Test code for shared memory message queue facility.

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.

contrib/Makefile

@@ -51,6 +51,7 @@ SUBDIRS = \
 		tablefunc	\
 		tcn		\
 		test_parser	\
+		test_shm_mq	\
 		tsearch2	\
 		unaccent	\
 		vacuumlo	\

contrib/test_shm_mq/.gitignore

+# Generated subdirectories
+/log/
+/results/
+/tmp_check/

contrib/test_shm_mq/Makefile

+# contrib/test_shm_mq/Makefile
+
+MODULE_big = test_shm_mq
+OBJS = test.o setup.o worker.o
+
+EXTENSION = test_shm_mq
+DATA = test_shm_mq--1.0.sql
+
+REGRESS = test_shm_mq
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = contrib/test_shm_mq
+top_builddir = ../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif

contrib/test_shm_mq/expected/test_shm_mq.out

+CREATE EXTENSION test_shm_mq;
+--
+-- These tests don't produce any interesting output.  We're checking that
+-- the operations complete without crashing or hanging and that none of their
+-- internal sanity tests fail.
+--
+SELECT test_shm_mq(32768, (select string_agg(chr(32+(random()*96)::int), '') from generate_series(1,400)), 10000, 1);
+ test_shm_mq 
+-------------
+ 
+(1 row)
+
+SELECT test_shm_mq_pipelined(16384, (select string_agg(chr(32+(random()*96)::int), '') from generate_series(1,270000)), 200, 3);
+ test_shm_mq_pipelined 
+-----------------------
+ 
+(1 row)
+

