Revert "Close lower file descriptors on spawn"

This reverts commit 6efb1f7.

Author: grynspan

Diff for: Sources/Testing/ExitTests/SpawnProcess.swift

@@ -117,7 +117,7 @@ func spawnExecutable(
       }
 
       // Forward standard I/O streams and any explicitly added file handles.
-      var inheritedFDs: Set = [STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO]
+      var highestFD = max(STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO)
       func inherit(_ fileHandle: borrowing FileHandle, as standardFD: CInt? = nil) throws {
         try fileHandle.withUnsafePOSIXFileDescriptor { fd in
           guard let fd else {
@@ -129,7 +129,7 @@ func spawnExecutable(
 #if SWT_TARGET_OS_APPLE
             _ = posix_spawn_file_actions_addinherit_np(fileActions, fd)
 #endif
-            inheritedFDs.insert(fd)
+            highestFD = max(highestFD, fd)
           }
         }
       }
@@ -152,15 +152,7 @@ func spawnExecutable(
 #if SWT_TARGET_OS_APPLE
       // Close all other file descriptors open in the parent.
       flags |= CShort(POSIX_SPAWN_CLOEXEC_DEFAULT)
-#else
-      // Close all file descriptors up to the highest inherited one (except, of
-      // course, the ones we want to keep open!)
-      let highestFD = inheritedFDs.max()!
-      for fd in 0 ..< highestFD where !inheritedFDs.contains(fd) {
-        _ = posix_spawn_file_actions_addclose(fileActions, fd)
-      }
-
-#if os(Linux)
+#elseif os(Linux)
 #if !SWT_NO_DYNAMIC_LINKING
       // This platform doesn't have POSIX_SPAWN_CLOEXEC_DEFAULT, but we can at
       // least close all file descriptors higher than the highest inherited one.
@@ -183,7 +175,6 @@ func spawnExecutable(
 #else
 #warning("Platform-specific implementation missing: cannot close unused file descriptors")
 #endif
-#endif
 
 #if SWT_TARGET_OS_APPLE && DEBUG
       // Start the process suspended so we can attach a debugger if needed.