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

[G. David Butler gdb@dbSystems.com]

I'm sure many of you wish this topic would go away, but...

As you can guess I have made the change I have been advocating in
this thread and have found an interesting race that affects everybody.

In /sys/src/cmd/rc/plan9.c in the function Execute the statement

Updenv();

Is in a race with the same call in /sys/src/cmd/rc/simple.c in
the function Xsimple().

During the fork and subsequent exec, both the parent and child
are writing the environment with sometimes spectacular results.

With the new create(2), I saw periodic rc: can't open #e/blah since
Create() in /sys/src/cmd/rc/plan9.c now has open(OWRITE|OTRUNC)
and only if that fails create(OWRITE).  If I add the additional
open(OWRITE|OTRUNC) call to emulate the old create(2) call, the
race stays hidden.

I took out the Updenv() in Execute to follow /sys/src/cmd/rc/unix.c
and because it makes sense.

Also, if anybody is interested, the OTRUNC handling in
/sys/src/9/port/devenv.c needs to include things other than OWRITE.

David Butler
gdb@dbSystems.com