contrib/test_shm_mq/setup.c

+/*--------------------------------------------------------------------------
+ *
+ * setup.c
+ *		Code to set up a dynamic shared memory segments and a specified
+ *		number of background workers for shared memory message queue
+ *		testing.
+ *
+ * Copyright (C) 2013, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		contrib/test_shm_mq/setup.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "miscadmin.h"
+#include "postmaster/bgworker.h"
+#include "storage/procsignal.h"
+#include "storage/shm_toc.h"
+#include "utils/memutils.h"
+
+#include "test_shm_mq.h"
+
+typedef struct
+{
+	int		nworkers;
+	BackgroundWorkerHandle *handle[FLEXIBLE_ARRAY_MEMBER];
+} worker_state;
+
+static void setup_dynamic_shared_memory(uint64 queue_size, int nworkers,
+							dsm_segment **segp,
+							test_shm_mq_header **hdrp,
+							shm_mq **outp, shm_mq **inp);
+static worker_state *setup_background_workers(int nworkers,
+											  dsm_segment *seg);
+static void cleanup_background_workers(dsm_segment *seg, Datum arg);
+static void wait_for_workers_to_become_ready(worker_state *wstate,
+								 volatile test_shm_mq_header *hdr);
+static bool check_worker_status(worker_state *wstate);
+
+/*
+ * Set up a dynamic shared memory segment and zero or more background workers
+ * for a test run.
+ */
+void
+test_shm_mq_setup(uint64 queue_size, int32 nworkers, dsm_segment **segp,
+				  shm_mq_handle **output, shm_mq_handle **input)
+{
+	dsm_segment *seg;
+	test_shm_mq_header *hdr;
+	shm_mq	   *outq;
+	shm_mq	   *inq;
+	worker_state	   *wstate;
+
+	/* Set up a dynamic shared memory segment. */
+	setup_dynamic_shared_memory(queue_size, nworkers, &seg, &hdr, &outq, &inq);
+	*segp = seg;
+
+	/* Register background workers. */
+	wstate = setup_background_workers(nworkers, seg);
+
+	/* Attach the queues. */
+	*output = shm_mq_attach(outq, seg, wstate->handle[0]);
+	*input = shm_mq_attach(inq, seg, wstate->handle[nworkers - 1]);
+
+	/* Wait for workers to become ready. */
+	wait_for_workers_to_become_ready(wstate, hdr);
+
+	/*
+	 * Once we reach this point, all workers are ready.  We no longer need
+	 * to kill them if we die; they'll die on their own as the message queues
+	 * shut down.
+	 */
+	cancel_on_dsm_detach(seg, cleanup_background_workers,
+						 PointerGetDatum(wstate));
+	pfree(wstate);
+}
+
+/*
+ * Set up a dynamic shared memory segment.
+ *
+ * We set up a small control region that contains only a test_shm_mq_header,
+ * plus one region per message queue.  There are as many message queues as
+ * the number of workers, plus one.
+ */
+static void
+setup_dynamic_shared_memory(uint64 queue_size, int nworkers,
+							dsm_segment **segp, test_shm_mq_header **hdrp,
+							shm_mq **outp, shm_mq **inp)
+{
+	shm_toc_estimator	e;
+	int					i;
+	uint64			segsize;
+	dsm_segment	   *seg;
+	shm_toc		   *toc;
+	test_shm_mq_header *hdr;
+
+	/* Ensure a valid queue size. */
+	if (queue_size < 0 || ((uint64) queue_size) < shm_mq_minimum_size)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("queue size must be at least " UINT64_FORMAT " bytes",
+					shm_mq_minimum_size)));
+
+	/*
+	 * Estimate how much shared memory we need.
+	 *
+	 * Because the TOC machinery may choose to insert padding of oddly-sized
+	 * requests, we must estimate each chunk separately.
+	 *
+	 * We need one key to register the location of the header, and we need
+	 * nworkers + 1 keys to track the locations of the message queues.
+	 */
+	shm_toc_initialize_estimator(&e);
+	shm_toc_estimate_chunk(&e, sizeof(test_shm_mq_header));
+	for (i = 0; i <= nworkers; ++i)
+		shm_toc_estimate_chunk(&e, queue_size);
+	shm_toc_estimate_keys(&e, 2 + nworkers);
+	segsize = shm_toc_estimate(&e);
+
+	/* Create the shared memory segment and establish a table of contents. */
+	seg = dsm_create(shm_toc_estimate(&e));
+	toc = shm_toc_create(PG_TEST_SHM_MQ_MAGIC, dsm_segment_address(seg),
+						 segsize);
+
+	/* Set up the header region. */
+	hdr = shm_toc_allocate(toc, sizeof(test_shm_mq_header));
+	SpinLockInit(&hdr->mutex);
+	hdr->workers_total = nworkers;
+	hdr->workers_attached = 0;
+	hdr->workers_ready = 0;
+	shm_toc_insert(toc, 0, hdr);
+
+	/* Set up one message queue per worker, plus one. */
+	for (i = 0; i <= nworkers; ++i)
+	{
+		shm_mq		   *mq;
+
+		mq = shm_mq_create(shm_toc_allocate(toc, queue_size), queue_size);
+		shm_toc_insert(toc, i + 1, mq);
+
+		if (i == 0)
+		{
+			/* We send messages to the first queue. */
+			shm_mq_set_sender(mq, MyProc);
+			*outp = mq;
+		}
+		if (i == nworkers)
+		{
+			/* We receive messages from the last queue. */
+			shm_mq_set_receiver(mq, MyProc);
+			*inp = mq;
+		}
+	}
+
+	/* Return results to caller. */
+	*segp = seg;
+	*hdrp = hdr;
+}
+
+/*
+ * Register background workers.
+ */
