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/*-------------------------------------------------------------------------
*
* tqueue.c
* Use shm_mq to send & receive tuples between parallel backends
*
* A DestReceiver of type DestTupleQueue, which is a TQueueDestReceiver
* under the hood, writes tuples from the executor to a shm_mq.
*
* A TupleQueueReader reads tuples from a shm_mq and returns the tuples.
*
* Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* src/backend/executor/tqueue.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/htup_details.h"
#include "executor/tqueue.h"
/*
* DestReceiver object's private contents
*
* queue is a pointer to data supplied by DestReceiver's caller.
*/
typedef struct TQueueDestReceiver
{
DestReceiver pub; /* public fields */
shm_mq_handle *queue; /* shm_mq to send to */
} TQueueDestReceiver;
/*
* TupleQueueReader object's private contents
*
* queue is a pointer to data supplied by reader's caller.
*
* "typedef struct TupleQueueReader TupleQueueReader" is in tqueue.h
*/
struct TupleQueueReader
{
shm_mq_handle *queue; /* shm_mq to receive from */
};
/*
* Receive a tuple from a query, and send it to the designated shm_mq.
*
* Returns true if successful, false if shm_mq has been detached.
*/
static bool
tqueueReceiveSlot(TupleTableSlot *slot, DestReceiver *self)
{
TQueueDestReceiver *tqueue = (TQueueDestReceiver *) self;
MinimalTuple tuple;
shm_mq_result result;
bool should_free;
/* Send the tuple itself. */
tuple = ExecFetchSlotMinimalTuple(slot, &should_free);
result = shm_mq_send(tqueue->queue, tuple->t_len, tuple, false, false);
if (should_free)
pfree(tuple);
/* Check for failure. */
if (result == SHM_MQ_DETACHED)
return false;
else if (result != SHM_MQ_SUCCESS)
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("could not send tuple to shared-memory queue")));
return true;
}
/*
* Prepare to receive tuples from executor.
*/
static void
tqueueStartupReceiver(DestReceiver *self, int operation, TupleDesc typeinfo)
{
/* do nothing */
}
/*
* Clean up at end of an executor run
*/
static void
tqueueShutdownReceiver(DestReceiver *self)
{
TQueueDestReceiver *tqueue = (TQueueDestReceiver *) self;
if (tqueue->queue != NULL)
shm_mq_detach(tqueue->queue);
tqueue->queue = NULL;
}
/*
* Destroy receiver when done with it
*/
static void
tqueueDestroyReceiver(DestReceiver *self)
{
TQueueDestReceiver *tqueue = (TQueueDestReceiver *) self;
/* We probably already detached from queue, but let's be sure */
if (tqueue->queue != NULL)
shm_mq_detach(tqueue->queue);
pfree(self);
}
/*
* Create a DestReceiver that writes tuples to a tuple queue.
*/
DestReceiver *
CreateTupleQueueDestReceiver(shm_mq_handle *handle)
{
TQueueDestReceiver *self;
self = (TQueueDestReceiver *) palloc0(sizeof(TQueueDestReceiver));
self->pub.receiveSlot = tqueueReceiveSlot;
self->pub.rStartup = tqueueStartupReceiver;
self->pub.rShutdown = tqueueShutdownReceiver;
self->pub.rDestroy = tqueueDestroyReceiver;
self->pub.mydest = DestTupleQueue;
self->queue = handle;
return (DestReceiver *) self;
}
/*
* Create a tuple queue reader.
*/
TupleQueueReader *
CreateTupleQueueReader(shm_mq_handle *handle)
{
TupleQueueReader *reader = palloc0(sizeof(TupleQueueReader));
reader->queue = handle;
return reader;
}
/*
* Destroy a tuple queue reader.
*
* Note: cleaning up the underlying shm_mq is the caller's responsibility.
* We won't access it here, as it may be detached already.
*/
void
DestroyTupleQueueReader(TupleQueueReader *reader)
{
pfree(reader);
}
/*
* Fetch a tuple from a tuple queue reader.
*
* The return value is NULL if there are no remaining tuples or if
* nowait = true and no tuple is ready to return. *done, if not NULL,
* is set to true when there are no remaining tuples and otherwise to false.
*
* The returned tuple, if any, is either in shared memory or a private buffer
* and should not be freed. The pointer is invalid after the next call to
* TupleQueueReaderNext().
*
* Even when shm_mq_receive() returns SHM_MQ_WOULD_BLOCK, this can still
* accumulate bytes from a partially-read message, so it's useful to call
* this with nowait = true even if nothing is returned.
*/
MinimalTuple
TupleQueueReaderNext(TupleQueueReader *reader, bool nowait, bool *done)
{
MinimalTuple tuple;
shm_mq_result result;
Size nbytes;
void *data;
if (done != NULL)
*done = false;
/* Attempt to read a message. */
result = shm_mq_receive(reader->queue, &nbytes, &data, nowait);
/* If queue is detached, set *done and return NULL. */
if (result == SHM_MQ_DETACHED)
{
if (done != NULL)
*done = true;
return NULL;
}
/* In non-blocking mode, bail out if no message ready yet. */
if (result == SHM_MQ_WOULD_BLOCK)
return NULL;
Assert(result == SHM_MQ_SUCCESS);
/*
* Return a pointer to the queue memory directly (which had better be
* sufficiently aligned).
*/
tuple = (MinimalTuple) data;
Assert(tuple->t_len == nbytes);
return tuple;
}
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