Re: [PATCH v2 0/2] Brown-bag fix on top of js/mingw-inherit-only-std-handles

From: Johannes Sixt <j6t@kdbg.org>
Date: Nov 30, 2019, 19:13 UTC
Message-ID: <6beaf25f-1369-3e77-fd54-fdb5a1e63707@kdbg.org>
In-Reply-To: <xmqq5zj12qc2.fsf@gitster-ct.c.googlers.com>

Am 30.11.19 um 19:04 schrieb Junio C Hamano:

Show 8 quoted lines

> "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
> writes:
> 
>>  * We now assign errno only when the call to CreateProcessW() failed.
> 
> Meaning the global variable 'errno' is left as it was (instead of
> getting cleared) when a system call succeeds?  That I think is the
> correct behaviour people who use the variable expect.

I hope you mean people who read the code. You cannot possibly mean developers who expect that the run-command API keeps errno unchanged if the calls were successful. I'm pretty sure they do not provide such a guarantee.

Show 14 quoted lines

> 
>>  * For good measure, we teach the err_win_to_posix() function to translate 
>>    ERROR_SUCCESS into the errno value 0.
> 
> So, I am not sure if this is a good idea---who are the callers of
> this function and why do they call it?  I would imagine that a
> caller who makes a system call, upon seeing a failure from the
> system call, calls this helper with the Windows error code it
> received from the system call so that errno can be updated with a
> POSIXy value.  If my imagination is correct, such a caller should
> not be assigning anything to errno if the underlying system call
> succeeds, i.e. returns ERROR_SUCCESS.  So a better solution might
> be for the function to BUG() when fed ERROR_SUCCESS to point fingers
> at the caller, no?

Just like on POSIX the value of errno is indeterminate after a successful system call, the value of GetLastError() indeterminate after a successful Windows API call. Therefore, the err_win_to_posix() would not be able to point at a bogus caller reliably. For this reason, let's consider the function as a simple error code translator, and then translating ERROR_SUCCESS to 0 (or is there ESUCCESS?) makes total sense.

-- Hannes