Re: ERANGE strikes again on my Windows build; RFH

[Alban Gruin <alban.gruin@gmail.com>]

Hi,

Le 29/12/2019 à 15:29, Torsten Bögershausen a écrit :
> On Sat, Dec 28, 2019 at 04:41:42PM +0100, Johannes Sixt wrote:
>> In sha1-file.c:read_object_file_extended() we have the following pattern:
>>
>> 	errno = 0;
>> 	data = read_object(r, repl, type, size);
>> 	if (data)
>> 		return data;
>>
>> 	if (errno && errno != ENOENT)
>> 		die_errno(_("failed to read object %s"), oid_to_hex(oid));
>>
>> That is, it is expected that read_object() does not change the value of
>> errno in the non-error case. I find it intriguing that we expect a quite
>> large call graph that is behind read_object() to behave this way.
>>
>> What if a subordinate callee starts doing
>>
>> 	if (some_syscall(...) < 0) {
>> 		if (errno == EEXIST) {
>> 			/* never mind, that's OK */
>> 			...
>> 		}
>> 	}
>>
>> Would it be required to reset errno to its previous value in this
>> failure-is-not-an-error case?
>>
>> The problem in my Windows build is that one of these subordinate
>> syscalls is vsnprintf() (as part of a strbuf_add variant, I presume),
>> and it fails with ERANGE when the buffer is too short. Do I have to
>> modify the vsnprintf emulation to restore errno?
> 
> If you ask me: I think so, yes.
> At least the documentation about vsnprintf does not mention that errno is touched at all.
> That is the man pages for Linux and Mac OS, or see here:
> https://linux.die.net/man/3/vsnprintf
> 
> It would make sense to analyze the complete callstack, I think.
> Is your problem reproducable ?
> 
> Changing the function strbuf_vaddf() strbuf.c seems to be straight forward to me.
> 

According to the standard, vsnprintf() _can_ change errno[1] (and the
BSDs do so[2][3][4].)  But apparently, not to ERANGE.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/vfprintf.html
[2]
https://www.freebsd.org/cgi/man.cgi?query=vsnprintf&manpath=FreeBSD+12.1-RELEASE
[3] https://www.freebsd.org/cgi/man.cgi?query=vsnprintf&manpath=NetBSD+8.1
[4] https://man.openbsd.org/vsnprintf

Cheers,
Alban