servers: []
+in_latency_window: []