+static worker_state *
+setup_background_workers(int nworkers, dsm_segment *seg)
+{
+	MemoryContext	oldcontext;
+	BackgroundWorker worker;
+	worker_state	*wstate;
+	int		i;
+
+	/*
+	 * We need the worker_state object and the background worker handles to
+	 * which it points to be allocated in CurTransactionContext rather than
+	 * ExprContext; otherwise, they'll be destroyed before the on_dsm_detach
+	 * hooks run.
+	 */
+	oldcontext = MemoryContextSwitchTo(CurTransactionContext);
+
+	/* Create worker state object. */
+	wstate = MemoryContextAlloc(TopTransactionContext,
+								offsetof(worker_state, handle) +
+								sizeof(BackgroundWorkerHandle *) * nworkers);
+	wstate->nworkers = 0;
+
+	/*
+	 * Arrange to kill all the workers if we abort before all workers are
+	 * finished hooking themselves up to the dynamic shared memory segment.
+	 *
+	 * If we die after all the workers have finished hooking themselves up
+	 * to the dynamic shared memory segment, we'll mark the two queues to
+	 * which we're directly connected as detached, and the worker(s)
+	 * connected to those queues will exit, marking any other queues to
+	 * which they are connected as detached.  This will cause any
+	 * as-yet-unaware workers connected to those queues to exit in their
+	 * turn, and so on, until everybody exits.
+	 *
+	 * But suppose the workers which are supposed to connect to the queues
+	 * to which we're directly attached exit due to some error before they
+	 * actually attach the queues.  The remaining workers will have no way of
+	 * knowing this.  From their perspective, they're still waiting for those
+	 * workers to start, when in fact they've already died.
+	 */
+	on_dsm_detach(seg, cleanup_background_workers,
+				  PointerGetDatum(wstate));
+
+	/* Configure a worker. */
+	worker.bgw_flags = BGWORKER_SHMEM_ACCESS;
+	worker.bgw_start_time = BgWorkerStart_ConsistentState;
+	worker.bgw_restart_time = BGW_NEVER_RESTART;
+	worker.bgw_main = NULL;		/* new worker might not have library loaded */
+	sprintf(worker.bgw_library_name, "test_shm_mq");
+	sprintf(worker.bgw_function_name, "test_shm_mq_main");
+	snprintf(worker.bgw_name, BGW_MAXLEN, "test_shm_mq");
+	worker.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(seg));
+	/* set bgw_notify_pid, so we can detect if the worker stops */
+	worker.bgw_notify_pid = MyProcPid;
+
+	/* Register the workers. */
+	for (i = 0; i < nworkers; ++i)
+	{
+		if (!RegisterDynamicBackgroundWorker(&worker, &wstate->handle[i]))
+			ereport(ERROR,
+					(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
+					 errmsg("could not register background process"),
+				 errhint("You may need to increase max_worker_processes.")));
+		++wstate->nworkers;
+	}
+
+	/* All done. */
+	MemoryContextSwitchTo(oldcontext);
+	return wstate;
+}
+
+static void
+cleanup_background_workers(dsm_segment *seg, Datum arg)
+{
+	worker_state *wstate = (worker_state *) DatumGetPointer(arg);
+
+	while (wstate->nworkers > 0)
+	{
+		--wstate->nworkers;
+		TerminateBackgroundWorker(wstate->handle[wstate->nworkers]);
+	}
+}
+
+static void
+wait_for_workers_to_become_ready(worker_state *wstate,
+								 volatile test_shm_mq_header *hdr)
+{
+	bool	save_set_latch_on_sigusr1;
+	bool	result = false;
+
+	save_set_latch_on_sigusr1 = set_latch_on_sigusr1;
+	set_latch_on_sigusr1 = true;
+
+	PG_TRY();
+	{
+		for (;;)
+		{
+			int workers_ready;
+
+			/* If all the workers are ready, we have succeeded. */
+			SpinLockAcquire(&hdr->mutex);
+			workers_ready = hdr->workers_ready;
+			SpinLockRelease(&hdr->mutex);
+			if (workers_ready >= wstate->nworkers)
+			{
+				result = true;
+				break;
+			}
+
+			/* If any workers (or the postmaster) have died, we have failed. */
+			if (!check_worker_status(wstate))
+			{
+				result = false;
+				break;
+			}
+
+  			/* Wait to be signalled. */
+			WaitLatch(&MyProc->procLatch, WL_LATCH_SET, 0);
+
+			/* An interrupt may have occurred while we were waiting. */
+			CHECK_FOR_INTERRUPTS();
+
+			/* Reset the latch so we don't spin. */
+			ResetLatch(&MyProc->procLatch);
+		}
+	}
+	PG_CATCH();
+	{
+		set_latch_on_sigusr1 = save_set_latch_on_sigusr1;
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
+
+	if (!result)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
+				 errmsg("one or more background workers failed to start")));
+}
+
+static bool
+check_worker_status(worker_state *wstate)
+{
+	int	n;
+
+	/* If any workers (or the postmaster) have died, we have failed. */
+	for (n = 0; n < wstate->nworkers; ++n)
+	{
+		BgwHandleStatus status;
+		pid_t	pid;
+
+		status = GetBackgroundWorkerPid(wstate->handle[n], &pid);
+		if (status == BGWH_STOPPED || status == BGWH_POSTMASTER_DIED)
+			return false;
+	}
+
+	/* Otherwise, things still look OK. */
+	return true;
+}

