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1 : : /*--------------------------------------------------------------------------
2 : : *
3 : : * worker.c
4 : : * Code for sample worker making use of shared memory message queues.
5 : : * Our test worker simply reads messages from one message queue and
6 : : * writes them back out to another message queue. In a real
7 : : * application, you'd presumably want the worker to do some more
8 : : * complex calculation rather than simply returning the input,
9 : : * but it should be possible to use much of the control logic just
10 : : * as presented here.
11 : : *
12 : : * Copyright (c) 2013-2026, PostgreSQL Global Development Group
13 : : *
14 : : * IDENTIFICATION
15 : : * src/test/modules/test_shm_mq/worker.c
16 : : *
17 : : * -------------------------------------------------------------------------
18 : : */
19 : :
20 : : #include "postgres.h"
21 : :
22 : : #include "miscadmin.h"
23 : : #include "storage/ipc.h"
24 : : #include "storage/latch.h"
25 : : #include "storage/proc.h"
26 : : #include "storage/procarray.h"
27 : : #include "storage/shm_mq.h"
28 : : #include "storage/shm_toc.h"
29 : : #include "tcop/tcopprot.h"
30 : :
31 : : #include "test_shm_mq.h"
32 : :
33 : : static void attach_to_queues(dsm_segment *seg, shm_toc *toc,
34 : : int myworkernumber, shm_mq_handle **inqhp,
35 : : shm_mq_handle **outqhp);
36 : : static void copy_messages(shm_mq_handle *inqh, shm_mq_handle *outqh);
37 : :
38 : : /*
39 : : * Background worker entrypoint.
40 : : *
41 : : * This is intended to demonstrate how a background worker can be used to
42 : : * facilitate a parallel computation. Most of the logic here is fairly
43 : : * boilerplate stuff, designed to attach to the shared memory segment,
44 : : * notify the user backend that we're alive, and so on. The
45 : : * application-specific bits of logic that you'd replace for your own worker
46 : : * are attach_to_queues() and copy_messages().
47 : : */
48 : : void
49 : 7 : test_shm_mq_main(Datum main_arg)
50 : : {
51 : : dsm_segment *seg;
52 : : shm_toc *toc;
53 : : shm_mq_handle *inqh;
54 : : shm_mq_handle *outqh;
55 : : volatile test_shm_mq_header *hdr;
56 : : int myworkernumber;
57 : : PGPROC *registrant;
58 : :
59 : : /* Unblock signals. The standard signal handlers are OK for us. */
60 : 7 : BackgroundWorkerUnblockSignals();
61 : :
62 : : /*
63 : : * Connect to the dynamic shared memory segment.
64 : : *
65 : : * The backend that registered this worker passed us the ID of a shared
66 : : * memory segment to which we must attach for further instructions. Once
67 : : * we've mapped the segment in our address space, attach to the table of
68 : : * contents so we can locate the various data structures we'll need to
69 : : * find within the segment.
70 : : *
71 : : * Note: at this point, we have not created any ResourceOwner in this
72 : : * process. This will result in our DSM mapping surviving until process
73 : : * exit, which is fine. If there were a ResourceOwner, it would acquire
74 : : * ownership of the mapping, but we have no need for that.
75 : : */
76 : 7 : seg = dsm_attach(DatumGetUInt32(main_arg));
77 [ - + ]: 7 : if (seg == NULL)
78 [ # # ]: 0 : ereport(ERROR,
79 : : (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
80 : : errmsg("unable to map dynamic shared memory segment")));
81 : 7 : toc = shm_toc_attach(PG_TEST_SHM_MQ_MAGIC, dsm_segment_address(seg));
82 [ - + ]: 7 : if (toc == NULL)
83 [ # # ]: 0 : ereport(ERROR,
84 : : (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
85 : : errmsg("bad magic number in dynamic shared memory segment")));
86 : :
87 : : /*
88 : : * Acquire a worker number.
89 : : *
90 : : * By convention, the process registering this background worker should
91 : : * have stored the control structure at key 0. We look up that key to
92 : : * find it. Our worker number gives our identity: there may be just one
93 : : * worker involved in this parallel operation, or there may be many.
