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4a4e6893aa
The shm_mq mechanism was built to send error (and notice) messages and
tuples between backends. However, shm_mq itself only deals in raw
bytes. Since commit 2bd9e412f9, we have
had infrastructure for one message to redirect protocol messages to a
queue and for another backend to parse them and do useful things with
them. This commit introduces a somewhat analogous facility for tuples
by adding a new type of DestReceiver, DestTupleQueue, which writes
each tuple generated by a query into a shm_mq, and a new
TupleQueueFunnel facility which reads raw tuples out of the queue and
reconstructs the HeapTuple format expected by the executor.
The TupleQueueFunnel abstraction supports reading from multiple tuple
streams at the same time, but only in round-robin fashion. Someone
could imaginably want other policies, but this should be good enough
to meet our short-term needs related to parallel query, and we can
always extend it later.
This also makes one minor addition to the shm_mq API that didn'
seem worth breaking out as a separate patch.
Extracted from Amit Kapila's parallel sequential scan patch. This
code was originally written by me, and then it was revised by Amit,
and then it was revised some more by me.
86 lines
2.6 KiB
C
86 lines
2.6 KiB
C
/*-------------------------------------------------------------------------
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*
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* shm_mq.h
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* single-reader, single-writer shared memory message queue
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*
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* Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/storage/shm_mq.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef SHM_MQ_H
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#define SHM_MQ_H
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#include "postmaster/bgworker.h"
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#include "storage/dsm.h"
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#include "storage/proc.h"
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/* The queue itself, in shared memory. */
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struct shm_mq;
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typedef struct shm_mq shm_mq;
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/* Backend-private state. */
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struct shm_mq_handle;
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typedef struct shm_mq_handle shm_mq_handle;
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/* Descriptors for a single write spanning multiple locations. */
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typedef struct
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{
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const char *data;
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Size len;
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} shm_mq_iovec;
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/* Possible results of a send or receive operation. */
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typedef enum
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{
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SHM_MQ_SUCCESS, /* Sent or received a message. */
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SHM_MQ_WOULD_BLOCK, /* Not completed; retry later. */
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SHM_MQ_DETACHED /* Other process has detached queue. */
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} shm_mq_result;
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/*
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* Primitives to create a queue and set the sender and receiver.
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*
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* Both the sender and the receiver must be set before any messages are read
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* or written, but they need not be set by the same process. Each must be
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* set exactly once.
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*/
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extern shm_mq *shm_mq_create(void *address, Size size);
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extern void shm_mq_set_receiver(shm_mq *mq, PGPROC *);
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extern void shm_mq_set_sender(shm_mq *mq, PGPROC *);
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/* Accessor methods for sender and receiver. */
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extern PGPROC *shm_mq_get_receiver(shm_mq *);
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extern PGPROC *shm_mq_get_sender(shm_mq *);
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/* Set up backend-local queue state. */
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extern shm_mq_handle *shm_mq_attach(shm_mq *mq, dsm_segment *seg,
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BackgroundWorkerHandle *handle);
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/* Associate worker handle with shm_mq. */
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extern void shm_mq_set_handle(shm_mq_handle *, BackgroundWorkerHandle *);
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/* Break connection. */
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extern void shm_mq_detach(shm_mq *);
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/* Get the shm_mq from handle. */
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extern shm_mq *shm_mq_get_queue(shm_mq_handle *mqh);
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/* Send or receive messages. */
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extern shm_mq_result shm_mq_send(shm_mq_handle *mqh,
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Size nbytes, const void *data, bool nowait);
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extern shm_mq_result shm_mq_sendv(shm_mq_handle *mqh,
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shm_mq_iovec *iov, int iovcnt, bool nowait);
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extern shm_mq_result shm_mq_receive(shm_mq_handle *mqh,
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Size *nbytesp, void **datap, bool nowait);
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/* Wait for our counterparty to attach to the queue. */
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extern shm_mq_result shm_mq_wait_for_attach(shm_mq_handle *mqh);
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/* Smallest possible queue. */
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extern PGDLLIMPORT const Size shm_mq_minimum_size;
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#endif /* SHM_MQ_H */
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