Some WIP linux network-namespace (reader) utils

To be re-orged once we likely formalize a new `.linux` subpkg but,
provide basic `ip netns show` and lower-level `man namespaces` as read
from the filesys under `/proc/{pid}/ns` once we get to supporting it
more "seriously".

tractor/ipc/_linux_ns.py

@@ -0,0 +1,88 @@
+# tractor: structured concurrent "actors".
+# Copyright 2018-eternity Tyler Goodlet.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+'''
+Utils for Linux namespaces.
+
+See `man namespaces` for the high level deats!
+
+'''
+import subprocess
+import os
+from pathlib import Path
+
+
+def get_ns_inode(
+    ns_type: str = 'net',
+    pid: int = os.getpid(),
+) -> int:
+    '''
+    Return the `inode: int` for the namespace type `ns_type: str` for
+    the current process.
+
+    By default we use the  "network namespace" since it is generallly
+    used to isolate all the networ-transport domains for all IPC
+    inside `tractor.ipc`.
+
+    See for linux:
+    - `man namespaces`,
+    - `man network_namespaces`,
+     > In addition, network namespaces isolate the UNIX domain
+       abstract socket name‐ space (see unix(7))
+
+    - `man unix`,
+
+    If pid: int` is passed then do the lookup to
+    `Path(f'/proc/{pid}/ns/{ns_type}') instead of for the current
+    pid.
+
+    '''
+    ns_dir = Path(f'/proc/{pid}/ns/')
+    ns_path: Path = ns_dir / ns_type
+    entry: str = os.readlink(f'{ns_path}')
+    _ns_type: str
+    boxed_inode: str
+    _ns_type, _, boxed_inode = entry.partition(':')
+    assert ns_type == _ns_type
+
+    inode_str: str = boxed_inode[1:-1]
+    return int(inode_str)
+
+
+def get_netns(pid: int = os.getpid()) -> tuple[str|None, int]:
+    '''
+    Return the network namespace key-inode pair for the given process
+    id.
+
+    '''
+    out: str = subprocess.check_ouput([
+        'ip',
+        'netns',
+        'identify',
+        pid,
+    ])
+    netns_key: str|None = None
+    if out.decode().strip():
+        netns_key: str = out
+
+    # signifies the "default" (really just not in use)
+    # net namespace.
+    return (
+        netns_key or 'default',
+        get_ns_inode(
+            ns_type='net',
+            pid=pid,
+        ),
+    )