Actually disable hostname validation on Apple platforms when asked (#502)

Motivation:

swift-nio-ssl has long had a flag to disable its own certificate
validation behaviour. This flag works fine, and had the desired effect.

However, this flag did not take account of the fact that
Security.framework _also_ validates the hostname. Due to an
implementation bug, setting .noHostnameVerification disabled only one of
the two checks.

Modifications:

- Correctly check the flag before passing a hostname to
Security.framework
- Write some tests to make sure the behaviour works in all modes.

Result:

Better, more consistent behaviour.

Author: Lukasa

Sources/NIOSSL/SecurityFrameworkCertificateVerification.swift

@@ -30,10 +30,12 @@ extension SSLConnection {
                 preconditionFailure("This callback should only be used if we are using the system-default trust.")
             }
 
+            let expectedHostname = self.validateHostnames ? self.expectedHostname : nil
+
             // This force-unwrap is safe as we must have decided if we're a client or a server before validation.
             var trust: SecTrust? = nil
             var result: OSStatus
-            let policy = SecPolicyCreateSSL(self.role! == .client, self.expectedHostname as CFString?)
+            let policy = SecPolicyCreateSSL(self.role! == .client, expectedHostname as CFString?)
             result = SecTrustCreateWithCertificates(peerCertificates as CFArray, policy, &trust)
             guard result == errSecSuccess, let actualTrust = trust else {
                 throw NIOSSLError.unableToValidateCertificate

Tests/NIOSSLTests/SecurityFrameworkVerificationTests.swift

@@ -73,6 +73,68 @@ final class SecurityFrameworkVerificationTests: XCTestCase {
         #endif
     }
 
+    func testDefaultVerificationCanValidateHostname() throws {
+        #if canImport(Darwin)
+        let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+        defer {
+            try! group.syncShutdownGracefully()
+        }
+
+        let p = group.next().makePromise(of: NIOSSLVerificationResult.self)
+        let context = try NIOSSLContext(configuration: .makeClientConfiguration())
+        let connection = context.createConnection()!
+        connection.setConnectState()
+        connection.expectedHostname = "www.apple.com"
+
+        connection.performSecurityFrameworkValidation(promise: p, peerCertificates: Self.appleComCertChain)
+        let result = try p.futureResult.wait()
+
+        XCTAssertEqual(result, .certificateVerified)
+        #endif
+    }
+
+    func testDefaultVerificationFailsOnInvalidHostname() throws {
+        #if canImport(Darwin)
+        let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+        defer {
+            try! group.syncShutdownGracefully()
+        }
+
+        let p = group.next().makePromise(of: NIOSSLVerificationResult.self)
+        let context = try NIOSSLContext(configuration: .makeClientConfiguration())
+        let connection = context.createConnection()!
+        connection.setConnectState()
+        connection.expectedHostname = "www.swift-nio.io"
+
+        connection.performSecurityFrameworkValidation(promise: p, peerCertificates: Self.appleComCertChain)
+        let result = try p.futureResult.wait()
+
+        XCTAssertEqual(result, .failed)
+        #endif
+    }
+
+    func testDefaultVerificationIgnoresHostnamesWhenConfiguredTo() throws {
+        #if canImport(Darwin)
+        let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+        defer {
+            try! group.syncShutdownGracefully()
+        }
+
+        let p = group.next().makePromise(of: NIOSSLVerificationResult.self)
+        var configuration = TLSConfiguration.makeClientConfiguration()
+        configuration.certificateVerification = .noHostnameVerification
+        let context = try NIOSSLContext(configuration: configuration)
+        let connection = context.createConnection()!
+        connection.setConnectState()
+        connection.expectedHostname = "www.swift-nio.io"
+
+        connection.performSecurityFrameworkValidation(promise: p, peerCertificates: Self.appleComCertChain)
+        let result = try p.futureResult.wait()
+
+        XCTAssertEqual(result, .certificateVerified)
+        #endif
+    }
+
     func testDefaultVerificationPlusAdditionalCanUseAdditionalRoot() throws {
         #if canImport(Darwin)
         let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)