[Toybox] tac issues and tests

[Elie De Brauwer]

Hi Rob and all,

I've been playing around with 'tac'. And as a result you can find two 
patches in attach. The first one removes some code from tac for that 
simple reason that the tac toy contains some logic (including a strlen) 
to remove a trailing \n (if present). But this \n is the seperator which 
get_line() internally passes to get_rawline() and that newline is 
already removed in get_line().

Next to this you can find some test for tac which are based upon the cat 
tests. I have however found two problems I'd like to throw in the group.

Problem 1:
On my system:
edb at lapedb:~/today$ echo -ne "abc\ndef" | tac
defabc\n
tac in toybox:
edb at lapedb:~/edb-stuff/toybox/toybox$ echo -ne "abc\ndef" | ./toybox tac
def\n
abc\n
(I added the \n here for visibility). This however cannot be trivially 
fixed because of the implicit separator in get_line() as stated above. 
Nevertheless the output of the 'default' tac is such a corner case that 
probably no sane human being would use it that way (I hope).

Problem 2:
cat has a test which writes to /dev/full to see if the error gets 
caught. cat however doesn't use xwrite but uses xputs to put its output 
(also a cause for problem 1). But take a look at the following strace 
output of

echo "boe" | strace ./toybox tac > /dev/full

read(0, "b", 1)                         = 1
read(0, "o", 1)                         = 1
read(0, "e", 1)                         = 1
read(0, "\n", 1)                        = 1
read(0, "", 1)                          = 0
fstat64(1, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 7), ...}) = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbfb6ae50) = -1 ENOTTY 
(Inappropriate ioctl for device)
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 
0) = 0xb773d000
write(1, "boe\n", 4)                    = -1 ENOSPC (No space left on 
device)
exit_group(0)                           = ?

True, xputs uses puts which ends up calling write() which returns -1 but 
this does not ripple through to puts() and thus the error condition in 
xputs() doesn't trigger (it just happily return a '4').

So in order to fix the issues above I would make 'tac' more like 'cat', 
which would probably mean not using get_line() but using get_rawline() 
(and thus reintroducing what I threw out in my patch ;-) ), not removing 
the newline and using xwrite() (eventually storing the length somewhere) 
to write the output. This would still not solve the root problem of 
xputs() not triggering when write throws and ENOSPC at you.

What do you guys think ?

(Btw, my patch containing the tests, has the /dev/full test commented 
out, and the abc\ndef case modeled as the current toybox behavior).

gr
E.



-- 
Elie De Brauwer

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