Make computeXCTestMinimumDeploymentTarget() more robust

This could assert with empty stdout from `xcrun` or if the output of
`xcrun` was not an absolute path. We should be robust against these
cases and return the fallback.

rdar://problem/62898201

Author: neonichu

Sources/PackageLoading/MinimumDeploymentTarget.swift

@@ -30,6 +30,12 @@ public struct MinimumDeploymentTarget {
         return PlatformVersion(versionString)
     }
 
+    static func computeXCTestMinimumDeploymentTarget(with runResult: ProcessResult) throws -> PlatformVersion? {
+        let sdkPath = try AbsolutePath(validating: try runResult.utf8Output().spm_chuzzle() ?? "/")
+        let xcTestPath = sdkPath.appending(RelativePath("Developer/Library/Frameworks/XCTest.framework/XCTest"))
+        return try computeMinimumDeploymentTarget(of: xcTestPath)
+    }
+
     static func computeXCTestMinimumDeploymentTarget(for platform: PackageModel.Platform) -> PlatformVersion {
         guard let sdkName = platform.sdkName else {
             return platform.oldestSupportedVersion
@@ -39,10 +45,8 @@ public struct MinimumDeploymentTarget {
         #if os(macOS)
         do {
             let runResult = try Process.popen(arguments: ["xcrun", "--sdk", sdkName, "--show-sdk-platform-path"])
-            let sdkPath = AbsolutePath(try runResult.utf8Output().spm_chuzzle() ?? "")
-            let xcTestPath = sdkPath.appending(RelativePath("Developer/Library/Frameworks/XCTest.framework/XCTest"))
 
-            if let version = try computeMinimumDeploymentTarget(of: xcTestPath) {
+            if let version = try computeXCTestMinimumDeploymentTarget(with: runResult) {
                 return version
             }
         } catch { } // we do not treat this a fatal and instead use the fallback minimum deployment target

Tests/PackageLoadingTests/MinimumDeploymentTargetTests.swift

@@ -0,0 +1,57 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See http://swift.org/LICENSE.txt for license information
+ See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+import TSCBasic
+import XCTest
+
+@testable import PackageLoading
+
+class MinimumDeploymentTargetTests: XCTestCase {
+#if os(macOS) // these tests eventually call `xcrun`.
+    func testDoesNotAssertWithNoOutput() throws {
+        let result = ProcessResult(arguments: [],
+                                   environment: [:],
+                                   exitStatus: .terminated(code: 0),
+                                   output: "".asResult,
+                                   stderrOutput: "xcodebuild: error: SDK \"macosx\" cannot be located.".asResult)
+
+        XCTAssertNil(try MinimumDeploymentTarget.computeXCTestMinimumDeploymentTarget(with: result))
+    }
+
+    func testThrowsWithNonPathOutput() throws {
+        let result = ProcessResult(arguments: [],
+                                   environment: [:],
+                                   exitStatus: .terminated(code: 0),
+                                   output: "some string".asResult,
+                                   stderrOutput: "".asResult)
+
+        XCTAssertThrowsError(try MinimumDeploymentTarget.computeXCTestMinimumDeploymentTarget(with: result))
+    }
+
+    func testThrowsWithErrorForOutput() throws {
+        let result = ProcessResult(arguments: [],
+                                   environment: [:],
+                                   exitStatus: .terminated(code: 0),
+                                   output: .failure(DummyError()),
+                                   stderrOutput: "".asResult)
+
+        XCTAssertThrowsError(try MinimumDeploymentTarget.computeXCTestMinimumDeploymentTarget(with: result))
+    }
+#endif
+}
+
+private struct DummyError: Error {
+}
+
+private extension String {
+    var asResult: Result<[UInt8], Error> {
+        return .success(Array(utf8))
+    }
+}