[9fans] create(2)/open(2) race for file creation

[G. David Butler gdb@dbSystems.com]

>From: "Dave Presotto" <presotto@plan9.bell-labs.com>
>This is why I got ken to introduce exclusive access files.  I
>use one as a lock for a directory/file/whatever.

I tried that with:

open(lock file in directory)
if open fail {
	create
	write
	close
}
close(lock file)

But the 9p overhead and the bottleneck created convinced me that
it was not a good idea.  The above becomes:

open(2)
	walk(5)
	open(5)
open(2)
	walk(5) [fails]
create(2)
	walk(5) [create(2) trying to determine to send the create(5)]
	create(5)
write(2)...
close(2)
	clunk(5)
close(2)
	clunk(5)

If we change create(2) to do create(5) then:

if create() success {
	write
	close
}

becomes:

create(2)
	create(5)
write(2)...
close(2)
	clunk(5)

Much better.

David Butler
gdb@dbSystems.com