Capture the current working directory early on OpenBSD.

Sources/Testing/Support/Additions/CommandLineAdditions.swift

@@ -10,6 +10,17 @@
 
 private import _TestingInternals
 
+#if os(OpenBSD)
+/// At process start (before `main()` is called), capture the current working
+/// directory.
+///
+/// -Note: This function is the only valid caller of `swt_captureEarlyCWD()`.
+@_section(".init_array.101") @_used
+private let _captureEarlyCWD: @convention(c) () -> Void = {
+  swt_captureEarlyCWD()
+}
+#endif
+
 extension CommandLine {
   /// The path to the current process' executable.
   static var executablePath: String {
@@ -57,11 +68,17 @@ 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/include/Stubs.h

@@ -180,6 +180,41 @@ static int swt_setfdflags(int fd, int flags) {
 }
 #endif
 
+#if defined(__OpenBSD__)
+/// Storage for the early working directory.
+static _Atomic(const char *_Nullable) swt_earlyCWD = NULL;
+
+/// 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.)
+///
+/// - Important: Do not call this function. It is automatically called by the
+///   loader when the process starts. To get the early current working
+///   directory, call ``swt_getEarlyCWD()`` instead.
+static void swt_captureEarlyCWD(void) {
+  static char buffer[PATH_MAX * 2];
+  if (getcwd(buffer, sizeof(buffer))) {
+    // stdatomic.h is missing on OpenBSD 7.7, so use clang's builtins instead.
+    const char *expectingNULL = NULL;
+    __c11_atomic_store(&swt_earlyCWD, buffer, __ATOMIC_SEQ_CST);
+  }
+}
+
+/// 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.)
+static const char *_Nullable swt_getEarlyCWD(void) {
+  // stdatomic.h is missing on OpenBSD 7.7, so use clang's builtins instead.
+  return __c11_atomic_load(&swt_earlyCWD, __ATOMIC_SEQ_CST);
+}
+#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