Re: testsuite under wine

[Jacob Bachmeyer <jcb62281@gmail.com>]

NightStrike wrote:
> On Fri, Dec 23, 2022 at 11:00 PM Jacob Bachmeyer <jcb62281@gmail.com> wrote:
>   
>> NightStrike wrote:
>>     
>>> On Wed, Dec 21, 2022 at 11:37 PM Jacob Bachmeyer <jcb62281@gmail.com> wrote:
>>>       
>>>> [...]
>>> So at least we know for sure that this particular instance of extra
>>> characters is coming from Wine.  Maybe Wine can be smart enough to
>>> only translate \n into \r\n instead of translating \r\n into \r\r\n.
>>> Jacek / Eric, comments here?  I'm happy to try another patch, the
>>> first one was great.
>>>
>>>       
>> I doubt that Wine is doing that translation.  MinGW libc produces output
>> conformant to Windows conventions, so printf("\n") on a text handle
>> emits "\r\n", which Wine passes along.  POSIX convention is that "\n" is
>> translated to "\r\n" in the kernel terminal driver upon output, so the
>> kernel translates the "\n" in the "\r\n" into /another/ "\r\n", yielding
>> "\r\r\n" at the pty master end.  This is why DejaGnu testsuites must be
>> prepared to discard excess carriage returns.  The first CR came from
>> MinGW libc; the second CR came from the kernel terminal driver; the LF
>> was ultimately passed through.
>>     
>
> Jacek and I have been digging into this on IRC, and he's been very
> helpful in trying to get further, but we're still stuck.  We tried to
> be more introspective, inserting strace both as "strace script wine"
> and as "script strace wine".  We tried running just "wine a.exe"
> without any extra glue, and logging the raw SSH packets from putty.
> After many iterations on these and other tests, Jacek finally had the
> idea to try removing Windows entirely from the equation, and we ran
> with a purely unix program / compiler combination:
>
> #include <unistd.h>
>
> int main()
> {
>         write(1, "test\r\n", 6);
>         return 0;
> }
>
> (and also as "test\n", 5)
>
> In both versions, the following was observed:
>
> case 1) ./a.out | xxd
> case 2) script -c ./a.out out; xxd out
> case 3) enable putting logging, ./a.out
>
> In case 1, xxd showed no extra \r's.  In cases 2 and 3, there was an
> extra \r (either 0d 0d 0a for test\r\n, or 0d 0a for test\n).
>
> So, is it possible after all of this back and forth regarding mingw,
> wine, and others, that it's down to the write() system call that's
> inserting extra \r's?  Is this expected?
>   

"This is why DejaGnu testsuites must be prepared to discard excess 
carriage returns."

The write(2) system call inserts nothing and simply hands off the buffer 
to the relevant part of the kernel I/O subsystem.  (The kernel in POSIX 
is *not* a monolithic black box.)  When stdout for your test program is 
a pty slave, that relevant part is the kernel terminal driver.  The 
kernel terminal driver is converting "\n" to "\r\n" upon output to the 
associated port, since hardware terminals typically *do* require CRLF.  
The associated port in this case is virtual and part of the kernel pty 
subsystem, which presents octets written to that port to its associated 
pty master device.  The user-visible pty slave device acts just like a 
serial terminal, including all translations normally done for handling 
serial terminals.

A pty is conceptually a null-modem cable connected between two 
infinitely-fast serial ports on the same machine, although the slave 
will still report an actual baud rate if queried.  (Run "stty" with no 
arguments under script(1), an ssh session, or an X11 terminal emulator 
to see what a pty slave looks like on your machine.)

In your case 1, the pty subsystem is not used and output is collected 
over a pipe.  Using "./a.out > out; xxd out" would produce the same 
results.  In cases 2 and 3, there is a pty involved, either set up by 
script(1) or by sshd (assuming you meant "enable putty logging" in case 
3) that performs the standard terminal translations.  In all cases, 
strace(1) will show the exact string written to the pty slave device, 
which will not include any extra CRs because *those* *are* *inserted* 
*by* *the* *kernel* *terminal* *driver* as the data is transferred to 
the pty master device's read queue.

This insertion of carriage returns is expected and standardized behavior 
in POSIX and is the reason Unix could use bare LF as end-of-line even 
though hardware terminals always needed CRLF.  CP/M (and therefore 
MS-DOS which began its existence as a cheap CP/M knockoff) did not have 
this translation layer and instead dumped the complexity of a two-octet 
end-of-line sequence on user programs, leading to much confusion even 
today.  This is not a Wine issue, although the terminal escape sequences 
in your original issue *were* from Wine.  Note that the number of excess 
carriage returns that a DejaGnu testsuite must be prepared to discard is 
unspecified because running tests on remote targets may result in *any* 
*number* of CRs preceding each LF by the time the results reach the test 
driver machine in more complex testing lab environments.


-- Jacob