Capture the current working directory early on OpenBSD. (#1388)

This PR adds some OpenBSD-specific code to capture the current working
directory as early as possible. We then use that directory when argv[0]
appears to be a relative path so that the following incantation works
correctly:

```sh
./.build/debug/myPackageTests.xctest --testing-library swift-testing
```

This logic is not necessary on other platforms because they all provide
a way to get the path to the current executable. OpenBSD has no such
API.

Resolves #1348.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Support/Additions/CommandLineAdditions.swift

@@ -57,11 +57,17 @@ extension CommandLine {
       }
 #elseif os(OpenBSD)
       // OpenBSD does not have API to get a path to the running executable. Use
-      // arguments[0]. We do a basic sniff test for a path-like string, but
-      // otherwise return argv[0] verbatim.
-      guard let argv0 = arguments.first, argv0.contains("/") else {
+      // arguments[0]. We do a basic sniff test for a path-like string, and
+      // prepend the early CWD if it looks like a relative path, but otherwise
+      // return argv[0] verbatim.
+      guard var argv0 = arguments.first, argv0.contains("/") else {
         throw CError(rawValue: ENOEXEC)
       }
+      if argv0.first != "/",
+         let earlyCWD = swt_getEarlyCWD().flatMap(String.init(validatingCString:)),
+         !earlyCWD.isEmpty {
+        argv0 = "\(earlyCWD)/\(argv0)"
+      }
       return argv0
 #elseif os(Windows)
       var result: String?

Sources/_TestingInternals/CMakeLists.txt

@@ -12,6 +12,7 @@ include(GitCommit)
 include(TargetTriple)
 add_library(_TestingInternals STATIC
   Discovery.cpp
+  ExecutablePath.cpp
   Versions.cpp
   WillThrow.cpp)
 target_include_directories(_TestingInternals PUBLIC

Sources/_TestingInternals/ExecutablePath.cpp

@@ -0,0 +1,35 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+#include "ExecutablePath.h"
+
+#include <atomic>
+
+#if defined(__OpenBSD__)
+static std::atomic<const char *> earlyCWD { nullptr };
+
+/// At process start (before `main()` is called), capture the current working
+/// directory.
+///
+/// This function is necessary on OpenBSD so that we can (as correctly as
+/// possible) resolve the executable path when the first argument is a relative
+/// path (which can occur when manually invoking the test executable.)
+__attribute__((__constructor__(101), __used__))
+static void swt_captureEarlyCWD(void) {
+  static char buffer[PATH_MAX * 2];
+  if (getcwd(buffer, sizeof(buffer))) {
+    earlyCWD.store(buffer);
+  }
+}
+
+const char *swt_getEarlyCWD(void) {
+  return earlyCWD.load();
+}
+#endif

Sources/_TestingInternals/include/ExecutablePath.h

@@ -0,0 +1,31 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+#if !defined(SWT_EXECUTABLE_PATH_H)
+#define SWT_EXECUTABLE_PATH_H
+
+#include "Defines.h"
+#include "Includes.h"
+
+SWT_ASSUME_NONNULL_BEGIN
+
+#if defined(__OpenBSD__)
+/// Get the current working directory as it was set shortly after the process
+/// started and before `main()` has been called.
+///
+/// This function is necessary on OpenBSD so that we can (as correctly as
+/// possible) resolve the executable path when the first argument is a relative
+/// path (which can occur when manually invoking the test executable.)
+SWT_EXTERN const char *_Nullable swt_getEarlyCWD(void);
+#endif
+
+SWT_ASSUME_NONNULL_END
+
+#endif

Tests/TestingTests/ExitTestTests.swift

@@ -625,6 +625,14 @@ private import _TestingInternals
     }
   }
 #endif
+
+#if os(OpenBSD)
+  @Test("Changing the CWD doesn't break exit tests")
+  func changeCWD() async throws {
+    try #require(0 == chdir("/"))
+    await #expect(processExitsWith: .success) {}
+  }
+#endif
 }
 
 // MARK: - Fixtures