NFS problems: ed(1) doesn't work on NFS mounted filesystem!?!?

[Dave Burton]

Disclaimer: ** This is not an official reply from ISC. **

In article <618 at hades.OZ> greyham at hades.OZ (Greyham Stoney) writes:
| SYMPTOM:
|	when using ed(1) on a file that is on a filesystem that is NFS mounted
|	from a remote host, every second 'w' (write) fails with a '?',
|	starting with the first. every other 'w' succeeds. Bizarre!
|
| SYSTEM:
|	386/ix 2.0.2 with NFS over TCP/IP.
| ...
| Anyone got any clues on why this happens and perhaps a fix?.

In article <3974 at labtam.oz> timr at labtam.oz (Tim Roper) replies:
: I think this is because ed has a "feaure" whereby it refuses to attempt to
: write to a file system that doesn't have enough space left.
: Unfortunately it thinks blocks are always 512 bytes so it can make the
: wrong decision for file systems with bigger blocks.  Maybe NFS filesystems
: are the ones you have with bigger blocks but not too many of them.
:
: Every second 'w' works because typing 'w' again is the ed way of saying
: you really mean it (as for writes to read-only files).

The version of ed distributed from AT&T does not have this "feature". In
the case of a read-only file, successive 'w' requests with fail with '?'.

The toggling behavior is due to the coding in ed(1), which does (essentially):

	static int toggle = 0;
	...
	ustat(st_dev of file to write, &struct ustat);
	if (!toggle && ustat.numblks_on_fs < (numblks for file + fudge)) {
		toggle = 1;
		report an error and restart input;
	}
	toggle = 0;

Every other pass through this code, the write will fail - assuming
numblks.fs < (numblks.file+fudge), which it is, because:

SVR3.2 uses the device number of a mounted filesystem to identify
non-local filesystems, e.g. those with st_dev < 0.  NFS reports a
negative st_dev to conform with this expectation.  ustat(2) (sic)
detects a negative st_dev and calls the RFS dependant code directly,
without going through the FSS.  The result is improper behavior for NFS
filesystems; i.e. calls to ustat(2) for NFS filesystems go through the
RFS code (please note the caveat at the bottom of the ustat(2) man
page).  Not surprisingly, ustat(2) fails for NFS (with ENOMEM),
although ed(1) doesn't check the return status.

One fix is to replace the ustat call with statfs(2), which uses the
FSS.  The workaround is to use ex(1) or vi(1) :-).
--
Dave Burton
uunet!ism780c!laidbak!daveb