Use a shared file-system lock in `create_server` (#11294)

With parallel run of tests, one gets "Address already in use" errors
as all tests attempt to bind to the same port. Fix it with a shared
file-system lock.

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

pyproject.toml

@@ -93,6 +93,7 @@ test = [
     "pytest>=4.6",
     "html5lib",
     "cython",
+    "filelock"
 ]
 
 [[project.authors]]

tests/test_build_linkcheck.py

@@ -99,6 +99,7 @@ def test_defaults(app):
 
 @pytest.mark.sphinx('linkcheck', testroot='linkcheck-too-many-retries', freshenv=True)
 def test_too_many_retries(app):
+    with http_server(DefaultsHandler):
         app.build()
 
     # Text output
@@ -688,6 +689,7 @@ def test_get_after_head_raises_connection_error(app):
 
 @pytest.mark.sphinx('linkcheck', testroot='linkcheck-documents_exclude', freshenv=True)
 def test_linkcheck_exclude_documents(app):
+    with http_server(DefaultsHandler):
         app.build()
 
     with open(app.outdir / 'output.json', encoding='utf-8') as fp:

tests/utils.py

@@ -4,10 +4,16 @@ import pathlib
 import threading
 from ssl import PROTOCOL_TLS_SERVER, SSLContext
 
+import filelock
+
 # Generated with:
 # $ openssl req -new -x509 -days 3650 -nodes -out cert.pem \
 #     -keyout cert.pem -addext "subjectAltName = DNS:localhost"
-CERT_FILE = str(pathlib.Path(__file__).parent / "certs" / "cert.pem")
+TESTS_ROOT = pathlib.Path(__file__).parent
+CERT_FILE = str(TESTS_ROOT / "certs" / "cert.pem")
+
+# File lock for tests
+LOCK_PATH = str(TESTS_ROOT / 'test-server.lock')
 
 
 class HttpServerThread(threading.Thread):
@@ -34,6 +40,8 @@ class HttpsServerThread(HttpServerThread):
 
 def create_server(thread_class):
     def server(handler):
+        lock = filelock.FileLock(LOCK_PATH)
+        with lock:
             server_thread = thread_class(handler, daemon=True)
             server_thread.start()
             try: