[Tarantool-patches] [PATCH v2 3/3] sql: update temporary file name format

[Leonid Vasiliev <lvasiliev@tarantool.org>]

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