94 : : */
95 : 7 : hdr = shm_toc_lookup(toc, 0, false);
96 : 7 : SpinLockAcquire(&hdr->mutex);
97 : 7 : myworkernumber = ++hdr->workers_attached;
98 : 7 : SpinLockRelease(&hdr->mutex);
99 [ - + ]: 7 : if (myworkernumber > hdr->workers_total)
100 [ # # ]: 0 : ereport(ERROR,
101 : : (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
102 : : errmsg("too many message queue testing workers already")));
103 : :
104 : : /*
105 : : * Attach to the appropriate message queues.
106 : : */
107 : 7 : attach_to_queues(seg, toc, myworkernumber, &inqh, &outqh);
108 : :
109 : : /*
110 : : * Indicate that we're fully initialized and ready to begin the main part
111 : : * of the parallel operation.
112 : : *
113 : : * Once we signal that we're ready, the user backend is entitled to assume
114 : : * that our on_dsm_detach callbacks will fire before we disconnect from
115 : : * the shared memory segment and exit. Generally, that means we must have
116 : : * attached to all relevant dynamic shared memory data structures by now.
117 : : */
118 : 7 : SpinLockAcquire(&hdr->mutex);
119 : 7 : ++hdr->workers_ready;
120 : 7 : SpinLockRelease(&hdr->mutex);
121 : 7 : registrant = BackendPidGetProc(MyBgworkerEntry->bgw_notify_pid);
122 [ - + ]: 7 : if (registrant == NULL)
123 : : {
124 [ # # ]: 0 : elog(DEBUG1, "registrant backend has exited prematurely");
125 : 0 : proc_exit(1);
126 : : }
127 : 7 : SetLatch(®istrant->procLatch);
128 : :
129 : : /* Do the work. */
130 : 7 : copy_messages(inqh, outqh);
131 : :
132 : : /*
133 : : * We're done. For cleanliness, explicitly detach from the shared memory
134 : : * segment (that would happen anyway during process exit, though).
135 : : */
136 : 7 : dsm_detach(seg);
137 : 7 : proc_exit(1);
138 : : }
139 : :
140 : : /*
141 : : * Attach to shared memory message queues.
142 : : *
143 : : * We use our worker number to determine to which queue we should attach.
144 : : * The queues are registered at keys 1..<number-of-workers>. The user backend
145 : : * writes to queue #1 and reads from queue #<number-of-workers>; each worker
146 : : * reads from the queue whose number is equal to its worker number and writes
147 : : * to the next higher-numbered queue.
148 : : */
149 : : static void
150 : 7 : attach_to_queues(dsm_segment *seg, shm_toc *toc, int myworkernumber,
151 : : shm_mq_handle **inqhp, shm_mq_handle **outqhp)
152 : : {
153 : : shm_mq *inq;
154 : : shm_mq *outq;
155 : :
156 : 7 : inq = shm_toc_lookup(toc, myworkernumber, false);
157 : 7 : shm_mq_set_receiver(inq, MyProc);
158 : 7 : *inqhp = shm_mq_attach(inq, seg, NULL);
159 : 7 : outq = shm_toc_lookup(toc, myworkernumber + 1, false);
160 : 7 : shm_mq_set_sender(outq, MyProc);
161 : 7 : *outqhp = shm_mq_attach(outq, seg, NULL);
162 : 7 : }
163 : :
164 : : /*
165 : : * Loop, receiving and sending messages, until the connection is broken.
166 : : *
167 : : * This is the "real work" performed by this worker process. Everything that
168 : : * happens before this is initialization of one form or another, and everything
169 : : * after this point is cleanup.
170 : : */
171 : : static void
172 : 7 : copy_messages(shm_mq_handle *inqh, shm_mq_handle *outqh)
173 : : {
174 : : Size len;
175 : : void *data;
176 : : shm_mq_result res;
177 : :
178 : : for (;;)
179 : : {
180 : : /* Notice any interrupts that have occurred. */
181 [ - + ]: 24608 : CHECK_FOR_INTERRUPTS();
182 : :
183 : : /* Receive a message. */
184 : 24608 : res = shm_mq_receive(inqh, &len, &data, false);
185 [ + + ]: 24608 : if (res != SHM_MQ_SUCCESS)
186 : 7 : break;
187 : :
188 : : /* Send it back out. */
189 : 24601 : res = shm_mq_send(outqh, len, data, false, true);
190 [ - + ]: 24601 : if (res != SHM_MQ_SUCCESS)
191 : 0 : break;
192 : : }
193 : 7 : }
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