fix: close task5 state and capability blockers

harness/tht/workspace_writer_lock.py

@@ -11,8 +11,6 @@ import fcntl
 import os
 import re
 import stat
-import struct
-import sys
 from dataclasses import dataclass
 
 
@@ -78,24 +76,11 @@ def verify_workspace_writer_fds(*, writer_fd: int = 3, root_fd: int = 4, env: di
         lock = _fstat(lock_fd)
         if (lock.st_dev, lock.st_ino) != (writer.st_dev, writer.st_ino) or not stat.S_ISREG(lock.st_mode) or lock.st_uid != uid or (lock.st_mode & 0o777) != 0o600 or lock.st_nlink != 1:
             raise WorkspaceWriterConflict()
-        # Probe using a *different* open file description.  An inherited FD 3
-        # is valid only when its lock is already held: the independent probe
-        # must therefore receive EWOULDBLOCK.  We deliberately never flock(3)
-        # here: doing so would turn an unheld, independently opened descriptor
-        # into an apparently valid capability.
+        # First prove that an independently opened description cannot acquire the
+        # lock.  Then probe the inherited description itself.  flock is an
+        # open-file-description lock: the second call succeeds only on the same
+        # description held by the backend and does not release it.
         try:
-            # Linux OFD locks identify the open file description rather than
-            # the process.  The inherited FD 3 must already own this lock;
-            # an independent unlocked description can acquire the probe and
-            # is rejected.  Darwin has no OFD constants, so retain flock's
-            # equivalent open-description probe there.
-            ofd_setlk = getattr(fcntl, "F_OFD_SETLK", None) if sys.platform.startswith("linux") else None
-            if ofd_setlk is not None:
-                lock_record = struct.pack("hhqqi", fcntl.F_WRLCK, os.SEEK_SET, 0, 0, 0)
-                fcntl.fcntl(lock_fd, ofd_setlk, lock_record)
-                unlock_record = struct.pack("hhqqi", fcntl.F_UNLCK, os.SEEK_SET, 0, 0, 0)
-                fcntl.fcntl(lock_fd, ofd_setlk, unlock_record)
-                raise WorkspaceWriterConflict()
             fcntl.flock(lock_fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
         except OSError as exc:
             if exc.errno not in (errno.EACCES, errno.EAGAIN, errno.EWOULDBLOCK):
@@ -106,6 +91,10 @@ def verify_workspace_writer_fds(*, writer_fd: int = 3, root_fd: int = 4, env: di
             except OSError:
                 pass
             raise WorkspaceWriterConflict()
+        try:
+            fcntl.flock(writer_fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+        except OSError as exc:
+            raise WorkspaceWriterConflict() from exc
     finally:
         try:
             os.close(lock_fd)