contrib/test_shm_mq/sql/test_shm_mq.sql

+CREATE EXTENSION test_shm_mq;
+
+--
+-- These tests don't produce any interesting output.  We're checking that
+-- the operations complete without crashing or hanging and that none of their
+-- internal sanity tests fail.
+--
+SELECT test_shm_mq(32768, (select string_agg(chr(32+(random()*96)::int), '') from generate_series(1,400)), 10000, 1);
+SELECT test_shm_mq_pipelined(16384, (select string_agg(chr(32+(random()*96)::int), '') from generate_series(1,270000)), 200, 3);

contrib/test_shm_mq/test.c

+/*--------------------------------------------------------------------------
+ *
+ * test.c
+ *		Test harness code for shared memory message queues.
+ *
+ * Copyright (C) 2013, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		contrib/test_shm_mq/test.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+#include "miscadmin.h"
+
+#include "test_shm_mq.h"
+
+PG_MODULE_MAGIC;
+PG_FUNCTION_INFO_V1(test_shm_mq);
+PG_FUNCTION_INFO_V1(test_shm_mq_pipelined);
+
+void		_PG_init(void);
+Datum		test_shm_mq(PG_FUNCTION_ARGS);
+Datum		test_shm_mq_pipelined(PG_FUNCTION_ARGS);
+
+static void verify_message(uint64 origlen, char *origdata, uint64 newlen,
+			   char *newdata);
+
+/*
+ * Simple test of the shared memory message queue infrastructure.
+ *
+ * We set up a ring of message queues passing through 1 or more background
+ * processes and eventually looping back to ourselves.  We then send a message
+ * through the ring a number of times indicated by the loop count.  At the end,
+ * we check whether the final message matches the one we started with.
+ */
+Datum
+test_shm_mq(PG_FUNCTION_ARGS)
+{
+	int64		queue_size = PG_GETARG_INT64(0);
+	text	   *message = PG_GETARG_TEXT_PP(1);
+	char	   *message_contents = VARDATA_ANY(message);
+	int			message_size = VARSIZE_ANY_EXHDR(message);
+	int32		loop_count = PG_GETARG_INT32(2);
+	int32		nworkers = PG_GETARG_INT32(3);
+	dsm_segment *seg;
+	shm_mq_handle *outqh;
+	shm_mq_handle *inqh;
+	shm_mq_result	res;
+	uint64		len;
+	void	   *data;
+
+	/* A negative loopcount is nonsensical. */
+	if (loop_count < 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("repeat count size must be a non-negative integer")));
+
+	/*
+	 * Since this test sends data using the blocking interfaces, it cannot
+	 * send data to itself.  Therefore, a minimum of 1 worker is required.
+	 * Of course, a negative worker count is nonsensical.
+	 */
+	if (nworkers < 1)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("number of workers must be a positive integer")));
+
+	/* Set up dynamic shared memory segment and background workers. */
+	test_shm_mq_setup(queue_size, nworkers, &seg, &outqh, &inqh);
+
+	/* Send the initial message. */
+	res = shm_mq_send(outqh, message_size, message_contents, false);
+	if (res != SHM_MQ_SUCCESS)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("could not send message")));
+
+	/*
+	 * Receive a message and send it back out again.  Do this a number of
+	 * times equal to the loop count.
+	 */
+	for (;;)
+	{
+		/* Receive a message. */
+		res = shm_mq_receive(inqh, &len, &data, false);
+		if (res != SHM_MQ_SUCCESS)
+			ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("could not receive message")));
+
+		/* If this is supposed to be the last iteration, stop here. */
+		if (--loop_count <= 0)
+			break;
+
+		/* Send it back out. */
+		res = shm_mq_send(outqh, len, data, false);
+		if (res != SHM_MQ_SUCCESS)
+			ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("could not send message")));
+	}
+
+	/*
+	 * Finally, check that we got back the same message from the last
+	 * iteration that we originally sent.
+	 */
+	verify_message(message_size, message_contents, len, data);
+
+	/* Clean up. */
+	dsm_detach(seg);
+
+	PG_RETURN_VOID();
+}
+
+/*
+ * Pipelined test of the shared memory message queue infrastructure.
+ *
+ * As in the basic test, we set up a ring of message queues passing through
+ * 1 or more background processes and eventually looping back to ourselves.
+ * Then, we send N copies of the user-specified message through the ring and
