Re: [HACKERS] Fix mdsync never-ending loop problem

I wrote:
> Actually, on second look I think the key idea here is Takahiro-san's
> introduction of a cancellation flag in the hashtable entries, to
> replace the cases where AbsorbFsyncRequests can try to delete entries.
> What that means is mdsync() doesn't need an outer retry loop at all:

I fooled around with this idea and came up with the attached patch.
It seems to do what's intended but could do with more eyeballs and
testing before committing. Comments please?

(Note: I ignored my own advice not to reindent. Sorry ...)

regards, tom lane