Re: [PATCHES] Infrastructure changes for recovery (v8) - Mailing list pgsql-hackers

Simon Riggs wrote:
> diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/
> index 063b366..5e64cb4 100644
> --- a/src/backend/access/transam/subtrans.c
> +++ b/src/backend/access/transam/subtrans.c
> @@ -226,6 +226,9 @@ ZeroSUBTRANSPage(int pageno)
>   *
>   * oldestActiveXID is the oldest XID of any prepared transaction, or nextXid
>   * if there are none.
> + *
> + * Note that this is not atomic and is not yet safe to perform while other
> + * processes might access subtrans.
>   */
>  void
>  StartupSUBTRANS(TransactionId oldestActiveXID)

I'm a bit confused by that comment. Does that need to be fixed? It 
sounds like it does, because other processes might access subtrans when 
StartupSUBTRANS is called, with the patch to allow read-only queries 
during recovery. Or is that done in the hot standby patch?

However, I don't see why that isn't safe. StartupSUBTRANS takes the 
SubtransControlLock in exclusive mode while it zeroes out subtrans.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com