Create an xunit results file even if no tests are run. (#7066)

This PR ensures that when `--xunit-output` is passed to `swift test
--parallel`, the output XML file is generated even if no tests are run.

Failure modes outside of test failures (i.e. errors thrown during the
build process or when trying to run the test process) will still prevent
the generation of this file.

Resolves #7065.

Author: grynspan

Fixtures/Miscellaneous/EmptyTestsPkg/.gitignore

@@ -0,0 +1,4 @@
+.DS_Store
+/.build
+/Packages
+/*.xcodeproj

Fixtures/Miscellaneous/EmptyTestsPkg/Package.swift

@@ -0,0 +1,14 @@
+// swift-tools-version:4.2
+import PackageDescription
+
+let package = Package(
+    name: "EmptyTestsPkg",
+    targets: [
+        .target(
+            name: "EmptyTestsPkg",
+            dependencies: []),
+        .testTarget(
+            name: "EmptyTestsPkgTests",
+            dependencies: ["EmptyTestsPkg"]),
+    ]
+)

Fixtures/Miscellaneous/EmptyTestsPkg/Sources/EmptyTestsPkg/EmptyTestsPkg.swift

@@ -0,0 +1,5 @@
+struct EmptyTests {
+
+    var text = "Hello, World!"
+    var bool = false
+}

Fixtures/Miscellaneous/EmptyTestsPkg/Tests/EmptyTestsPkgTests/EmptyTestsTests.swift

@@ -0,0 +1,2 @@
+import XCTest
+@testable import EmptyTestsPkg

Sources/Commands/SwiftTestTool.swift

@@ -241,6 +241,7 @@ public struct SwiftTestTool: SwiftCommand {
             // If there were no matches, emit a warning and exit.
             if tests.isEmpty {
                 swiftTool.observabilityScope.emit(.noMatchingTests)
+                try generateXUnitOutputIfRequested(for: [], swiftTool: swiftTool)
                 return
             }
 
@@ -264,14 +265,7 @@ public struct SwiftTestTool: SwiftCommand {
 
             let testResults = try runner.run(tests)
 
-            // Generate xUnit file if requested
-            if let xUnitOutput = options.xUnitOutput {
-                let generator = XUnitGenerator(
-                    fileSystem: swiftTool.fileSystem,
-                    results: testResults
-                )
-                try generator.generate(at: xUnitOutput)
-            }
+            try generateXUnitOutputIfRequested(for: testResults, swiftTool: swiftTool)
 
             // process code Coverage if request
             if self.options.enableCodeCoverage, runner.ranSuccessfully {
@@ -325,6 +319,19 @@ public struct SwiftTestTool: SwiftCommand {
         }
     }
 
+    /// Generate xUnit file if requested.
+    private func generateXUnitOutputIfRequested(for testResults: [ParallelTestRunner.TestResult], swiftTool: SwiftTool) throws {
+        guard let xUnitOutput = options.xUnitOutput else {
+            return
+        }
+
+        let generator = XUnitGenerator(
+            fileSystem: swiftTool.fileSystem,
+            results: testResults
+        )
+        try generator.generate(at: xUnitOutput)
+    }
+
     // MARK: - swift-testing
 
     private func swiftTestingRun(_ swiftTool: SwiftTool) throws {

Tests/CommandsTests/TestToolTests.swift

@@ -128,6 +128,22 @@ final class TestToolTests: CommandsTestCase {
         }
     }
 
+    func testSwiftTestXMLOutputWhenEmpty() throws {
+        try fixture(name: "Miscellaneous/EmptyTestsPkg") { fixturePath in
+            let xUnitOutput = fixturePath.appending("result.xml")
+            // Run tests in parallel with verbose output.
+            let stdout = try SwiftPM.Test.execute(["--parallel", "--verbose", "--xunit-output", xUnitOutput.pathString], packagePath: fixturePath).stdout
+            // in "swift test" test output goes to stdout
+            XCTAssertNoMatch(stdout, .contains("passed"))
+            XCTAssertNoMatch(stdout, .contains("failed"))
+
+            // Check the xUnit output.
+            XCTAssertFileExists(xUnitOutput)
+            let contents: String = try localFileSystem.readFileContents(xUnitOutput)
+            XCTAssertMatch(contents, .contains("tests=\"0\" failures=\"0\""))
+        }
+    }
+
     func testSwiftTestFilter() throws {
         try fixture(name: "Miscellaneous/SkipTests") { fixturePath in
             let (stdout, _) = try SwiftPM.Test.execute(["--filter", ".*1"], packagePath: fixturePath)