Re: chained transactions

Hi,

On 2019-01-02 16:02:21 +0100, Peter Eisentraut wrote:
> +++ b/src/backend/access/transam/xact.c
> @@ -189,6 +189,7 @@ typedef struct TransactionStateData
>  	bool		startedInRecovery;	/* did we start in recovery? */
>  	bool		didLogXid;		/* has xid been included in WAL record? */
>  	int			parallelModeLevel;	/* Enter/ExitParallelMode counter */
> +	bool		chain;			/* start a new block after this one */
>  	struct TransactionStateData *parent;	/* back link to parent */
>  } TransactionStateData;
>  
> @@ -2760,6 +2761,36 @@ StartTransactionCommand(void)
>  	MemoryContextSwitchTo(CurTransactionContext);
>  }
>  
> +
> +/*
> + * Simple system for saving and restoring transaction characteristics
> + * (isolation level, read only, deferrable).  We need this for transaction
> + * chaining, so that we can set the characteristics of the new transaction to
> + * be the same as the previous one.  (We need something like this because the
> + * GUC system resets the characteristics at transaction end, so for example
> + * just skipping the reset in StartTransaction() won't work.)
> + */
> +static int	save_XactIsoLevel;
> +static bool	save_XactReadOnly;
> +static bool	save_XactDeferrable;

We normally don't define variables in the middle of a file?  Also, why
do these need to be global vars rather than defined where we do
chaining? I'm imagining a SavedTransactionState struct declared on the
stack that's then passed to Save/Restore?

Not crucial, but I do wonder if we can come up with a prettier approach
for this.

Greetings,

Andres Freund