+ * receive them all back.  Since this might fill up all message queues in the
+ * ring and then stall, we must be prepared to begin receiving the messages
+ * back before we've finished sending them.
+ */
+Datum
+test_shm_mq_pipelined(PG_FUNCTION_ARGS)
+{
+	int64		queue_size = PG_GETARG_INT64(0);
+	text	   *message = PG_GETARG_TEXT_PP(1);
+	char	   *message_contents = VARDATA_ANY(message);
+	int			message_size = VARSIZE_ANY_EXHDR(message);
+	int32		loop_count = PG_GETARG_INT32(2);
+	int32		nworkers = PG_GETARG_INT32(3);
+	bool		verify = PG_GETARG_BOOL(4);
+	int32		send_count = 0;
+	int32		receive_count = 0;
+	dsm_segment *seg;
+	shm_mq_handle *outqh;
+	shm_mq_handle *inqh;
+	shm_mq_result	res;
+	uint64		len;
+	void	   *data;
+
+	/* A negative loopcount is nonsensical. */
+	if (loop_count < 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("repeat count size must be a non-negative integer")));
+
+	/*
+	 * Using the nonblocking interfaces, we can even send data to ourselves,
+	 * so the minimum number of workers for this test is zero.
+	 */
+	if (nworkers < 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("number of workers must be a non-negative integer")));
+
+	/* Set up dynamic shared memory segment and background workers. */
+	test_shm_mq_setup(queue_size, nworkers, &seg, &outqh, &inqh);
+
+	/* Main loop. */
+	for (;;)
+	{
+		bool		wait = true;
+
+		/*
+		 * If we haven't yet sent the message the requisite number of times,
+		 * try again to send it now.  Note that when shm_mq_send() returns
+		 * SHM_MQ_WOULD_BLOCK, the next call to that function must pass the
+		 * same message size and contents; that's not an issue here because
+		 * we're sending the same message every time.
+		 */
+		if (send_count < loop_count)
+		{
+			res = shm_mq_send(outqh, message_size, message_contents, true);
+			if (res == SHM_MQ_SUCCESS)
+			{
+				++send_count;
+				wait = false;
+			}
+			else if (res == SHM_MQ_DETACHED)
+				ereport(ERROR,
+						(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+						 errmsg("could not send message")));
+		}
+
+		/*
+		 * If we haven't yet received the message the requisite number of
+		 * times, try to receive it again now.
+		 */
+		if (receive_count < loop_count)
+		{
+			res = shm_mq_receive(inqh, &len, &data, true);
+			if (res == SHM_MQ_SUCCESS)
+			{
+				++receive_count;
+				/* Verifying every time is slow, so it's optional. */
+				if (verify)
+					verify_message(message_size, message_contents, len, data);
+				wait = false;
+			}
+			else if (res == SHM_MQ_DETACHED)
+				ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("could not receive message")));
+		}
+		else
+		{
+			/*
+			 * Otherwise, we've received the message enough times.  This
+			 * shouldn't happen unless we've also sent it enough times.
+			 */
+			if (send_count != receive_count)
+				ereport(ERROR,
+						(errcode(ERRCODE_INTERNAL_ERROR),
+						 errmsg("message sent %d times, but received %d times",
+							send_count, receive_count)));
+			break;
+		}
+
+		if (wait)
+		{
+			/*
+			 * If we made no progress, wait for one of the other processes
+			 * to which we are connected to set our latch, indicating that
+			 * they have read or written data and therefore there may now be
+			 * work for us to do.
+			 */
+			WaitLatch(&MyProc->procLatch, WL_LATCH_SET, 0);
+			CHECK_FOR_INTERRUPTS();
+			ResetLatch(&MyProc->procLatch);
+		}
+	}
+
+	/* Clean up. */
+	dsm_detach(seg);
+
+	PG_RETURN_VOID();
+}
+
+/*
+ * Verify that two messages are the same.
+ */
+static void
+verify_message(uint64 origlen, char *origdata, uint64 newlen, char *newdata)
+{
+	uint64	i;
+
+	if (origlen != newlen)
+		ereport(ERROR,
+				(errmsg("message corrupted"),
+				 errdetail("The original message was " UINT64_FORMAT " bytes but the final message is " UINT64_FORMAT " bytes.",
+					 origlen, newlen)));
+
+	for (i = 0; i < origlen; ++i)
+		if (origdata[i] != newdata[i])
+			ereport(ERROR,
+					(errmsg("message corrupted"),
+					 errdetail("The new and original messages differ at byte " UINT64_FORMAT " of " UINT64_FORMAT ".", i, origlen)));
+}

