Read-only locks should close fd before opening for write. (#1906)

- Fixes bad file descriptor error in lock acquire, #1904
- Fix bug introduced in previous PR #1857
- Backported fix from soon-to-be merged fine-grained DB locking branch.

lib/spack/llnl/util/lock.py

@@ -69,6 +69,14 @@ def _lock(self, op, timeout):
         start_time = time.time()
         while (time.time() - start_time) < timeout:
             try:
+                # If this is already open read-only and we want to
+                # upgrade to an exclusive write lock, close first.
+                if self._fd is not None:
+                    flags = fcntl.fcntl(self._fd, fcntl.F_GETFL)
+                    if op == fcntl.LOCK_EX and flags | os.O_RDONLY:
+                        os.close(self._fd)
+                        self._fd = None
+
                 if self._fd is None:
                     mode = os.O_RDWR if op == fcntl.LOCK_EX else os.O_RDONLY
                     self._fd = os.open(self._file_path, mode)