fix(auth): serialise the token refresh across processes

The login cache is shared by every mdrs process, but the refresh was
guarded by a lock that only reaches inside one. Concurrent runs each
sent the same refresh token, and a provider that rotates them accepts
the first and refuses the rest.

- hold a lock that spans processes across the whole read-refresh-
  write, checking cheaply first so ordinary requests never take it
- write the cache through a temporary file: opening it for writing
  truncates it, and a reader landing in that window found it empty
  and threw the session away
- take the lock for every write, not just the refresh, so a login
  running beside one cannot be silently reverted
- wait for a busy lock on Windows rather than giving up after the ten
  attempts msvcrt allows, and retry the rename it refuses while a
  reader still holds the file open
- bound the refresh request on its own, so a provider that goes quiet
  cannot hold the lock indefinitely
- accept bare hostnames such as localhost, store URLs without the
  trailing slash, and join download paths through one helper

mdrsclient/utils.py

@@ -1,5 +1,6 @@
 import os
-from typing import IO, Any
+import time
+from typing import IO, Any, Final
 from urllib.parse import parse_qs, urlparse
 
 if os.name == "nt":
@@ -9,10 +10,23 @@ elif os.name == "posix":
 
 
 class FileLock:
+    # Long enough to outlast a token refresh, which is what the lock is held across.
+    WAIT_SECONDS: Final[float] = 60.0
+
     @staticmethod
     def lock(file: IO[Any]) -> None:
         if os.name == "nt":
-            msvcrt.locking(file.fileno(), msvcrt.LK_LOCK, 1)
+            # msvcrt.LK_LOCK gives up after ten one-second attempts, which is shorter
+            # than the refresh it now has to wait for, so do the waiting here instead.
+            deadline = time.monotonic() + FileLock.WAIT_SECONDS
+            while True:
+                try:
+                    msvcrt.locking(file.fileno(), msvcrt.LK_NBLCK, 1)
+                    return
+                except OSError:
+                    if time.monotonic() >= deadline:
+                        raise
+                    time.sleep(0.1)
         elif os.name == "posix":
             fcntl.flock(file.fileno(), fcntl.LOCK_EX)