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

From: Johannes Sixt <j6t@kdbg.org>
Date: Sep 8, 2018, 18:31 UTC
Message-ID: <f207bc28-a303-5d63-e9f4-da8e4d466bd5@kdbg.org>
In-Reply-To: <1c524f56-2021-a961-168f-e5c6d7914ec2@kdbg.org>

Am 08.09.2018 um 11:26 schrieb Johannes Sixt:

Show 31 quoted lines

> 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