Re: [Tarantool-patches] [PATCH 2/2] sql: update temporary file name format

[Leonid Vasiliev <lvasiliev@tarantool.org>]

Hi! Thank you for review.

On 10.12.2020 19:39, Sergey Ostanevich wrote:
> Thanks for the patch!
> 
> I tend to the 1st alternative, although the code using the name generated
> is hairy. I believe the same resolution as for the first part: if we’re
> in a rush - LGTM, better solution is desirable otherwise.

As I wrote in the commit message, I think using `O_TMPFILE` or
`tmpfile()` is the best solution to work with temporary files.
But we already have the code in which the filename is passed from one 
function to another, and some logic over all this. So, replacing the use 
of a named file with a real temporary file will lead to refactoring,
changing some logic and increasing the number of differences with
SQLite. This can lead to degradation and complication of the review.
Instead, I propose a simple one line change. Could you please describe
the reproduction of the probable problem step by step?

> Sergos
>   
> 
> 
>> On 8 Dec 2020, at 22:59, Leonid Vasiliev <lvasiliev@tarantool.org> wrote:
>>
>> The bug was consisted in fail when working with temporary files
>> created by VDBE to sort large result of a `SELECT` statement with
>> `ORDER BY`, `GROUP BY` clauses.
>>
>> Whats happen (step by step):
>> - We have two instances on one node (sharded cluster).
>> - A query is created that executes on both.
>> - The first instance creates the name of the temporary file and
>>   checks a file with such name on existence.
>> - The second instance creates the name of the temporary file
>>   (the same as in  first instance) and checks a file with such name
>>   on existence.
>> - The first instance creates a file with the `SQL_OPEN_DELETEONCLOSE`
>>   flag.
>> - The second instance opens(try to open) the same file.
>> - The first instance closes (and removes) the temporary file.
>> - The second instance tries to work with the file and fails.
>>
>> Why did it happen:
>> The temporary file name format has a random part, but the random
>> generator uses a fixed seed.
>>
>> When it was decided to use a fixed seed:
>> 32cb1ad298b2b55d8536a85bdfb3827c8c8739e1
>>
>> How the patch fixes the problem:
>> The patch injects the PID in the temporary file name format.
>> The generated name is unique for a single process (due to a random part)
>> and unique between processes (due to the PID part).
>>
>> Alternatives:
>> 1) Use `O_TMPFILE` or `tmpfile()` (IMHO the best way to work with
>>   temporary files). In both cases, we need to update a significant
>>   part of the code, and some degradation can be added. It's hard to
>>   review.
>> 2) Return a random seed for the generator. As far as I understand,
>>   we want to have good reproducible system behavior, in which case
>>   it's good to use a fixed seed.
>> 3) Add reopening file with the flags `O_CREAT | O_EXCL` until we
>>   win the fight. Now we set such flags when opening a temporary file,
>>   but after that we try to open the file in `READONLY` mode and
>>   if ok - return the descriptor. This is strange logic for me and I
>>   don't want to add any aditional logic here. Also, such solution will
>   
>>   add additional attempts to open the file.
>>
>> So, it look like such minimal changes will work fine and are simple
>> to review.
>>
>> Co-authored-by: Mergen Imeev<imeevma@gmail.com>
>>
>> Fixes #5537
>> ---
>> src/box/sql/os_unix.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/box/sql/os_unix.c b/src/box/sql/os_unix.c
>> index 557d709..ce415cb 100644
>> --- a/src/box/sql/os_unix.c
>> +++ b/src/box/sql/os_unix.c
>> @@ -1483,8 +1483,8 @@ unixGetTempname(int nBuf, char *zBuf)
>> 		assert(nBuf > 2);
>> 		zBuf[nBuf - 2] = 0;
>> 		sql_snprintf(nBuf, zBuf,
>> -				 "%s/" SQL_TEMP_FILE_PREFIX "%llx%c", zDir,
>> -				 r, 0);
>> +				 "%s/" SQL_TEMP_FILE_PREFIX "%ld_%llx%c", zDir,
>> +				 (long)randomnessPid, r, 0);
>> 		if (zBuf[nBuf - 2] != 0 || (iLimit++) > 10)
>> 			return -1;
>> 	} while (access(zBuf, 0) == 0);
>> -- 
>> 2.7.4
>>
>