SWIFT-549 Test that $readPreference isn't sent to standalone servers (#691)

Author: kmahar

Sources/MongoSwift/SDAM.swift

@@ -249,7 +249,7 @@ extension ServerDescription: Equatable {
 /// which servers are up, what type of servers they are, which is primary, and so on.
 public struct TopologyDescription: Equatable {
     /// The possible types for a topology.
-    public struct TopologyType: RawRepresentable, Equatable {
+    public struct TopologyType: RawRepresentable, Equatable, CustomStringConvertible {
         /// A single mongod server.
         public static let single = TopologyType(.single)
 
@@ -295,6 +295,10 @@ public struct TopologyDescription: Equatable {
             }
             self._topologyType = _type
         }
+
+        public var description: String {
+            self.rawValue
+        }
     }
 
     /// The type of this topology.

Tests/LinuxMain.swift

@@ -268,6 +268,7 @@ extension ReadConcernTests {
 extension ReadPreferenceOperationTests {
     static var allTests = [
         ("testOperationReadPreference", testOperationReadPreference),
+        ("testReadPreferenceIsntSentToStandalones", testReadPreferenceIsntSentToStandalones),
     ]
 }
 

Tests/MongoSwiftSyncTests/ReadPreferenceOperationTests.swift

@@ -41,4 +41,27 @@ final class ReadPreferenceOperationTests: MongoSwiftTestCase {
             options: DistinctOptions(readPreference: .secondaryPreferred)
         )).toNot(throwError())
     }
+
+    func testReadPreferenceIsntSentToStandalones() throws {
+        try withTestNamespace { client, _, coll in
+            guard try client.topologyType() == .single else {
+                print(unsupportedTopologyMessage(testName: self.name))
+                return
+            }
+
+            let opts = FindOneOptions(readPreference: .secondary)
+            let monitor = client.addCommandMonitor()
+            try monitor.captureEvents {
+                _ = try coll.findOne(options: opts)
+            }
+
+            guard let event = monitor.commandStartedEvents(withNames: ["find"]).first else {
+                XCTFail("Unexpectedly missing find event")
+                return
+            }
+
+            expect(event.command.keys)
+                .toNot(contain("$readPreference"), description: "Command unexpectedly contained $readPreference key")
+        }
+    }
 }