Merge bitcoin/bitcoin#29034: test: detect OS in functional tests consistently using platform.system()

878d914777 doc: test: mention OS detection preferences in style guideline (Sebastian Falbesoner)
4c65ac96f8 test: detect OS consistently using `platform.system()` (Sebastian Falbesoner)
37324ae3df test: use `skip_if_platform_not_linux` helper where possible (Sebastian Falbesoner)

Pull request description:

  There are at least three ways to detect the operating system in Python3:
  - `os.name` (https://docs.python.org/3.9/library/os.html#os.name)
  - `sys.platform` (https://docs.python.org/3.9/library/sys.html#sys.platform)
  - `platform.system()` (https://docs.python.org/3.9/library/platform.html#platform.system)

  We are currently using all of them in functional tests (both in individual tests and shared test framework code), which seems a bit messy. This PR consolidates into using `platform.system()`, as it appears to be one most consistent and easy to read (see also [IRC discussion](https://bitcoin-irc.chaincode.com/bitcoin-core-dev/2023-12-08#989301;) and table below). `sys.platform` is inconsistent as it has the major version number encoded for BSD systems, which doesn't make much sense for e.g. OpenBSD, where there is no concept of major versions, but instead the version is simply increased by 0.1 on each release.

  Note that `os.name` is still useful to detect whether we are running a POSIX system (see `BitcoinTestFramework.skip_if_platform_not_posix`), so for this use-case it is kept as only exception. The following table shows values for common operating systems, found via
  ```
  $ python3 -c "import os; import sys; import platform; print(os.name, sys.platform, platform.system())"
  ```

  |     OS       | os.name | sys.platform | platform.system()  |
  |--------------|---------|--------------|--------------------|
  | Linux 6.2.0  |  posix  |   linux      |      Linux         |
  | MacOS*       |  posix  |   darwin     |      Darwin        |
  | OpenBSD 7.4  |  posix  |   openbsd7   |      OpenBSD       |
  | Windows*     |  nt     |   win32      |      Windows       |

  \* = I neither have a MacOS nor a Windows machine available, so I extracted the values from documentation and our current code. Also I'm relying on CI for testing the relevant code-paths. Having reviewers to this this locally would be very appreciated, if this gets Concept ACKed.

ACKs for top commit:
  kevkevinpal:
    ACK [878d914](878d914777)
  achow101:
    ACK 878d914777
  hebasto:
    ACK 878d914777, I have reviewed the code and it looks OK.
  pablomartin4btc:
    tACK 878d914777

Tree-SHA512: 24513d493e47f572028c843260b81c47c2c29bfb701991050255c9f9529cd19065ecbc7b3b6e15619da7f3f608b4825c345ce6fee30d8fd1eaadbd08cff400fc

test/functional/test_framework/p2p.py

@@ -24,6 +24,7 @@ import asyncio
 from collections import defaultdict
 from io import BytesIO
 import logging
+import platform
 import struct
 import sys
 import threading
@@ -592,7 +593,7 @@ class NetworkThread(threading.Thread):
 
         NetworkThread.listeners = {}
         NetworkThread.protos = {}
-        if sys.platform == 'win32':
+        if platform.system() == 'Windows':
             asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
         NetworkThread.network_event_loop = asyncio.new_event_loop()
 

test/functional/test_framework/test_node.py

@@ -12,6 +12,7 @@ import http.client
 import json
 import logging
 import os
+import platform
 import re
 import subprocess
 import tempfile
@@ -19,7 +20,6 @@ import time
 import urllib.parse
 import collections
 import shlex
-import sys
 from pathlib import Path
 
 from .authproxy import (
@@ -566,7 +566,7 @@ class TestNode():
                 cmd, shell=True,
                 stderr=subprocess.DEVNULL, stdout=subprocess.DEVNULL) == 0
 
-        if not sys.platform.startswith('linux'):
+        if platform.system() != 'Linux':
             self.log.warning("Can't profile with perf; only available on Linux platforms")
             return None
 

test/functional/test_framework/util.py

@@ -13,8 +13,8 @@ import json
 import logging
 import os
 import pathlib
+import platform
 import re
-import sys
 import time
 
 from . import coverage
@@ -414,12 +414,12 @@ def get_temp_default_datadir(temp_dir: pathlib.Path) -> tuple[dict, pathlib.Path
     """Return os-specific environment variables that can be set to make the
     GetDefaultDataDir() function return a datadir path under the provided
     temp_dir, as well as the complete path it would return."""
-    if sys.platform == "win32":
+    if platform.system() == "Windows":
         env = dict(APPDATA=str(temp_dir))
         datadir = temp_dir / "Bitcoin"
     else:
         env = dict(HOME=str(temp_dir))
-        if sys.platform == "darwin":
+        if platform.system() == "Darwin":
             datadir = temp_dir / "Library/Application Support/Bitcoin"
         else:
             datadir = temp_dir / ".bitcoin"