In Plan9 the create syscall fallback on a open(OTRUNC) if the
path provided already exists. This is actually a common requirement
as most programs (editors, cat...) simply requires that a file is
there and is empty, and doesn't care overwriting existing contents
(note that this is particularily sensible with something like fossil).
In Jehanne the application is responsible of actually handle this
"file exists" error but libc provides ocreate() to mimic the Plan9
behaviour. Note that ocreate introduce a subtle race too: the path
is walked several times if the file exists, thus it could misbehave
on concurrent namespace changes. However I guess this is not going to
happen often enough to care now.
NOTE we will probably address this rare race too, with a more drammatic change
to syscalls: a new walk() syscall that will provide an unopen fd.
Added wdir to devself and devproc:
- read '#0/wdir' to get the working directory of the calling process
NOTE that a read(fd, nil, -1) will return the negated length
of the working directory, just in case you want to
allocate the memory required
- read '/proc/n/wdir' to know the working directory of process n
(read(fd, nil, -1) still returns the negated length)
- write '#0/wdir' to change the working directory of the calling process
NOTE: no offset is allowed and the provided string must
be null terminated
- write '/proc/n/wdir' to change the working directory of process n
NOTE: no offset is allowed and the provided string must
be null terminated; moreover if another process change the working
directory change during the write, the current process will
receive an error.
In libc updated getwd() and chdir().
Also modified pwd to get advantage of the new file.
To test, run /arch/amd64/qa/kern/wdir.rc or simply try
% pwd
/usr/glenda
% echo -n /tmp > /proc/$pid/wdir
% pwd
/tmp
% cat '#0/wdir' && echo
/tmp
The expected use cases for wdir in devproc are rio and acme.
Also, note that we could theoretically remove the cd builtin
from rc and simply implement it as a rc function.
We don't do that to preserve rc portability to other OS.
