Trying to run a gocryptfs encrypted file system mount within a rclone mount remote file system itself accessing the remote host over SFTP, I ran into an annoying issue that writes would fail with a “Operation not permitted” error, but the file in question would appear within the gocryptfs file system (so clearly something was working).
Slothing through the output of running rclone mount with -vv but without --daemon to try to find clues as to what was actually going on, I came across these two lines:
yyyy/mm/dd hh:mm:ss ERROR : <encrypted filename>: WriteFileHandle: ReadAt: Can't read and write to file without --vfs-cache-mode >= minimal
yyyy/mm/dd hh:mm:ss DEBUG : &{<encrypted filename> (w)}: >Read: read=0, err=operation not permitted
Well, there’s the error being returned, and the cause.
Per the documentation, rclone mount --vfs-cache-mode takes one of off, minimal, writes, full; and the default, lo and behold, is off, which is clearly less than minimal.
Adding a --vfs-cache-mode minimal to the end of the rclone mount command seems to have fixed it, insofar as the error is gone and writes appear to go through fully as intended.