/*
* --------------------------------------------------------------
* EnableNotifyInterrupt
*
* This is called by the PostgresMain main loop just before waiting
* for a frontend command. If we are truly idle (ie, *not* inside
* a transaction block), then process any pending inbound notifies,
* and enable the signal handler to process future notifies directly.
*
* NOTE: the signal handler starts out disabled, and stays so until
* PostgresMain calls this the first time.
* --------------------------------------------------------------
*/
void
EnableNotifyInterrupt(void)
{
if (IsTransactionOrTransactionBlock())
return; /* not really idle */
/*
* This code is tricky because we are communicating with a signal
* handler that could interrupt us at any point. If we just checked
* notifyInterruptOccurred and then set notifyInterruptEnabled, we
* could fail to respond promptly to a signal that happens in between
* those two steps. (A very small time window, perhaps, but Murphy's
* Law says you can hit it...) Instead, we first set the enable flag,
* then test the occurred flag. If we see an unserviced interrupt has
* occurred, we re-clear the enable flag before going off to do the
* service work. (That prevents re-entrant invocation of
* ProcessIncomingNotify() if another interrupt occurs.) If an
* interrupt comes in between the setting and clearing of
* notifyInterruptEnabled, then it will have done the service work and
* left notifyInterruptOccurred zero, so we have to check again after
* clearing enable. The whole thing has to be in a loop in case
* another interrupt occurs while we're servicing the first. Once we
* get out of the loop, enable is set and we know there is no
* unserviced interrupt.
*
* NB: an overenthusiastic optimizing compiler could easily break this
* code. Hopefully, they all understand what "volatile" means these
* days.
*/
for (;;)
{
notifyInterruptEnabled = 1;
if (!notifyInterruptOccurred)
break;
notifyInterruptEnabled = 0;
if (notifyInterruptOccurred)
{
if (Trace_notify)
elog(DEBUG1, "EnableNotifyInterrupt: perform async notify");
ProcessIncomingNotify();
if (Trace_notify)
elog(DEBUG1, "EnableNotifyInterrupt: done");
}
}
}
/*
*--------------------------------------------------------------
* NotifyInterruptHandler
*
* This is the signal handler for SIGUSR2.
*
* If we are idle (notifyInterruptEnabled is set), we can safely invoke
* ProcessIncomingNotify directly. Otherwise, just set a flag
* to do it later.
*
* Results:
* none
*
* Side effects:
* per above
*--------------------------------------------------------------
*/
void
NotifyInterruptHandler(SIGNAL_ARGS)
{
int save_errno = errno;
/*
* Note: this is a SIGNAL HANDLER. You must be very wary what you do
* here. Some helpful soul had this routine sprinkled with TPRINTFs, which
* would likely lead to corruption of stdio buffers if they were ever
* turned on.
*/
/* Don't joggle the elbow of proc_exit */
if (proc_exit_inprogress)
return;
if (notifyInterruptEnabled)
{
bool save_ImmediateInterruptOK = ImmediateInterruptOK;
/*
* We may be called while ImmediateInterruptOK is true; turn it off
* while messing with the NOTIFY state. (We would have to save and
* restore it anyway, because PGSemaphore operations inside
* ProcessIncomingNotify() might reset it.)
*/
ImmediateInterruptOK = false;
/*
* I'm not sure whether some flavors of Unix might allow another
* SIGUSR2 occurrence to recursively interrupt this routine. To cope
* with the possibility, we do the same sort of dance that
* EnableNotifyInterrupt must do --- see that routine for comments.
*/
notifyInterruptEnabled = 0; /* disable any recursive signal */
notifyInterruptOccurred = 1; /* do at least one iteration */
for (;;)
{
notifyInterruptEnabled = 1;
if (!notifyInterruptOccurred)
break;
notifyInterruptEnabled = 0;
if (notifyInterruptOccurred)
{
/* Here, it is finally safe to do stuff. */
if (Trace_notify)
elog(DEBUG1, "NotifyInterruptHandler: perform async notify");
ProcessIncomingNotify();
if (Trace_notify)
elog(DEBUG1, "NotifyInterruptHandler: done");
}
}
/*
* Restore ImmediateInterruptOK, and check for interrupts if needed.
*/
ImmediateInterruptOK = save_ImmediateInterruptOK;
if (save_ImmediateInterruptOK)
CHECK_FOR_INTERRUPTS();
}
else
{
/*
* In this path it is NOT SAFE to do much of anything, except this:
*/
notifyInterruptOccurred = 1;
}
errno = save_errno;
}
