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opening /fd, /srv and /shr
The OCEXEC flag used to be maintained per channel,
making it shared between all the file desciptors.
This has a unexpected side effects with regard to
channel passing drivers such as devdup (/fd),
devsrv (/srv) and devshr (/shr).
For example, opening a /srv file with OCEXEC
makes it impossible to be remounted by exportfs
as it internally does a exec() to mount and
re-export it. There is no way to reset the flag.
This change makes the OCEXEC flag per file descriptor,
so a open with the OCEXEC flag only affects the fd
group of the calling process, and not the channel
itself.
On rfork(RFFDG), the per file descriptor flags get
copied.
On dup(), the per file descriptor flags are reset.
The second modification is that /fd, /srv and /shr
should reject the ORCLOSE flag, as the files that
are returned have already been opend.
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devdir internally replicates the qid in ther perm stat field
already and the practice of explicitely passing just causing
confusion when done inconsistently.
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to allow bytewise access to /proc/#/fd, the contents of the file where
recreated on each call. if fd's had been closed or reassigned between
the reads, the offset would be inconsistent and a read could start off
in the middle of a line. this happens when you cat /proc/#/fd file of
a busy process that mutates its filedescriptor table.
to fix this, we now return one line record at a time. if the line
fits in the read size, then this means the next read will always start
at the beginning of the next line record. we remember the consumed
byte count in Chan.mrock and the current record in Chan.nrock. (these
fields are free to usefor non-directory files)
if a read comes in and the offset is the same as c->mrock, we do not
need to regenerate the file and just render the next c->nrock's record.
for reads smaller than the line count, we have to regenerate the content
up to the offset and the race is still possible, but this should not
be the common case.
the same algorithm is now used for /proc/#/ns file, allowing a simpler
reimplementation and getting rid of Mntwalk state strcture.
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