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

[G. David Butler gdb@dbSystems.com]

>From: "G. David Butler" <gdb@dbSystems.com>

>The current create(2) call, IMHO, is broken.  When the call was
>implemented, it introduced a race condition that was not necessary
>and can't be easily avoided.  Since Plan9 is a "research" system,
>I don't consider the API set in stone and was trying to open a
>discussion on the subject.  I offered two solutions, an option
>to open (POSIX O_EXCL) or a fix to create(2).  There is a third

I have found some interesting information while considering
the change of create(2).

Look at the code in /sys/src/cmd/exportfs/exportsrv.c and
/sys/src/cmd/iostats/statsrv.c for the comment with the word
"race" in it.  It seems that any user space "file server"
laying over a real file server can't transfer the Tcreate
correctly.  Big surprise!

Another comment in /sys/src/cmd/samterm/plan9.c about an
"existing guy" is interesting in that it assumes that create(2)
fails if the file exists!

It doesn't take much to make the change in the kernel and a simple
change in /sys/src/cmd/rc/plan9.c will let the system boot.  Beyond
that the changes are mechanical and I have listed most of them below.
Overall, the change of create(2) is not very hard.

The actual change in the cpu/terminal kernel is:

delete from the comment in namec() in /sys/src/9/port/chan.c near
line 582
	/*
	* Walk the element
to near line 600
	poperror();

delete from the comment near line 610
	/*
	* protect against the open/create race.
to near line 620
	}

delete the line near line 625
	poperror();

also remove the local variables cc and createrr and the
label Open: to keep the compile warning free.

In the following list, if there is no comment then the change is
create becomes open/create/open as previously discussed.

/sys/man/2
open	documentation change

/sys/man/5
open	documentation change

/sys/src/9/boot
aux.c

/sys/src/alef/8
output.c

/sys/src/alef/k
output.c

/sys/src/alef/lib/libauth
newns.l

/sys/src/alef/lib/libbio
binit.l

/sys/src/alef/lib/libg
binit.l	create should be open OTRUNC

/sys/src/alef/lib/p9
putenv.l

/sys/src/alef/test
test16.l

/sys/src/alef/test/Y
files.l

/sys/src/alef/v
output.c

/sys/src/ape/9src
ptyfs.c	create there is correct to keep two from running

/sys/src/ape/lib/ap/plan9
execve.c	fix _CREATE
mkdir.c	access is race, but _CREATE is correct
open.c	access is race, but _CREATE is correct
rename.c	fix _CREATE
tmpfile.c	fix _CREATE

/sys/src/cmd
ar.c
char.c
cp.c
dd.c
ed.c
fortune.c
init.c
mkdir.c	access is race but create is correct
mv.c
news.c
ramfs.c	create there is correct to keep two from running
sed.c	create there is correct
sh.C
sort.c	to avoid races, some create become open OTRUNC
split.c
srv.c		access is race but create is correct
srvfs.c	create there is correct to keep two from running
strip.c
swap.c	fix create on env but swapfd create is correct
tar.c
tee.c
touch.c
tweak.c
½char.c

/sys/src/cmd/2l
obj.c

/sys/src/cmd/6l
obj.c

/sys/src/cmd/8l
obj.c

/sys/src/cmd/8½
main.c	fix create on env but srv create is correct

/sys/src/cmd/9660srv
main.c	create there is correct to keep two from running

/sys/src/cmd/acid
builtin.c	already does the create/open trick

/sys/src/cmd/acme
disk.l	stat is race but create is correct
exec.l
rows.l
util.l

/sys/src/cmd/art
fileio.c

/sys/src/cmd/auth
adduser.c		create is correct
changeuser.c	create is correct
cron.c		create is correct

/sys/src/cmd/aux
consolefs.l	create there is correct to keep two from running
depend.l		create there is correct to keep two from running

/sys/src/cmd/aux/icmp
icmp.c		create there is correct to keep two from running

/sys/src/cmd/cc
compat.c

/sys/src/cmd/chdb
sub.c

/sys/src/cmd/chdb/cdb
cdb.c

/sys/src/cmd/con
xmr.c

/sys/src/cmd/cpp
nlist.c

/sys/src/cmd/db
output.c	create ok
setup.c	create ok
trcrun.c

/sys/src/cmd/diff
main.c	create ok

/sys/src/cmd/disk
format.c
mkext.c	create ok
mkfs.c	create ok

/sys/src/cmd/disk/kfs
main.c	create there is correct to keep two from running

/sys/src/cmd/disk/pip
disk.c

/sys/src/cmd/dossrv
xfssrv.c	create there is correct to keep two from running

/sys/src/cmd/exportfs
exportsrv.c	create now will work like it should! see race comment

/sys/src/cmd/fax
file.c		create is correct

/sys/src/cmd/fone
plan9.c	create there is correct to keep two from running

/sys/src/cmd/ftpfs
file.c		stat is race but create is correct

/sys/src/cmd/hp
hp.c

/sys/src/cmd/hp/hp-vt
main.c

/sys/src/cmd/iostats
iostats.c	create there maybe correct if two run in debug mode
statsrv.c	create now will work like it should! see race comment

/sys/src/cmd/ip
tftpd.c

/sys/src/cmd/kl
obj.c

/sys/src/cmd/lex
sub1.c

/sys/src/cmd/lp
LOCK.c	create now will work like it should!
lpdaemon.c	create ok
lpsend.c	create ok

/sys/src/cmd/mk
main.c	create ok
plan9.c
t_ar.c	create ok
t_file.c	create ok

/sys/src/cmd/mothra
gopher2html.c	create ok
http.c	create ok
mothra.c

/sys/src/cmd/ndb
cs.c	create there is correct to keep two from running
dns.c	create there is correct to keep two from running
mkhash.c
mkhosts.c

/sys/src/cmd/plot
plot.c	create there is correct to keep two from running

/sys/src/cmd/postscript/postio
postio.l	create ok

/sys/src/cmd/postscript/tcpostio
tcpostio.l	create ok

/sys/src/cmd/rc
plan9.c

/sys/src/cmd/rschar
rschar.c

/sys/src/cmd/sam
io.c
mesg.c
plan9.c	create ok
sam.c
shell.c

/sys/src/cmd/samterm
plan9.c	one create ok (see comment) other open OTRUNC

/sys/src/cmd/scuzz
scuzz.c

/sys/src/cmd/service
ftp.c		two creates need changing, not all

/sys/src/cmd/service/nfs
chat.c	create there is correct to keep two from running

/sys/src/cmd/spin
pangen1.h	create ok

/sys/src/cmd/telco
telco.c	create there is correct to keep two from running

/sys/src/cmd/upas/common
libsys.c	fix create in syscreate

/sys/src/cmd/upas/q
qer.c		look at more
runq.c	fix create on /dev/user

/sys/src/cmd/vl
obj.c

/sys/src/cmd/xl
obj.c

/sys/src/fb
cvt2pic.c	create ok

/sys/src/games/gps
gps55.c	create ok

/sys/src/games/plumb
pced.c

/sys/src/games/smiley
mkfont.c

/sys/src/libauth
newns.c

/sys/src/libbio
binit.c

/sys/src/libc/9sys
putenv.c

/sys/src/libc/port
profile.c

/sys/src/libfb
picopen_w.c

/sys/src/libg
binit.c

/sys/src/libstdio
freopen.c

David Butler
gdb@dbSystems.com