contrib/test_shm_mq/test_shm_mq--1.0.sql

+/* contrib/test_shm_mq/test_shm_mq--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION test_shm_mq" to load this file. \quit
+
+CREATE FUNCTION test_shm_mq(queue_size pg_catalog.int8,
+					   message pg_catalog.text,
+					   repeat_count pg_catalog.int4 default 1,
+					   num_workers pg_catalog.int4 default 1)
+    RETURNS pg_catalog.void STRICT
+	AS 'MODULE_PATHNAME' LANGUAGE C;
+
+CREATE FUNCTION test_shm_mq_pipelined(queue_size pg_catalog.int8,
+					   message pg_catalog.text,
+					   repeat_count pg_catalog.int4 default 1,
+					   num_workers pg_catalog.int4 default 1,
+					   verify pg_catalog.bool default true)
+    RETURNS pg_catalog.void STRICT
+	AS 'MODULE_PATHNAME' LANGUAGE C;

contrib/test_shm_mq/test_shm_mq.control

+comment = 'Test code for shared memory message queues'
+default_version = '1.0'
+module_pathname = '$libdir/test_shm_mq'
+relocatable = true

contrib/test_shm_mq/test_shm_mq.h

+/*--------------------------------------------------------------------------
+ *
+ * test_shm_mq.h
+ *		Definitions for shared memory message queues
+ *
+ * Copyright (C) 2013, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		contrib/test_shm_mq/test_shm_mq.h
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#ifndef TEST_SHM_MQ_H
+#define TEST_SHM_MQ_H
+
+#include "storage/dsm.h"
+#include "storage/shm_mq.h"
+#include "storage/spin.h"
+
+/* Identifier for shared memory segments used by this extension. */
+#define		PG_TEST_SHM_MQ_MAGIC		0x79fb2447
+
+/*
+ * This structure is stored in the dynamic shared memory segment.  We use
+ * it to determine whether all workers started up OK and successfully
+ * attached to their respective shared message queues.
+ */
+typedef struct
+{
+	slock_t			mutex;
+	int				workers_total;
+	int				workers_attached;
+	int				workers_ready;
+} test_shm_mq_header;
+
+/* Set up dynamic shared memory and background workers for test run. */
+extern void test_shm_mq_setup(uint64 queue_size, int32 nworkers,
+							  dsm_segment **seg, shm_mq_handle **output,
+							  shm_mq_handle **input);
+
+/* Main entrypoint for a worker. */
+extern void	test_shm_mq_main(Datum);
+
+#endif

