RE: some concern about the fix of " tail: consistently output all data for truncated files"

[Lian, George (Nokia - CN/Hangzhou)]

Hi,

Add one more suggestion, if we have not a perfect solution to consider all the 
case of truncate, could we add an option to tail, such like tail -no-truncate
If tail run with this option, than application not consider any truncate case.

For example, I suppose syslog output file will not have any truncate case in 
our environment, then the tail could use the option to avoid the mis-truncated 
case?

Thanks & Best Regards,
George

_____________________________________________
From: Lian, George (Nokia - CN/Hangzhou)
Sent: Monday, November 07, 2016 1:17 PM
To: 'address@hidden' <address@hidden>
Cc: Zizka, Jan (Nokia - CZ/Prague) <address@hidden>; Zhang, Bingxuan (Nokia - 
CN/Hangzhou) <address@hidden>; Li, Deqian (Nokia - CN/Hangzhou) 
<address@hidden>; Bao, Xiaohui (Nokia - CN/Hangzhou) <address@hidden>
Subject: some concern about the fix of " tail: consistently output all data for 
truncated files"


Hi,

There are a fix about truncated for tail application of coreutils,

The change messages is "tail: consistently output all data for truncated files",

Now I have some concern for this case

1)      I suppose Truncate maybe not always truncate to ZERO, for example, 
sometimes a file maybe only has been truncate the bottom 100 lines of 1M lines 
, in this case , SEEK from head will be confused by the users.
2)      Sometimes truncated maybe not really happen even "stats.st_size < 
fspec->size" is meet,  for example, a network based filesystems, for it's 
current implementation, it will active the read data-cache first and then 
update the meta-data cache, it is not an atomic operation, I suppose it is 
reasonable that the "tail application" will read the data-cache with new 
updated data, but get stat size with old value, in this case, the tail assume 
it is truncated and dump the data from the begin, it also make more confuse to 
users.
3)      As the comments in your changes, the stat size is bigger thand the 
length has read, it also maybe happ truncate.
4)      Why can't get size of fstat first, and read not more than the size of 
fstat during the tail_forever loop?

Could you please share your comments for the above concerns?

Thanks & Best Regards,
George