Re: [PATCH] copy.c: make copy_fd preserve meaningful errno

>>> Update copy_fd to return a meaningful errno on failure and also
>>> preserve the existing errno variable.
>>
>> Some functions in git make errno meaningful on error and others don't.
>> In general, the more we only use errno immediately after a system
>> call, the better, so based on the above description this seems like a
>> step in the wrong direction.
>
> I did reword the commit message badly. Before it just read
> "Update copy_fd to return a meaningful errno".
>
> In fact the proposed patch doesn't guarantee the errno, which is set
> at the beginning of the function to be the same at the end.
>
> What it really should preserve is the errno from xread, while
> evaluating error("copy-fd: read returned %s", strerror(errno));
> So the function call of error(...) or strerror(...) may change the errno.

>> Can we do better?  E.g., if the signature were
>>
>>         int copy_fd(int ifd, int ofd, struct strbuf *err);
>>
>> then we could write the error message to the err strbuf for the
>> caller to print.  The error handling would be more explicit so
>> there would be no need to protect errno from clobbering by other
>> system calls (both here and in callers).
>>
>> Something like this:
>
> I like this approach, though your patch is not about the original
> intention as this one
> (having strerror(...) not messing with the errno), but rather
> accumulating the errors not
> in numbers but string buffers?