contrib/test_shm_mq/worker.c

+/*--------------------------------------------------------------------------
+ *
+ * worker.c
+ *		Code for sample worker making use of shared memory message queues.
+ *		Our test worker simply reads messages from one message queue and
+ *		writes them back out to another message queue.  In a real
+ *		application, you'd presumably want the worker to do some more
+ *		complex calculation rather than simply returning the input,
+ *		but it should be possible to use much of the control logic just
+ *		as presented here.
+ *
+ * Copyright (C) 2013, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		contrib/test_shm_mq/worker.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "miscadmin.h"
+#include "storage/ipc.h"
+#include "storage/procarray.h"
+#include "storage/shm_mq.h"
+#include "storage/shm_toc.h"
+#include "utils/resowner.h"
+
+#include "test_shm_mq.h"
+
+static void handle_sigterm(SIGNAL_ARGS);
+static void attach_to_queues(dsm_segment *seg, shm_toc *toc,
+							 int myworkernumber, shm_mq_handle **inqhp,
+							 shm_mq_handle **outqhp);
+static void copy_messages(shm_mq_handle *inqh, shm_mq_handle *outqh);
+
+/*
+ * Background worker entrypoint.
+ *
+ * This is intended to demonstrate how a background worker can be used to
+ * facilitate a parallel computation.  Most of the logic here is fairly
+ * boilerplate stuff, designed to attach to the shared memory segment,
+ * notify the user backend that we're alive, and so on.  The
+ * application-specific bits of logic that you'd replace for your own worker
+ * are attach_to_queues() and copy_messages().
+ */
+void
+test_shm_mq_main(Datum main_arg)
+{
+	dsm_segment *seg;
+	shm_toc	   *toc;
+	shm_mq_handle *inqh;
+	shm_mq_handle *outqh;
+	volatile test_shm_mq_header *hdr;
+	int			myworkernumber;
+	PGPROC	   *registrant;
+
+	/*
+	 * Establish signal handlers.
+	 *
+	 * We want CHECK_FOR_INTERRUPTS() to kill off this worker process just
+	 * as it would a normal user backend.  To make that happen, we establish
+	 * a signal handler that is a stripped-down version of die().  We don't
+	 * have any equivalent of the backend's command-read loop, where interrupts
+	 * can be processed immediately, so make sure ImmediateInterruptOK is
+	 * turned off.
+	 */
+	pqsignal(SIGTERM, handle_sigterm);
+	ImmediateInterruptOK = false;
+	BackgroundWorkerUnblockSignals();
+
+	/*
+	 * Connect to the dynamic shared memory segment.
+	 *
+	 * The backend that registered this worker passed us the ID of a shared
+	 * memory segment to which we must attach for further instructions.  In
+	 * order to attach to dynamic shared memory, we need a resource owner.
+	 * Once we've mapped the segment in our address space, attach to the table
+	 * of contents so we can locate the various data structures we'll need
+	 * to find within the segment.
+	 */
+	CurrentResourceOwner = ResourceOwnerCreate(NULL, "test_shm_mq worker");
+	seg = dsm_attach(DatumGetInt32(main_arg));
+	if (seg == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("unable to map dynamic shared memory segment")));
+	toc = shm_toc_attach(PG_TEST_SHM_MQ_MAGIC, dsm_segment_address(seg));
+	if (toc == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("bad magic number in dynamic shared memory segment")));
+
+	/*
+	 * Acquire a worker number.
+	 *
+	 * By convention, the process registering this background worker should
+	 * have stored the control structure at key 0.  We look up that key to
+	 * find it.  Our worker number gives our identity: there may be just one
