[Flightgear-devel] Windows nav-cache permission issue

[James T. <zak...@ma...>]

Hi,

Had a bit of insight on the commonly reported failures on the forum about the Nav-cache being read-only. It looks as if the whole nav-cache thing may have been completely mis-leading - the problem is nothing to do with the nav-cache at all, it’s unfortunate that the error mentioned it, since that has rather distracted from finding the solution. (All of the above assuming we have found the solution, which is still uncertain but looking quite likely).

The actual problem seems to be the fgfs.pid file we use to ensure only one copy of FGFS is running with write-access to FG_HOME - all other copies run with read-only access. On POSIX I use standard open+unlink behaviour to create the PID file but ensure we don’t ever get stale ones, even if the system crashes. I then used Google / StackOverflow to find equivalent code for Windows - CreateFile with a ‘delete file on close’ flag. However, it looks as if this behaviour may be fragile in the case of non-clean exits / power-offs. I’ve just pushed a patch to next to change to a named-mutex-based solution on Windows (POSIX is unchanged), which can’t ever get stuck with stale data, since there is no file at all.

The reason I think this may be the culprit:

 - the affected FG_HOMEs I’ve been sent contain a fgfs.pid
 - none of the error output that would indicate a problem opening the cache file is produced, suggesting the Sqlite startup is running on its normal codepath, not some error path
 - in the case of a read-only home, we *don’t* log to fgfs.log (so once there is a bad fgfs.pid, we don’t see any useful logging at all)

This still doesn’t entirely make sense, because for example while developing on Windows I kill the application all the time while developing and don’t see a stale fgfs.pid : so the file-based approach must work some of the time. But switching to the named mutex seems like a pragmatic solution, given the amount of pain this issue causes.

My intention is to let this bake on the nightlies for a few days, and once it’s confirmed as working there, merge to the release branch. 

Did I mention I hate Windows? :)

Kind regards,
James