process shared robust mutexes for tdb

[Rusty Russell]

Rusty Russell <rusty at samba.org> writes:
> Will finish review this afternoon, but wanted to send this off now.

OK, part II:

OK, I really like the new tests you wrote.

> +	tdb = tdb_open_ex("mutex.tdb", 0, tdb_flags,
> +			  O_RDWR|O_CREAT, 0755, &log_ctx, NULL);
> +	ok(tdb, "tdb_open_ex should succeed\n");

I'd recommend changing the name "mutex.tdb" for each test, so we can run
them in parallel.  The simple stuff we have at the moment doesn't of
course, but it should.

> +	ret = tdb_chainlock(tdb, key);
> +	ok(ret == 0, "tdb_chainlock should succeed\n");

I use ok1() everywhere: saves typing.

> @@ -279,6 +281,7 @@ static void usage(void)
>  	printf("   -s suffix     set the backup suffix\n");
>  	printf("   -v            verify mode (restore if corrupt)\n");
>  	printf("   -n hashsize   set the new hash size for the backup\n");
> +	printf("   -l            open without locking to back up mutex dbs\n");
>  }

I'm not comfortable with tying CLEAR_IF_FIRST to TDB_MUTEX_LOCKING.
This example shows why: it tempts people to do backup without any
locking, which isn't really a backup.

I think we can do better: why not always hold the ACTIVE_LOCK (fcntl
lock) if TDB_MUTEX_LOCKING?  If they *don't* specify CLEAR_IF_FIRST, and
we get a write lock, then reset the mutexes (in theory we could resize
them at this point if we needed to, too).

> diff --git a/lib/tdb/tools/tdbtorture.c b/lib/tdb/tools/tdbtorture.c
> index a23d154..94bfe80 100644
> --- a/lib/tdb/tools/tdbtorture.c
> +++ b/lib/tdb/tools/tdbtorture.c
> @@ -17,7 +17,7 @@
>  #define DELETE_PROB 8
>  #define STORE_PROB 4
>  #define APPEND_PROB 6
> -#define TRANSACTION_PROB 10
> +#define TRANSACTION_PROB 100
>  #define TRANSACTION_PREPARE_PROB 2
>  #define LOCKSTORE_PROB 5
>  #define TRAVERSE_PROB 20

This makes transaction 10x less likely?

Cheers,
Rusty.