+	 * worker involved in this parallel operation, or there may be many.
+	 */
+	hdr = shm_toc_lookup(toc, 0);
+	SpinLockAcquire(&hdr->mutex);
+	myworkernumber = ++hdr->workers_attached;
+	SpinLockRelease(&hdr->mutex);
+	if (myworkernumber > hdr->workers_total)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("too many message queue testing workers already")));
+
+	/*
+	 * Attach to the appropriate message queues.
+	 */
+	attach_to_queues(seg, toc, myworkernumber, &inqh, &outqh);
+
+	/*
+	 * Indicate that we're fully initialized and ready to begin the main
+	 * part of the parallel operation.
+	 *
+	 * Once we signal that we're ready, the user backend is entitled to assume
+	 * that our on_dsm_detach callbacks will fire before we disconnect from
+	 * the shared memory segment and exit.  Generally, that means we must have
+	 * attached to all relevant dynamic shared memory data structures by now.
+	 */
+	SpinLockAcquire(&hdr->mutex);
+	++hdr->workers_ready;
+	SpinLockRelease(&hdr->mutex);
+	registrant = BackendPidGetProc(MyBgworkerEntry->bgw_notify_pid);
+	if (registrant == NULL)		
+	{
+		elog(DEBUG1, "registrant backend has exited prematurely");
+		proc_exit(1);
+	}
+	SetLatch(&registrant->procLatch);
+
+	/* Do the work. */
+	copy_messages(inqh, outqh);
+
+	/*
+	 * We're done.  Explicitly detach the shared memory segment so that we
+	 * don't get a resource leak warning at commit time.  This will fire any
+	 * on_dsm_detach callbacks we've registered, as well.  Once that's done,
+	 * we can go ahead and exit.
+	 */
+	dsm_detach(seg);
+	proc_exit(1);
+}
+
+/*
+ * Attach to shared memory message queues.
+ *
+ * We use our worker number to determine to which queue we should attach.
+ * The queues are registered at keys 1..<number-of-workers>.  The user backend
+ * writes to queue #1 and reads from queue #<number-of-workers>; each worker
+ * reads from the queue whose number is equal to its worker number and writes
+ * to the next higher-numbered queue.
+ */
+static void
+attach_to_queues(dsm_segment *seg, shm_toc *toc, int myworkernumber,
+				 shm_mq_handle **inqhp, shm_mq_handle **outqhp)
+{
+	shm_mq	   *inq;
+	shm_mq	   *outq;
+
+	inq = shm_toc_lookup(toc, myworkernumber);
+	shm_mq_set_receiver(inq, MyProc);
+	*inqhp = shm_mq_attach(inq, seg, NULL);
+	outq = shm_toc_lookup(toc, myworkernumber + 1);
+	shm_mq_set_sender(outq, MyProc);
+	*outqhp = shm_mq_attach(outq, seg, NULL);
+}
+
+/*
+ * Loop, receiving and sending messages, until the connection is broken.
+ *
+ * This is the "real work" performed by this worker process.  Everything that
+ * happens before this is initialization of one form or another, and everything
+ * after this point is cleanup.
+ */
+static void
+copy_messages(shm_mq_handle *inqh, shm_mq_handle *outqh)
+{
+	uint64		len;
+	void	   *data;
+	shm_mq_result res;
+
+	for (;;)
+	{
+		/* Notice any interrupts that have occurred. */
+		CHECK_FOR_INTERRUPTS();
+
+		/* Receive a message. */
+		res = shm_mq_receive(inqh, &len, &data, false);
+		if (res != SHM_MQ_SUCCESS)
+			break;
+
+		/* Send it back out. */
+		res = shm_mq_send(outqh, len, data, false);
+		if (res != SHM_MQ_SUCCESS)
+			break;
+	}
+}
+
+/*
+ * When we receive a SIGTERM, we set InterruptPending and ProcDiePending just
+ * like a normal backend.  The next CHECK_FOR_INTERRUPTS() will do the right
+ * thing.
+ */
+static void
+handle_sigterm(SIGNAL_ARGS)
+{
+	int			save_errno = errno;
+
+	if (MyProc)
+		SetLatch(&MyProc->procLatch);
+
+	if (!proc_exit_inprogress)
+	{
+		InterruptPending = true;
+		ProcDiePending = true;
+	}
+
+	errno = save_errno;
+}