Re: ERANGE strikes again on my Windows build; RFH

[Johannes Sixt <j6t@kdbg.org>]

Am 29.12.19 um 18:25 schrieb Alban Gruin:
> 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.

I am not worried about errno being set (or to what value) when there
actually is an error. I am asking what to do when there is actually *no*
error. In my vsnprintf emulation, the case where ERANGE happens is *not*
an error as far as the emulation is concerned.

What if in the huge call graph behind read_object() some function
changes errno to, say, EEXIST, EISDIR, or ENODIR and the condition under
which this happens is *not* an error in that context? Is the function
required to restore the original errno?

Consider the task to create file "foo/bar.c". We would have to
mkdir("foo"), but it is *not* an error when mkdir() fails with errno ==
EEXIST. Are we required to reset errno back to its old value?

(I know, read_object() is unlikely to allocate files, but I think I have
to explain in some way that the context may define that there is no
error -- even though a lower-level function failed and modified errno.)

-- Hannes