Re: [PATCH 2/2] mingw: fix mingw_open_append to work with named pipes

[Jeff Hostetler <git@jeffhostetler.com>]

On 9/8/2018 2:31 PM, Johannes Sixt wrote:
> Am 08.09.2018 um 11:26 schrieb Johannes Sixt:
>> Am 07.09.2018 um 20:19 schrieb Jeff Hostetler via GitGitGadget:
>>> diff --git a/compat/mingw.c b/compat/mingw.c
>>> index 858ca14a57..ef03bbe5d2 100644
>>> --- a/compat/mingw.c
>>> +++ b/compat/mingw.c
>>> @@ -355,7 +355,7 @@ static int mingw_open_append(wchar_t const 
>>> *wfilename, int oflags, ...)
>>>        * FILE_SHARE_WRITE is required to permit child processes
>>>        * to append to the file.
>>>        */
>>> -    handle = CreateFileW(wfilename, FILE_APPEND_DATA,
>>> +    handle = CreateFileW(wfilename, FILE_WRITE_DATA | FILE_APPEND_DATA,
>>>               FILE_SHARE_WRITE | FILE_SHARE_READ,
>>>               NULL, create, FILE_ATTRIBUTE_NORMAL, NULL);
>>>       if (handle == INVALID_HANDLE_VALUE)
>>>
>>
>> I did not go with this version because the documentation 
>> https://docs.microsoft.com/en-us/windows/desktop/fileio/file-access-rights-constants 
>> says:
>>
>> FILE_APPEND_DATA: For a file object, the right to append data to the 
>> file. (For local files, write operations will not overwrite existing 
>> data if this flag is specified without FILE_WRITE_DATA.) [...]
>>
>> which could be interpreted as: Only if FILE_WRITE_DATA is not set, we 
>> have the guarantee that existing data in local files is not 
>> overwritten, i.e., new data is appended atomically.
>>
>> Is this interpretation too narrow and we do get atomicity even when 
>> FILE_WRITE_DATA is set?
> 
> Here is are some comments on stackoverflow which let me think that 
> FILE_APPEND_DATA with FILE_WRITE_DATA is no longer atomic:
> 
> https://stackoverflow.com/questions/20093571/difference-between-file-write-data-and-file-append-data#comment29995346_20108249 
> 
> 
> -- Hannes

Yeah, this whole thing is a little under-documented for my tastes.
Let's leave it as you have it.  I'll re-roll with a fix to route
named pipes to the existing _wopen() code.

thanks
Jeff