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This code is intended as a demonstration of how the dynamic shared memory and dynamic background worker facilities can be used to establish a group of coooperating processes which can coordinate their activities using the shared memory message queue facility. By itself, the code does nothing particularly interesting: it simply allows messages to be passed through a loop of workers and back to the original process. But it's a useful unit test, in addition to its demonstration value.
225 lines
6.9 KiB
C
225 lines
6.9 KiB
C
/*--------------------------------------------------------------------------
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*
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* worker.c
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* Code for sample worker making use of shared memory message queues.
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* Our test worker simply reads messages from one message queue and
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* writes them back out to another message queue. In a real
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* application, you'd presumably want the worker to do some more
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* complex calculation rather than simply returning the input,
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* but it should be possible to use much of the control logic just
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* as presented here.
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*
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* Copyright (C) 2013, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* contrib/test_shm_mq/worker.c
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*
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* -------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "miscadmin.h"
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#include "storage/ipc.h"
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#include "storage/procarray.h"
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#include "storage/shm_mq.h"
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#include "storage/shm_toc.h"
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#include "utils/resowner.h"
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#include "test_shm_mq.h"
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static void handle_sigterm(SIGNAL_ARGS);
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static void attach_to_queues(dsm_segment *seg, shm_toc *toc,
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int myworkernumber, shm_mq_handle **inqhp,
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shm_mq_handle **outqhp);
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static void copy_messages(shm_mq_handle *inqh, shm_mq_handle *outqh);
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/*
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* Background worker entrypoint.
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*
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* This is intended to demonstrate how a background worker can be used to
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* facilitate a parallel computation. Most of the logic here is fairly
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* boilerplate stuff, designed to attach to the shared memory segment,
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* notify the user backend that we're alive, and so on. The
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* application-specific bits of logic that you'd replace for your own worker
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* are attach_to_queues() and copy_messages().
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*/
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void
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test_shm_mq_main(Datum main_arg)
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{
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dsm_segment *seg;
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shm_toc *toc;
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shm_mq_handle *inqh;
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shm_mq_handle *outqh;
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volatile test_shm_mq_header *hdr;
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int myworkernumber;
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PGPROC *registrant;
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/*
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* Establish signal handlers.
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*
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* We want CHECK_FOR_INTERRUPTS() to kill off this worker process just
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* as it would a normal user backend. To make that happen, we establish
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* a signal handler that is a stripped-down version of die(). We don't
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* have any equivalent of the backend's command-read loop, where interrupts
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* can be processed immediately, so make sure ImmediateInterruptOK is
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* turned off.
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*/
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pqsignal(SIGTERM, handle_sigterm);
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ImmediateInterruptOK = false;
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BackgroundWorkerUnblockSignals();
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/*
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* Connect to the dynamic shared memory segment.
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*
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* The backend that registered this worker passed us the ID of a shared
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* memory segment to which we must attach for further instructions. In
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* order to attach to dynamic shared memory, we need a resource owner.
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* Once we've mapped the segment in our address space, attach to the table
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* of contents so we can locate the various data structures we'll need
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* to find within the segment.
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*/
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CurrentResourceOwner = ResourceOwnerCreate(NULL, "test_shm_mq worker");
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seg = dsm_attach(DatumGetInt32(main_arg));
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if (seg == NULL)
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ereport(ERROR,
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(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
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errmsg("unable to map dynamic shared memory segment")));
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toc = shm_toc_attach(PG_TEST_SHM_MQ_MAGIC, dsm_segment_address(seg));
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if (toc == NULL)
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ereport(ERROR,
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(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
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errmsg("bad magic number in dynamic shared memory segment")));
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/*
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* Acquire a worker number.
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*
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* By convention, the process registering this background worker should
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* have stored the control structure at key 0. We look up that key to
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* find it. Our worker number gives our identity: there may be just one
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* worker involved in this parallel operation, or there may be many.
