Use a random port in MockFTPServer. (#7402)

tests/mockserver/ftp.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import re
 import sys
 from argparse import ArgumentParser
 from pathlib import Path
@@ -15,20 +16,36 @@ from tests.utils import get_script_run_env
 
 
 class MockFTPServer:
-    """Creates an FTP server on port 2121 with a default passwordless user
+    """Creates an FTP server on a random port with a default passwordless user
     (anonymous) and a temporary root path that you can read from the
     :attr:`path` attribute."""
 
+    def __init__(self) -> None:
+        self.proc: Popen[str] | None = None
+        self.host: str = "127.0.0.1"
+        self.port: int | None = None
+        self.path: Path | None = None
+
     def __enter__(self):
         self.path = Path(mkdtemp())
         self.proc = Popen(
             [sys.executable, "-u", "-m", "tests.mockserver.ftp", "-d", str(self.path)],
             stderr=PIPE,
             env=get_script_run_env(),
+            text=True,
         )
         for line in self.proc.stderr:
-            if b"starting FTP server" in line:
+            if "starting FTP server" in line and (
+                m := re.search(r"starting FTP server on ([^ :]+):(\d+),", line)
+            ):
+                self.port = int(m.group(2))
                 break
+        else:
+            self.proc.kill()
+            self.proc.communicate()
+            raise RuntimeError(
+                "The FTP server failed to start or the output is unrecognized"
+            )
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):
@@ -37,12 +54,12 @@ class MockFTPServer:
         self.proc.communicate()
 
     def url(self, path):
-        return "ftp://127.0.0.1:2121/" + path
+        return f"ftp://{self.host}:{self.port}/{path}"
 
 
 def main() -> None:
     parser = ArgumentParser()
-    parser.add_argument("-d", "--directory")
+    parser.add_argument("-d", "--directory", required=True)
     args = parser.parse_args()
 
     authorizer = DummyAuthorizer()
@@ -50,7 +67,7 @@ def main() -> None:
     authorizer.add_anonymous(args.directory, perm=full_permissions)
     handler = FTPHandler
     handler.authorizer = authorizer
-    address = ("127.0.0.1", 2121)
+    address = ("127.0.0.1", 0)
     server = FTPServer(address, handler)
     server.serve_forever()