expand use of FileDescriptor and add FileDescriptorLike type alias

When accepting file descriptors, non-int objects with a fileno method
are now generally accepted. When returning a file descriptor, a
FileDescriptor instance is now returned.

1 files changed, 22 insertions, 3 deletions

diff --git a/linuxnamespaces/filedescriptor.py b/linuxnamespaces/filedescriptor.py
index 2cf8442..d11d4d6 100644
--- a/linuxnamespaces/filedescriptor.py
+++ b/linuxnamespaces/filedescriptor.py
@@ -8,11 +8,28 @@ import os
 import typing
 
 
+@typing.runtime_checkable
+class HasFileno(typing.Protocol):
+    def fileno(self) -> int:
+        ...
+
+
+FileDescriptorLike = int | HasFileno
+
+
 class FileDescriptor(int):
     """Type tag for integers that represent file descriptors. It also provides
     a few very generic file descriptor methods.
     """
 
+    def __new__(cls, value: FileDescriptorLike) -> typing.Self:
+        """Construct a FileDescriptor from an int or HasFileno."""
+        if isinstance(value, cls):
+            return value  # No need to copy, it's immutable.
+        if not isinstance(value, int):
+            value = value.fileno()
+        return super(FileDescriptor, cls).__new__(cls, value)
+
     def __enter__(self) -> "FileDescriptor":
         """When used as a context manager, close the file descriptor on scope
         exit.
@@ -37,16 +54,18 @@ class FileDescriptor(int):
             return FileDescriptor(os.dup(self))
         return FileDescriptor(fcntl.fcntl(self, fcntl.F_DUPFD_CLOEXEC, 0))
 
-    def dup2(self, fd2: int, inheritable: bool = True) -> "FileDescriptor":
+    def dup2(
+        self, fd2: FileDescriptorLike, inheritable: bool = True
+    ) -> "FileDescriptor":
         """Duplicate the file to the given file descriptor number."""
-        return FileDescriptor(os.dup2(self, fd2, inheritable))
+        return FileDescriptor(os.dup2(self, FileDescriptor(fd2), inheritable))
 
     @classmethod
     def pidfd_open(cls, pid: int, flags: int = 0) -> typing.Self:
         """Convenience wrapper for os.pidfd_open."""
         return cls(os.pidfd_open(pid, flags))
 
-    def fileno(self) -> int:
+    def fileno(self) -> FileDescriptor:
         """Return self such that it satisfies the HasFileno protocol."""
         return self