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*/
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hdr = shm_toc_lookup(toc, 0);
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SpinLockAcquire(&hdr->mutex);
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myworkernumber = ++hdr->workers_attached;
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SpinLockRelease(&hdr->mutex);
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if (myworkernumber > hdr->workers_total)
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ereport(ERROR,
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(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
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errmsg("too many message queue testing workers already")));
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/*
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* Attach to the appropriate message queues.
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*/
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attach_to_queues(seg, toc, myworkernumber, &inqh, &outqh);
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/*
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* Indicate that we're fully initialized and ready to begin the main
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* part of the parallel operation.
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*
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* Once we signal that we're ready, the user backend is entitled to assume
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* that our on_dsm_detach callbacks will fire before we disconnect from
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* the shared memory segment and exit. Generally, that means we must have
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* attached to all relevant dynamic shared memory data structures by now.
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*/
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SpinLockAcquire(&hdr->mutex);
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++hdr->workers_ready;
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SpinLockRelease(&hdr->mutex);
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registrant = BackendPidGetProc(MyBgworkerEntry->bgw_notify_pid);
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if (registrant == NULL)
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{
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elog(DEBUG1, "registrant backend has exited prematurely");
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proc_exit(1);
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}
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SetLatch(®istrant->procLatch);
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/* Do the work. */
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copy_messages(inqh, outqh);
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/*
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* We're done. Explicitly detach the shared memory segment so that we
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* don't get a resource leak warning at commit time. This will fire any
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* on_dsm_detach callbacks we've registered, as well. Once that's done,
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* we can go ahead and exit.
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*/
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dsm_detach(seg);
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proc_exit(1);
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}
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/*
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* Attach to shared memory message queues.
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*
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* We use our worker number to determine to which queue we should attach.
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* The queues are registered at keys 1..<number-of-workers>. The user backend
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* writes to queue #1 and reads from queue #<number-of-workers>; each worker
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* reads from the queue whose number is equal to its worker number and writes
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* to the next higher-numbered queue.
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*/
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static void
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attach_to_queues(dsm_segment *seg, shm_toc *toc, int myworkernumber,
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shm_mq_handle **inqhp, shm_mq_handle **outqhp)
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{
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shm_mq *inq;
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shm_mq *outq;
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inq = shm_toc_lookup(toc, myworkernumber);
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shm_mq_set_receiver(inq, MyProc);
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*inqhp = shm_mq_attach(inq, seg, NULL);
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outq = shm_toc_lookup(toc, myworkernumber + 1);
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shm_mq_set_sender(outq, MyProc);
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*outqhp = shm_mq_attach(outq, seg, NULL);
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}
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/*
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* Loop, receiving and sending messages, until the connection is broken.
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*
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* This is the "real work" performed by this worker process. Everything that
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* happens before this is initialization of one form or another, and everything
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* after this point is cleanup.
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*/
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static void
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copy_messages(shm_mq_handle *inqh, shm_mq_handle *outqh)
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{
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uint64 len;
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void *data;
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shm_mq_result res;
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for (;;)
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{
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/* Notice any interrupts that have occurred. */
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CHECK_FOR_INTERRUPTS();
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/* Receive a message. */
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res = shm_mq_receive(inqh, &len, &data, false);
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if (res != SHM_MQ_SUCCESS)
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break;
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/* Send it back out. */
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res = shm_mq_send(outqh, len, data, false);
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if (res != SHM_MQ_SUCCESS)
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break;
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}
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}
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/*
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* When we receive a SIGTERM, we set InterruptPending and ProcDiePending just
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* like a normal backend. The next CHECK_FOR_INTERRUPTS() will do the right
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* thing.
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*/
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static void
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handle_sigterm(SIGNAL_ARGS)
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{
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int save_errno = errno;
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if (MyProc)
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SetLatch(&MyProc->procLatch);
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if (!proc_exit_inprogress)
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{
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InterruptPending = true;
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ProcDiePending = true;
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}
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errno = save_errno;
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}
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