Expose the available devices from the _ExecutionContext.

Sources/TensorFlow/Core/Runtime.swift

@@ -550,7 +550,7 @@ public final class _ExecutionContext {
     /// List of devices available to this execution context.
     /// Devices are represented by their names in TensorFlow notation.
     /// See documentation for `withDevice(named:perform:)` to learn about device names.
-    private var deviceNames: [String] = []
+    public private(set) var deviceNames: [String] = []
 
     /// The buffer storing a serialized TensorFlow config proto.
     public let tensorFlowConfig: UnsafeMutablePointer<TF_Buffer>

Tests/TensorFlowTests/RuntimeTests.swift

@@ -0,0 +1,31 @@
+// Copyright 2019 The TensorFlow Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import XCTest
+import TensorFlow  // Note: not imported as @testable in order to test the public API.
+
+final class RuntimeTests: XCTestCase {
+  func testDeviceNames() {
+    let deviceNames = _ExecutionContext.global.deviceNames
+    XCTAssert(deviceNames.count > 0, "Missing CPU device, got: \(deviceNames)")
+    let cpu0 = deviceNames.filter { $0.hasSuffix("/device:CPU:0") }
+    XCTAssertEqual(cpu0.count, 1, "All devices: \(deviceNames)")
+  }
+}
+
+extension RuntimeTests {
+  static var allTests = [
+    ("testDeviceNames", testDeviceNames),
+  ]
+}

Tests/TensorFlowTests/XCTestManifests.swift

@@ -16,25 +16,27 @@ import XCTest
 
 #if !os(macOS)
 public func allTests() -> [XCTestCaseEntry] {
+    // Please ensure the test cases remain alphabetized.
     return [
-        testCase(UtilitiesTests.allTests),
-        testCase(LossTests.allTests),
-        testCase(PRNGTests.allTests),
-        testCase(TrivialModelTests.allTests),
-        testCase(SequentialTests.allTests),
-        testCase(LayerTests.allTests),
-        testCase(TensorTests.allTests),
-        testCase(TensorGroupTests.allTests),
         testCase(BasicOperatorTests.allTests),
         testCase(ComparisonOperatorTests.allTests),
         testCase(DatasetTests.allTests),
-        testCase(MathOperatorTests.allTests),
+        testCase(LayerTests.allTests),
         testCase(LazyTensorTests.allTests),
         testCase(LazyTensorTraceTests.allTests),
         testCase(LazyTensorExplicitTraceTests.allTests),
         testCase(LazyTensorOperationTests.allTests),
         testCase(LazyTensorTFFunctionBuilderTests.allTests),
         testCase(LazyTensorEvaluationTests.allTests),
+        testCase(LossTests.allTests),
+        testCase(MathOperatorTests.allTests),
+        testCase(PRNGTests.allTests),
+        testCase(RuntimeTests.allTests),
+        testCase(SequentialTests.allTests),
+        testCase(TensorTests.allTests),
+        testCase(TensorGroupTests.allTests),
+        testCase(TrivialModelTests.allTests),
+        testCase(UtilitiesTests.allTests),
     ]
 }
 #endif